ANTI-CORROSIVE PAINT SYSTEMS
BASED ON CONDUCTING POLYMERS
Master of Science in Engineering Thesis
by M.Sc.Eng. Student
Marie Louise Petersen
Supervisor: Per Møller
The Danish Technical University – Marts 2006
I
ABSTRACT
The corrosion protection of anti-corrosive paint systems based on conducting polymers is
examined in the following four application areas: shopprimers, ballast tank protection, protection
in off-shore environment and protection of aluminium. The conducting polymers used as an
additive are polyaniline and polythiophene. Five possible hypotheses for the protection
mechanism of conducting polymers are stated, and the aim is to examine the credibility of these
hypotheses.
For shopprimer applications, no improvement of the corrosion protection is established by the
addition of polyaniline to a conventional shopprimer. In some of the applied shopprimers,
polyaniline seems to accelerate the degradation of the paint film instead. The main explanation for
these observations is a very low dispersibility of polyaniline in the paint.
For protection of ballast tanks, addition of polyaniline to a conventional paint system seems to
increase the resistance to cathodic disbonding. The assumption is made that polyaniline acts as an
amphoteric material and reacts with the hydroxide ions, which are produced in the cathodic
reaction. During this reaction, polyaniline is reduced to a non-conductive base form.
For protection in off-shore environment, no improvement of the corrosion resistance is
established by adding polyaniline to a conventional zinc primer. The effect of the cathodic
protection is lowered, therefore polyaniline is not a suitable conductor between the zinc particles.
Some positive effect is registered in the water based paint system with the addition of
polythiophene in the primer. It is evaluated that a higher degree of dispersibility is the main
explanation for this observation.
Finally, the protective character of a conventional polyaniline containing primer is examined on
aluminium. The efficiency is compared to other surface treatments that are especially designed for
protection of aluminium. The results show that the polyaniline containing primer has a low
adhesion to the substrate material, and the resistance to filiform corrosion is also not acceptable.
The best corrosion protection is established by a conventional coating system with an anodizing
pre-treatment.
II
PREFACE
This thesis, commenced in the autumn of 2005 at the Institute of Production and
Management (IPL) at the Danish Technical University in Lyngby, is a 30 ECTS points project.
This thesis is part of the education of Master of Science in Chemical Engineering, specializing in
material science. The preparation and execution of the project is done in collaboration with the
Danish company Hempel.
Several people and companies in and outside IPL have been very helpful during the project period.
I would among them like to thank my supervisor, professor Per Møller.
I would like to thank the companies Chemetall, for providing standardized aluminium test panels,
and Altitech, for performing the coating process altizizing. Thanks are also given to the German
company Ormecon, which have provided paint to the project. Finally, I would like to thank Claus
Weinell from Hempel for providing expertise and guidance, and the test department for their
assistance during execution of the experiments.
_______________________________
Marie Louise Petersen, s031909
Lyngby, 1st of marts 2006
III
TABLE OF CONTENTS
Abstract .................................................................................................................................. I
Preface ................................................................................................................................... II
Table of contents................................................................................................................. III
List of figures ........................................................................................................................ V
List of tables .........................................................................................................................VI
1
Introduction.................................................................................................................... 1
1.1
Motivation..................................................................................................................................... 1
1.2
Problem formulation ................................................................................................................... 1
2
Theory............................................................................................................................. 2
2.1
Conductive polymers................................................................................................................... 2
2.1.1
Introduction to conductive polymers ........................................................................................... 2
2.1.2
Electrical conductivity .............................................................................................................. 3
2.1.3
Conjugated polymers ................................................................................................................ 4
2.1.4
Chemical structure of polyaniline and polythiophene .................................................................. 5
2.1.5
Doping of conjugated polymers.................................................................................................. 6
2.1.6
Mechanism of polymer conductivity ........................................................................................... 7
2.2
Corrosion theory........................................................................................................................ 11
2.2.1
What is corrosion?................................................................................................................. 11
2.2.2
Potential................................................................................................................................ 12
2.2.3
Pourbaix diagram ................................................................................................................. 14
2.2.4
Corrosion of steel ................................................................................................................... 15
2.2.5
Corrosion of aluminium ......................................................................................................... 16
2.2.6
Corrosion of painted steel and aluminium ............................................................................... 18
2.3
Anti-corrosive paint................................................................................................................... 21
2.3.1
What is paint? ...................................................................................................................... 21
2.3.2
Binders.................................................................................................................................. 22
2.3.3
Pigments................................................................................................................................ 23
2.3.4
Solvents ................................................................................................................................. 24
2.3.5
Additives............................................................................................................................... 25
2.4
Corrosion protection by conductive polymers...................................................................... 26
2.4.1
Hypotheses of anti-corrosive mechanisms ................................................................................. 26
2.4.2
Article review of conducting polymers ...................................................................................... 28
2.4.3
Research from Ormecon.......................................................................................................... 31
3
Experimental procedure............................................................................................... 33
3.1
Applied paint systems................................................................................................................ 33
3.1.1
Test panels and surface preparation ........................................................................................ 33
3.1.2
Shopprimer............................................................................................................................ 34
3.1.3
Paint for ballast tank ............................................................................................................ 35
3.1.4
Paint for off-shore environment ............................................................................................... 36
3.1.5
Paint for aluminium substrates .............................................................................................. 37
3.2
Paint film characterization........................................................................................................ 38
3.2.1
Electromagnetic induction....................................................................................................... 38
3.2.2
Eddy Current Test ................................................................................................................ 39
IV
3.2.3
Scanning Electron Microscopy ................................................................................................ 39
3.2.4
Pull-off test ............................................................................................................................ 40
3.3
Corrosion resistance .................................................................................................................. 40
3.3.1
Open Circuit Potential........................................................................................................... 41
3.3.2
Anodic polarization............................................................................................................... 41
3.3.3
Natural Weathering Test ...................................................................................................... 42
3.3.4
Mebon Prohesion Test ........................................................................................................... 42
3.3.5
Cathodic Protection Test ........................................................................................................ 43
3.3.6
Immersion Test...................................................................................................................... 43
3.3.7
Salt Spray Test ..................................................................................................................... 44
3.3.8
Cyclic Test............................................................................................................................. 44
3.3.9
Filiform Corrosion Test ......................................................................................................... 45
4
Results and discussion ................................................................................................. 46
4.1
Shopprimer ................................................................................................................................. 46
4.1.1
PVB shopprimers.................................................................................................................. 46
4.1.2
Zinc phosphate shopprimers ................................................................................................... 48
4.1.3
Zinc silicate shopprimers ........................................................................................................ 49
4.2
Paint for ballast tanks ................................................................................................................ 50
4.3
Paint for off-shore environments............................................................................................ 53
4.3.1
Solvent based paint system ..................................................................................................... 53
4.3.2
Water based paint system....................................................................................................... 57
4.4
Paint for aluminium substrates ................................................................................................ 62
5
Conclusion .................................................................................................................... 67
6
Perspective.................................................................................................................... 68
Abbreviations ..........................................................................................................................i
Bibliography...........................................................................................................................ii
List of appendix..................................................................................................................... vi
V
LIST OF FIGURES
Figure 1. Conductivity of conducting polymers compared to other materials. .................................... 3
Figure 2. Chemical structure of polythiophene and polyaniline. ............................................................ 5
Figure 3. Outline of the redox-reactions that combine the emeraldine form and the leuco form of
polyaniline and their matching chemical structures. ........................................................................ 5
Figure 4. Chemical structure of poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate)................ 6
Figure 5. General energy band gap diagram for an insulator, a semi-conductor and a conductor. ... 8
Figure 6 Formation of a radical cation (polaron) by removal of a electron on the 5th carbon atom
of an undecahexaene chain (a→b). Migration of a polaron is shown in (c→e). ......................... 9
Figure 7. A polaron on a polyparaphenylene chain. ................................................................................. 9
Figure 8. Illustration of the energy band gap of a quinoid and a benzene unit. ................................. 10
Figure 9. Intersoliton hopping between two undecahexaene chains.................................................... 10
Figure 10. Pourbaix diagram for aluminium in water............................................................................. 14
Figure 11. Pourbaix diagram for iron in water......................................................................................... 16
Figure 12. Illustration of a blister............................................................................................................... 18
Figure 13. Illustration of undercutting corrosion.................................................................................... 18
Figure 14. Reaction mechanism for filiform corrosion. ......................................................................... 19
Figure 15. Model for cathodic disbondment of a painted steel specimen........................................... 20
Figure 16. Reaction mechanism for passivation of iron using polyaniline according to Ormecon. 32
Figure 17. OCP potential of PVB based shopprimers in 3,5w% NaCl. .............................................. 47
Figure 18. Polarization curves of PVB based shopprimers in 3,5w% NaCl. ...................................... 47
Figure 19. Cathodic disbonding after exposure in CPT......................................................................... 50
Figure 20. Cathodic disbonding after seawater immersion.................................................................... 52
Figure 21. Undercutting corrosion of solvent based paint system.. ..................................................... 55
Figure 22. OCP measurements of water based paint in 3,5w% NaCl. ................................................ 58
Figure 23. Polarization curves of water based paint with 85w% Zn in 3,5w% NaCl........................ 58
Figure 24. Polarization curves of water based paint with 60w% Zn in 3,5w% NaCl........................ 59
Figure 25. Undercutting corrosion of water based paint system. ......................................................... 60
Figure 26. SEM pictures of a zinc primer in secondary signal and backscattered signal................... 61
Figure 27. Surface topography of altizized and anodized aluminium panels. ..................................... 64
Figure 28. OCP potential of aluminium specimens in 3,5w% NaCl.................................................... 65
Figure 29. Polarization curves of aluminium specimens in 3,5w% NaCl.......................................65
VI
LIST OF TABLES
Table 1. Standard electrode potential of various metals......................................................................... 12
Table 2. Classification scheme of binders according to their chemical reactions............................... 22
Table 3. Classification scheme of pigments according to their functionality...................................... 23
Table 4. Classification scheme of solvents with a description of their application range. ................ 24
Table 5. Classification scheme of additives according to functionality................................................ 25
Table 6. Review of articles related to corrosion protection of aluminium by polyaniline................. 29
Table 7. Review of articles related to corrosion protection of steel by polyaniline. .......................... 30
Table 8. Review of articles related to corrosion protection of steel by polythiophene. .................... 31
Table 9. Requirements and specifications of shopprimers. ................................................................... 34
Table 10. Overview of applied shopprimers............................................................................................ 35
Table 11. Requirements and specifications of paints for ballast tanks................................................. 35
Table 12. Overview of applied paints for ballast tank protection......................................................... 36
Table 13. Requirements and specifications of paint systems for off-shore protection. .................... 36
Table 14. Overview of applied paint systems for off-shore protection. .............................................. 37
Table 15. Overview of water based paint systems for off-shore applications. ................................... 37
Table 16. Overview of applied coating systems for protection of aluminium.................................... 38
Table 17. Test methods used for characterization of the corrosive behaviour in the different
application areas. ................................................................................................................................. 41
Table 18. Test conditions of the Mebon Prohesion Test. ..................................................................... 42
Table 19. Test description and applied conditions for the cyclic test. ................................................. 44
Table 20. Measurements of the cathodic disbonding after the cathodic protection test. ................. 51
Table 21. Measurements of the cathodic disbonding after the immersion test. ................................. 52
Table 22. Measurements of adhesion by pull-off test after the salt spray test. ................................... 54
Table 23. Measurements of undercutting corrosion after the cyclic test. ............................................ 56
Table 24. Measurements of the adhesion by pull-off test after the cyclic test. ................................... 56
Table 25. Measurements of undercutting corrosion after the cyclic test. ............................................ 61
Table 26. Evaluation of the adhesion after the salt spray test. .............................................................. 63
Table 27. Evaluation of the adhesion after the cyclic test...................................................................... 63
1 INTRODUCTION
1
1 INTRODUCTION
1.1
Motivation
The corrosion and degradation of metals is an important issue and constitutes a big
expense to society. In order to improve the corrosion resistance of metals, continuous research is
necessary in order to develop new types of protective surface treatments.
One type of protective surface treatment is the application of anti-corrosive paint systems. The
protective character of these paint systems has, up until now, primarily been based on zinc rich
paints which provide cathodic protection of, for instance, steel. However, this protection is only
present if unused zinc particles are present in the paint. The lifetime is therefore limited and
continuous maintenance is needed in order to retain the cathodic protection.
The development of anti-corrosive paint systems based on electrically conducting polymers is
therefore an interesting alternative to conventional zinc paints. These new paint systems are also
more environmentally friendly and cheaper to manufacture than zinc paints. Besides these
advantages, there is a great scepticism about the efficiency of the paint systems and the application
is still at research level. This report will try to uncover some of the problems related to these new
paint systems and find possible application areas in relation to corrosion protection of metals.
1.2 Problem formulation
The aim of this project is to examine the corrosion protection of steel and aluminium by
paint systems based on conducting polymers. Different paint systems are produced and tested in
order to uncover the possible application areas of these paint systems. The conducting polymers
used in this project are polyaniline and polythiophene, and the different paint formulations are
prepared at the facilities of Hempel. A commercial paint system named CORRPASSIV from
Ormecon, which contains polyaniline, is also tested and evaluated.
On the basis of a thorough information retrieval, possible hypotheses about the anti-corrosive
effect of polyaniline and polythiophene are stated. This project will examine the credibility of these
hypotheses by several experiments performed at Hempel and DTU. Finally, the efficiency of the
formulated paint systems is evaluated in relation to other anti-corrosive paint systems and surface
treatments. In the case of steel protection, zinc rich paint systems are used for comparison, and for
protection of aluminium, the efficiency is evaluated in comparison to three surface treatment
methods named anodizing, chromatizing and altizizing.
2 THEORY
2
2 THEORY
This introductory theory section tries to give the reader a deeper understanding of the
basic concepts of conducting polymers and the application of conducting polymers in anticorrosive paint systems for the protection of steel and aluminium. The theory section is divided
into 4 sections, each with their own subject area. Section 2.1 concerns general aspects of
conducting polymers, section 2.2 describes basic corrosion theory and mechanisms, and section
2.3 describes the composition and formulation of anti-corrosive paint system. Section 2.4 presents
previous results and hypotheses about corrosion protection by conducting polymers.
2.1 Conductive polymers
This section describes the basic chemical and physical characteristics of conducting
polymers, exemplified by polyaniline and polythiophene. In particular, the chemical structure and
its importance for different physical and chemical properties are presented.
2.1.1 Introduction to conductive polymers
What is a conductive polymer and how is it possible that a polymer can behave as an
electrically conductive material? These are the basic questions when conductive polymers are
introduced as a new subject. This introductory chapter attempts to explain the basic principles of
conductive polymers and describes how conductive polymers act and how they differentiate from
other polymer materials.
A polymer is, in chemical terms, a long molecule formed of repeating smaller units called
monomers. Most of the polymers are known as electrical insulators due to their chemical structure
and nature. Polymers normally contain electrons which are closely packed to the atoms by
covalent bonds. This lack of mobility of the electrons is the reason why most of the polymers
behave like insulators.
An interesting discovery was however made in 1977 by Hideki Shirakawa, Alan MacDiarmid and
Alan Heeger. These three scientists discovered that an oxidation of polyacetylene with iodide,
bromide or chloride vapour increases the conductivity of the material by a factor 109. This is due
to the fact that electrons are jerked out of the polymer chain, leaving holes in form of positive
charges that can move along the chain. The experiments thus demonstrated that polyacetylene has
the ability to possess both insolating and conductive properties, depending on the degree of
2 THEORY
3
oxidation. The three scientists received in 2000 the Nobel Prize in chemistry for the discovery and
development of electrically conductive polymers. [1]
Other polymers such as polyaniline and polythiophene possess the same conductive properties as
polyacetylene. Each of these polymers is characterized by a chemical structure based on
conjugated double bonds and named conjugated polymers. Figure 1 illustrates the conductivity
range of these conjugated polymers ranging from insulating to conductive materials.
Figure 1. Conductivity of conducting polymers compared to other materials from quartz (insulator) to copper
(conductor). [1]
The potential of these conductive polymers is very broad and includes different fields such as
corrosion inhibitors, antistatic coatings, sensing devices and electromagnetic shielding. The main
advantage of these conductive polymers is their cheap manufacturing cost and their high degree of
flexibility. This makes them suitable in many fields and they will probably be used as a replacement
for metals and other materials in the future. [1]
2.1.2 Electrical conductivity
Electrical conductivity can be described as the ability of a substance to conduct an
electrical current. The conductivity is defined by Ohm’s law:
U = R·I
(2.1)
where U is the drop in potential (in Volts), I is the current (in Amperes) and R is the resistance of
the system (in Ohms). It is important to note that Ohm’s law is an empirical law and not all
2 THEORY
4
materials obey Ohm’s law. Examples of materials that deviate from Ohm’s law are vacuum tubes,
semiconductors and one-dimensional conductors. For Ohmic materials the resistance is
proportional to the length of the sample (l) and inverse proportional to the cross-section area (A),
according to equation 2.2:
R = ρ · (l/A) = (1/ σ ) · (l/A)
(2.2)
where ρ is a proportionality factor called the resistivity. The inverse value of the resistivity is the
conductivity (σ) with the unit S/m.
Generally, the conductivity depends on the number of charge carriers (n), the mobility (µ) and the
charge (q) of these carriers according to equation 2.3.
σ = n· µ ·q
(2.3)
For highly conductive materials such as metals, the conductivity will increase as the temperature
decreases, while the opposite is valid for semiconductors and insulators. Conductive polymers
such as polyacetylene follow the same temperature dependence as semiconductors i.e. decreasing
conductivity with falling temperature. [1]
2.1.3 Conjugated polymers
Each of the conducting polymers is characterized from a structural point of view as being
a conjugated polymer. Conjugated polymers are materials which contain conjugated π-bonds in
their chemical structure. These π-bonds are essential for the ability of the polymer to behave as an
electrically conductive material.
Conjugated polymers are constructed by conjugated double bonds, i.e. double bonds between
every second C-atom. By visualizing one of the bonds in these double bonds, one finds that the
electrons are moving in a special shell called a p-orbital. This type of orbital differentiates a lot
from the molecule shells in which the remaining binding electrons travel through. A p-orbital has
the ability to overlap similar p-orbitals from the following C-atoms, and forms a big coherent shell
spread along the carbon chain. The electrons in this coherent shell are described as π-electrons and
are characterized by their free mobility in the polymer chain.
This free mobility of the π-electrons is important if the polymers shall behave as a conductive
material. If a charge is introduced to the polymer chain, it can move along the π-electron clouds.
The charge can also move from one polymer chain to another if the π-electron clouds from each
2 THEORY
5
polymer chain are overlapping each other. These properties are essential in order to create a
conducting polymer. [2], [3]
2.1.4 Chemical structure of polyaniline and polythiophene
The two polymers polyaniline
and polythiophene are used in this
project and are examined according to
their anti-corrosive effect. Both polymers
are conjugated polymers which contain
conjugated
π-bonds.
structure
of
The
polyaniline
chemical
and
polythiophene are illustrated in figure 2.
Figure 2. Chemical structure of polythiophene (top) and
polyaniline (bottom).
The chemical structure of polyaniline is arranged by coherent aniline monomers. The illustrated
structure of polyaniline in figure 2 is called leucoemeraldine or leuco due to its oxidation stage.
Polyaniline is proven to assess three different oxidation stages with leucoemeraldine as the fully
reduced form. If polyaniline appears in a fully oxidized form it is termed pernigraniline, while the
half oxidized form is termed emeraldine. Polyaniline normally exists as leucoemeraldine or
emeraldine. The transformation between these two oxidation forms takes place through a series of
redox-reactions according to the reaction scheme on figure 3. [4], [5]
Figure 3. Outline of the redox-reactions that combine the emeraldine form and the leuco form of polyaniline
and their matching chemical structures. [4]
2 THEORY
6
The illustrated forms of polyaniline on figure 3 will in practice possess very different chemical and
physical properties. Only polyaniline on an emeraldine salt form has the functionality of an
electrically conductive material with a conductivity of 10-1000 S/cm. In comparison, the
conductivity of the base form is about 10-10 S/cm. [6]
Polythiophene has a chemical structure which is arranged by coherent units of thiophene, as
recalled on figure 2. A modified polythiophene product is used in this project in order to achieve a
better degree of dispersion in the paint formulation. The applied polythiophene product is called
Baytron
P
and
is
an
aqueous
dispersion
of
poly(3,4-ethylenedioxythiophene)
poly(styrenesulfonate), in short PEDOT/PSS. Figure 4 shows the chemical structure of the
complex between poly(3,4-ethylenedioxythiophene) in the bottom and poly(styrenesulfonate) in
the top.
Figure 4. Chemical structure of poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate). [7]
The conductivity of PEDOT/PSS is maximum 10 S/cm, but is strongly dependant on the type of
coating formulation. [7]
2.1.5 Doping of conjugated polymers
In order for a conjugated polymer to behave as an electroactive conducting material, it
must be exposed to a process called doping. During the doping process, an ionization of the
polymer chain takes place. This introduces charge carriers to the chain, which in turn increases the
conductivity of the polymer.
2 THEORY
7
Different kinds of doping methods exist, but common to all of them is that they are reversible
processes. The doping can be divided into two sub-classes called p-doping and n-doping.
In p-doping an oxidation of the polymer chain takes place for example by means of halogens such
as I2. In contrast, n-doping causes a reduction of the polymer chain and normally used reduction
mediums are alkali metals such as lithium and potassium. P-doping is the most useful method to
ionize the polymer chain because experiments have revealed that p-doping leads to more stable
compounds compared to n-doping. Alkali metals used in n-doping are unstable in the atmosphere,
and furthermore oxygen acts as an oxidant which neutralizes the n-doping. [2]
The doping process can also be described as a redox process and is carried out by either chemical
or electrochemical processes. In the context of the application of polyaniline in anti-corrosive
paint systems, doping will normally take place chemically. A very useful chemical method for
doping of polyaniline is by addition of acid which liberates protons to the polymer chain. Doping
of polyaniline by means of camphor sulphonic acid has proved to give a very high conductivity
(200-400 S/cm) and is stable at the same time. [8]
A de-doping of the conducting polymers can also take place. In the case of polyaniline, addition of
base (e.g. NH4OH) is the normal method for de-doping. De-doping can also take place due to pHchanges in the surrounding environment. An increase in the degree of protonation will appear if
the pH is dropping in the surrounding environment, making the polymer more conductive. At the
same time, an alkaline environment causes a de-doping and lowers the conductivity. Polyaniline is
therefore able to adjust its conductivity from the chemical changes in the environment during
application.
2.1.6 Mechanism of polymer conductivity
Materials can be classified according to their electrical conductivity as conductors, semiconductors and insulators. Metals are typically good conductors, while polymers often act as
insulators. The development of conducting polymers is therefore an important step in the polymer
technology.
The electrical properties of a material can always be determined according to its electronic
structure. A method to describe the electrical conductivity of a material is the energy band gap
theory. This theory describes the energy states of the electrons in a material. The outer shell of
electrons in a material contains the so called valence electrons. Valance electrons can be placed
theoretically in the valence band which defines the lowest energy state. The valence band describes
2 THEORY
8
the highest occupied band, while the lowest unoccupied band is called the conduction band. In
order for a material to conduct electricity, the electrons in the valence band must contain sufficient
energy to reach the conduction band and the difference between the valence and conduction band
must be small. The difference between the two bands is called the band gap. In a metal, the density
of electronic states is very high and the electrons which have relative low binding energy can move
freely from one atom to another. According to the energy band gap theory, this means that the
valance band and the conduction band are partly overlapping each other and the electrons can
easily pass into the conduction band. In insulating materials, the band gap is so big that the
electrons are fixed into the valence band. Therefore, an electrical current can not be conducted
through the material. Finally, semi-conductors will contain a moderate band gap. This allows a
partial transfer of electrons to the conductive band, leaving an electron hole in the valence band.
The three energy band gap states are illustrated in figure 5. [9], [10]
Figure 5. General energy band gap diagram for an insulator, a semi-conductor and a
conductor. [11]
Conducting polymers obtain their electrically conductive properties through a combination of a
conjugated π-electron structure and the doping of the polymer chain. When the polymer is doped,
for instance by an oxidation with iodine, an electron is removed from the top of the valence band.
This creates a vacancy (hole) which does not delocalize completely. If an electron is removed
locally from one carbon atom on the polymer chain, a radical cation will be formed (see figure 6
b).
2 THEORY
9
Figure 6 Formation of a radical cation (polaron) by removal of a
electron on the 5th carbon atom of an undecahexaene chain (a→b).
Migration of a polaron is shown in (c→e). [1]
A radical cation, which is also called a polaron, is localized by a Coulomb attraction to its
counterion (e.g. I3-) and by a local change in the equilibrium geometry of the polaron relative to the
neutrale molecule. The mobility of the polaron is normally quite high and the charge is carried
along the polymer chain according to figure 6 c-e. [1]
Figure 7. A polaron on a polyparaphenylene chain. [12]
If another electron is removed from the oxidized polymer chain, a second independent polaron or
a new bipolaron is created. Figure 7 shows the presence of a bipolaron in polyparaphenylene. A
bipolaron is more stable than two polarons in spite of the repelling coulomb forces between the
two ions. A bipolaron has the charge +2e and spin 0 while a positive polaron will have the charge
+e and spin ½. The spin condition refers to where the bonding defect is placed in the band gap.
Thus, spin ½ means that the defect is placed in the middle of the band gap. [12]
Other important polymer chain defects for the conductivity of conjugated polymers are solitary
wave defects called solitons. Solitons can either be neutral or charged, both having a spin of 0. A
2 THEORY
10
soliton is characterized by its free mobility and that no disorder of the structure is created when a
soliton moves along the polymer chain. [2], [13]
When defects such as solitons, polarons and
bipolarons are present in the polymer chain,
they will act as charge carriers and increase the
conductivity. In the case of polyaniline, the
movement of defects will cause the conjugated
bindings to move about and form quinoid units
instead of benzene units. Quinoid units will
have a smaller band gap and a higher energy in
the valance band than benzene units (see figure
8). A doping of polyaniline will therefore
increase the amount of quinoid units, and thus
the conductivity. [14], [15]
Figure 8. Illustration of the energy band gap of a
quinoid and a benzene unit. [14]
The actual conductivity of the polymer is limited by intermolecular charge transfer reactions, i.e.
the need for the electrons to jump from one polymer chain to another. Other important factors
are related to macroscopic terms such as bad contact between different crystalline domains in the
material. One mechanism to account for the conductivity of conjugated polymers is called
“intersoliton hopping”. This describes the charge-hopping between different polymer chains.
Figure 9 illustrates the principle where a charged soliton (bottom) is trapped by two dopant
counterions and a neutral soliton (top) are free to move. When a neutral soliton is placed on a
chain close to another chain with a charged soliton, interactions can take place. Interactions
between the two polymer chains will cause the electron to jump from one defect to the other. [1]
Figure 9. Intersoliton hopping between two undecahexaene chains. [1]
2 THEORY
11
2.2 Corrosion theory
This section is a pre-study of the basic theory about corrosion kinetics and
thermodynamics. The general corrosion behaviour of steel and aluminium is described, and the
most common failure mechanisms of paint are explained in the end of the section.
2.2.1 What is corrosion?
Corrosion of metals is defined as an electrochemical reaction between the metal and the
surrounding environment. These electrochemical reactions are driven by charged and neutral
particles, in both the metal and the environment. Free electrons are, for instance, located in the
metal and are able to move through the metal. In the surrounding environment, positive and
negative ions as well as neutral molecules are present, leading to a corrosive environment.
Corrosion of metals can only occur if an anode/cathode relation is established, where the metal
works as an anode and oxidizes. During oxidation, metal ions will form and be released to the
surrounding environment, in the course of electron release:
M → Mn+ + ne-
(2.4)
The anodic reaction will thus drive an anodic current, ia, running in the direction metal → solution.
To compensate for the anodic reaction, a cathodic reaction will simultaneously take place.
Electrons, when released in the anodic reaction, will form part of the cathodic reaction and reduce
another chemical substance. Examples of cathodic reactions are the reduction of hydrogen or
oxygen. In the cathodic reaction, a cathodic current, ic, will run in the direction solution → metal.
Anodic and cathodic reactions will take place simultaneously and in electrical equilibrium so that
the electron- and current fluxes are in balance, i.e. ∑ia = ∑ik.
The electrochemical reactions will result in an electrical current in any given system. This current
depends on the potential difference between the metal and the surrounding environment. The
kinetics of the different anodic and cathodic reactions is therefore an expression of the relation
between the potential, E, and the reaction rate with the corresponding electrical intensity, i. An E-i
mapping is used to graph the anodic and cathodic polarization curves through which the corrosion
potential and corrosion rate can be determined. The point where the anodic and cathodic
polarization curves cross each other is the point where the anodic and cathodic currents are equal.
A reading of the corrosion potential, E, and the corrosion current, icor, can be done at the point of
intersection.
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Faradays law can subsequently calculate an estimate of the corrosion rate:
m=
1
A
⋅ ⋅ I ⋅t
96500 n
(2.5)
where m is the weight loss, A is the atomic mass of the metal, n is the valence, I is the intensity ( =
icor) and t is the time. It is important to point out that the weight loss determination in equation 2.5
is only valid for uniform corrosion. If the metal is exposed to local corrosion such as pitting and
crevice corrosion, a true estimate is not possible by using this equation. [16], [21]
2.2.2 Potential
The potential difference between the metal and surrounding environment is called the
absolute potential or just the metal potential. In order to determine the absolute potential of a
given metal, a connection between the metal and a stable electrode must be established. A
reference electrode with a constant potential in stable conditions of temperature and pressure is
therefore used for potential measurements. The hydrogen equilibrium in reaction 2.6 is, per
definition, chosen to have a potential of 0V at 1013 bar and 20 °C.
2H+ + ne- ↔ H2
(2.6)
The standard electrode potential can be arranged for a number of different metals and their
respective oxidation reactions. A selection of different standard electrode potentials is listed in
table 1.
Table 1. Standard electrode potential of various metals. [21]
Reaction
Potential vs. SHE (V)
Au ↔ Au3+ + 3e-
+1,42
Ag ↔ Ag2+ + 2e-
+0,80
Cu ↔ Cu2+ + 2e-
+0,34
Fe ↔ Fe2+ + 2e-
-0,44
Zn ↔ Zn2+ + 2e-
-0,76
Al ↔ Al3+ + 3e-
-1,66
Mg ↔ Mg2+ + 2e-
-2,38
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The standard electrode potentials in table 1 can be used to determine the electrochemical character
of different metals. Metals with a high and positive potential, such as gold, have a very poor
tendency to oxidize and are therefore noble metals. In contrast, metals with a strongly
electronegative potential, such as magnesium, will have a great affinity for oxidation, and are
therefore un-noble metals.
Table 1 is, in practice, a useful tool for anticipating whether two metals will evolve a galvanic
coupling. As an example, zinc will function as an anode in connection with more noble metals
such as iron and copper. This property is employed for cathodic protection of, for example, steel
tanks, where sacrificial anodes of zinc are mounted to the tank. Sacrificial anodes will therefore
corrode in favour of the steel tank.
It is, however, important to state that the standard electrode potential is a thermodynamic
parameter which only expresses whether a reaction is possible or not in theory. Standard electrode
potentials are not valid for determining the kinetics of a given reaction. Aluminium is a very good
example, which, according to table 1, is very unstable and easily corrodes. But general practice
shows that the corrosion of aluminium is reduced by a natural oxide layer which is formed on the
surface of aluminium. Aluminium is therefore a so-called passive metal. [21]
Reality, however, rarely relies on standard conditions and the metal potential therefore deviates
from the standard electrode potential. To determine the metal potential of non-standard activities
or concentrations, an expression called Nernst equation can be used:
E = E0 +
RT
⋅ log(a M n +1 )
nF
(2.7)
where E0 is the standard electrode potential, R is the gas constant, T is the temperature, n is the
valence of the ion, F is Faradays constant and a is the activity of metal ions. At 20 °C, equation 2.7
can be rewritten as the following expression:
E = E0 +
0,058
⋅ log(a M n +1 )
n
(2.8)
If the activity is 1, it is clear that Nernst equation is reduced to E = E0.
The “real” potential is often influenced by different factors which complicate the application of
Nernst equation. These factors are primarily related to changes in the surrounding environment,
but different material properties also have a major effect on the potential. [16]
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14
2.2.3 Pourbaix diagram
A way to determine the corrosion properties of a metal is by means of the so called
Pourbaix diagram. A Pourbaix diagram illustrates the connection between E and pH for a metal in
an aqueous environment at a given pressure, temperature and concentration. The diagram is based
on thermodynamic equilibrium data and considers three types of equilibrium states: 1) equilibrium
between solid substances, 2) equilibrium between two substances in solution and 3) equilibrium
between a solid and a substance in solution. A Pourbaix diagram is therefore based on three
domain areas: a) corrosion, b) passivity and c) immunity.
Figure 10 shows the Pourbaix diagram for aluminium and water at 25 °C. The red areas mark the
conditions where aluminium has a risk of corroding and changing to an ion state as Al3+ or AlO2depending on the environment. In the light blue area, aluminium forms a protective oxide layer
that is insoluble in water. Finally, aluminium remains immune in surroundings that are strongly
reducing i.e. where the potential is very low.
Figure 10. Pourbaix diagram for aluminium in water.
It is important to state that the Pourbaix diagram is based on thermodynamic equilibrium data,
where the stable compounds and their domain areas and reaction ways are calculated. The
diagrams do not give information about reaction kinetics and make no allowances for impurities or
additives in the liquid or the metal. [21]
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2.2.4 Corrosion of steel
Corrosion of steel is a well-known problem, and can normally be recognized by the
formation of red rust on the surface. In order for the steel to corrode and form rust, anodic and
cathodic reactions must occur. In the anodic reaction, iron is oxidized and forms divalent cation
Fe2+ with the liberation of two electrons (cf. reaction 2.9). In reality, the most frequent cathodic
reaction is the reduction of oxygen, as shown in reaction 2.10.
Fe → Fe2+ + 2e-
(2.9)
O2 + 2H2O + 4e- → 4OH-
(2.10)
The iron ions given in reaction 2.9 will subsequently react with the hydroxide ions and form the
complex iron hydroxide:
Fe2+ + 2OH- → Fe(OH)2
(2.11)
Since oxygen dissolves readily in water, there is normally an excess of it. A further reaction with
the formation of the well known red rust can therefore take place according to reaction 12.
4Fe(OH)2 + O2 + 2H2O → 2Fe2O3·H2O + 4H2O
(2.12)
The stability of iron can be illustrated by means of a Pourbaix diagram. Figure 11 on the following
page shows the Pourbaix diagram for iron in water at 30°C with an ion molarity of 1,0E-06.
As illustrated on figure 11, iron is only immune at a potential below -650 mV. Above the immune
area, iron will corrode in acidic environments with the formation of Fe2+ or Fe3+. In neutral
environments, iron will have the tendency to form oxides normally as Fe2O3 or Fe3O4. In strongly
alkaline environments, iron will corrode and form either FeO2- or HFeO2-.
It is important to state that the presence of chloride ions in the environment has a big influence on
the stability of iron and increases the risk of active corrosion. The ability of iron to form oxides
will additionally decrease, and the passive properties are harder to reach. [16], [17]
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Figure 11. Pourbaix diagram for iron in water.
2.2.5 Corrosion of aluminium
Aluminium and its alloys are normally resistant to corrosive attack. This is due to the fact
that aluminium is a passive metal which forms a protective oxide layer on the substrate surface.
Corrosion can however occur in conditions where the passive layer is unstable.
In aqueous environments, corrosion of aluminium can occur according to the succeeding
reactions. Reaction 2.13 shows the anodic reaction where aluminium with oxidation stage 0
oxidizes and forms trivalent cation Al3+ in the course of electron release.
Al → Al3+ + 3e-
(2.13)
In order to obtain balance in the system, a matching cathodic reaction takes place simultaneously.
Two types of reduction reactions will normally dominate in aqueous and neutral environments:
3H+ + 3e- → 3/2H2
(2.14)
O2 + 2H2O + 4e- → 4OH-
(2.15)
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Adding written anodic and cathodic reactions results in a total reaction scheme. Reactions 2.16 and
2.17 show the two possible reactions for corrosion of aluminium in aqueous neutral and acidic
environments.
Al + 3H+ → Al3+ + 3/2H2
(2.16)
Al + 3H2O → Al(OH)3 + 3/2H2
(2.17)
As it appears in reaction 2.17, corrosion of aluminium will in some cases cause a formation of
aluminium hydroxide, Al(OH)3, which is insoluble in water and precipitates as a white gel on the
metal surface.
As mentioned earlier, aluminium is a metal that easily passivates. Aluminium, when in contact with
oxygen, will spontaneously form a natural oxide layer that functions as a protective barrier layer,
which reduces the substrate corrosion. Reaction 2.18 shows the reaction between aluminium and
oxygen with the formation of aluminium oxide.
2Al + 3/2O2 → Al2O3
(2.18)
Reaction 2.18 has a very high free energy of -1675 kJ which indicates that aluminium is a metal
with great affinity to oxygen. [21]
The structure of this oxide layer is based on two layers, with a total thickness of 4 to 10 nm. The
first layer is a compact and amorphous layer that works as a barrier layer. It is formed
instantaneously by contact with oxygen and the reaction rate is independent of the oxygen partial
pressure. This means that the layer is able to repair itself if, for instance, the metal is exposed to
mechanical stress in different machining operations. The growth of the layer is due to migration of
Al3+ ions through the oxide layer and a maximum thickness of about 4 nm can be achieved.
The second layer is contrary to the first, porous and less compact. This layer is formed by a
reaction between the first layer and the surrounding environment. The growth rate depends on
different factors and a maximum thickness of about 10 nm can only be achieved after several
weeks. Temperature and humidity have proved to be effective promoters on the second layer. The
composition of this layer can be very complex and contains different contaminants from the
surroundings. [19]
The corrosion of aluminium depends strongly on the stability and solubility of the natural oxide
layer which is a function of different factors, e.g. pH. The solubility of the oxide layer will normally
be greatest in strong acidic or alkaline environments, but the type of acid or base is also of great
2 THEORY
18
importance. For instance, a solution of hydrochloride acid is much more aggressive than a solution
of acetic acid at the same pH value.
2.2.6 Corrosion of painted steel and aluminium
The protective nature of anti-corrosive paint systems is only valid for a given time, and the
destruction of the paint film will eventually take place leaving free entrance for corrosive medias to
the substrate material. Normal failure mechanisms related to organic coatings include blistering
and undercutting corrosion, which are catastrophic for the protective character of the coating, and
chalking or fading, which are less risky.
Blisters in the paint are formed when water and other corrosive
elements penetrate the paint film during time of wetness. Access
of corrosive medias cause a corrosive reaction to take place
beneath the coating and cause the paint film to swell, as
illustrated on figure 12. As the blister grows, it often tends to
combine with other blisters and the paint will eventually fall off
Figure 12. Illustration of a blister.
in flakes. Three types of blisters can be formed: 1) Osmotic blister 2) Anodic blister and 3)
Cathodic blister. The osmotic blister is most common and is created near contaminants on the
metal surface. An osmotic blister is formed when water penetrates the paint and dissolves soluble
substances in the paint. This creates a dense fluid beneath the paint film which is drawn to the
water outside the paint by osmosis. The blister is formed as an attempt to equalize the density
between these two fluids. [18]
Undercutting corrosion is formed near scratches or sheared edges as illustrated on figure 13. The
corrosion will normally occur either by a chemical
reaction in the interface between substrate material
and paint, or by corrosion of the substrate material
itself. Both situations will decrease the adhesion, or
Figure 13. Illustration of undercutting corrosion.
in worst case, completely disconnect the paint film.
The mechanism of undercutting corrosion can be
described as a crevice corrosion scenario. Crevice corrosion is characterized by the presence of a
small local anode and a large external cathode. Formation of an acidic environment and gaseous
hydrogen in the crevice will further accelerate the corrosion. [19]
Another kind of crevice corrosion is often present on painted aluminium and is called filiform
corrosion. Filiform corrosion is initiated near defects and mechanical damages in the paint from
2 THEORY
19
which fine tunnels containing corrosive products are spread in a streaked pattern. The mechanism
behind filiform corrosion is driven by an active “head”, which acts as an anode, while the “tail”
and the surrounding regions acts as a cathode. A potential difference of 0,1 to 0,2 V is typically
obtained between the head and tail region. The presence of oxygen is essential for the maintenance
of cathodic reactions, and is therefore the driving force behind the mechanism of filiform
corrosion. The cathodic reactions take place in the tail region, which is supplied with oxygen and
condensed vapour through cracks and crevices in the coating. The head of the filament is filled
with floating flakes of opal aluminium gel which are moving towards the tail region. Reactions
between aluminium ions (Al3+) and hydroxide ions (OH-) will also take place in the tail region,
producing aluminium trihydroxide (Al(OH)3) and aluminium oxide (Al2O3). Figure 14 illustrates
the reactions which take place during filiform corrosion of aluminium. [20]
Figure 14. Reaction mechanism for filiform corrosion. [21]
Filiform corrosion is mostly dependent on the relative humidity and the quality of the applied
surface treatment. Serious attacks appear in warm coast areas, where the combination of saltwater
and high relative humidity increases the development of filiform corrosion. Filiform corrosion is
only present in the atmosphere and occurs especially at a relative humidity of 85 to 95%. [22]
Cathodic protection of steel by means of sacrificial anodes (e.g. zinc) or impressed current can
result in a cathodic disbondment which decreases the adhesion of the paint film. The loss of
adhesion between the paint and substrate material, is caused by formation of cathodic reaction
products (e.g. OH-) near damaged areas in the paint film. Cathodic protection thus increases the
2 THEORY
20
risk of disbondment since a cathodic polarized steel surface has a higher exposure to cathodic
reactions. A model for the mechanism of cathodic disbondment is illustrated in figure 15, with the
presence of cathodic reaction products beneath the paint film.
Figure 15. Model for cathodic disbondment of a painted steel specimen.
Studies of the mechanism behaviour have shown that production of hydroxyl ions is the primary
cause of cathodic disbondment. A direct proportionality is present between the OH- concentration
and the rate of disbondment. It is believed that the generated hydroxyl ions interact with the paint
and thus weaken and break the bonds between the paint and the steel substrate. The oxygen
concentration in the environment is another factor that enhances the risk of cathodic
disbondment. [23]
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2.3 Anti-corrosive paint
An introduction to general aspects of paint composition and properties is needed in order
to understand the mechanism of anti-corrosive paint systems. Paint is a complex formulation
based on many different components which contribute to the overall efficiency of the system. This
section describes the different components and their mode of operation.
2.3.1 What is paint?
Paint is an organic coating that primarily protects equipment and constructions from
environmental damage and provides a decorative surface. For protection purposes, paint is used in
a wide range of industrial applications such as ships, ballast tanks, drilling rigs and concrete
constructions. Selecting the right kind of paint system requires a comprehensive knowledge about
the environment and its demands. Protection is a wide area of expertise and requires information
about problems related to corrosion, fouling, contaminations, wear and abrasion.
Paint is, per definition, composed of a liquid material which transforms to a thin coherent and
adherent film when applied to a surface. The composition of paint can generally be divided into
four main components: binder, pigment, solvent and additive. The binder is the backbone of a
paint system and is therefore used to classify the system.
Generally, organic solvent based paint can be divided into two groups: physically drying paint and
chemically curing paint. This categorization concerns the action of film formation and describes
whether the transition from liquid to solid state takes place either by evaporation or by chemical
reaction.
Physically drying paints form a film exclusively by evaporation of solvents. The binder molecules
have therefore the same composition and size both before and after solidification of the film. The
mechanism of physically drying paint is a physical process where the solvents evaporate and leave
behind long chains of resin molecules, which pack together and form a coherent plastic film.
Chemically curing paint is based on a curing mechanism where the film formation takes place by a
chemical reaction between the binder and a curing agent. The final binder molecules in the dry
film are therefore different from the initial binder molecules. The final binder molecules are much
bigger and contain a high degree of cross-linking, forming a strong and non-reversible paint film.
[24]
A description of the four main components is given in the following four sub-sections. Each
section describes the most common components, their mechanism and application.
2 THEORY
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2.3.2 Binders
The binder is an essential component that provides uniformity and coherence to the paint
system. It holds the pigments together when a dry film is formed, and provides adhesion to the
substrate material. The type of binder often determines the durability of the final product. The
ability of a binder to form a dense and tight film is directly related to its molecular weight and
complexity. Binders with a high molecular weight often tend to form film by evaporation, while
low-molecular binders generally will form film by a reaction in situ. A way to classify a binder is
according to its chemical reactions. Table 2 lists some important binders, their grouping and
chemical reactions. [25]
Table 2. Classification scheme of binders according to their chemical reactions. [25]
Classification
Examples
Chemical reactions
Oxygen reactive
Alkyds
The binder molecules react with oxygen
binders
Epoxy esters
and a cross-linking of the resin molecules
Urethane alkyds
takes place.
Polyvinyl chloride polymers
Drying mechanism by solvent
Chlorinated rubbers
evaporation. The long chain resins
Acrylics
entangle with each other but no cross-
Lacquers
linking exists.
Heat conversion
Hot melts
Curing takes place upon heating as the
binders
Organisols and plastisols
components melt. Both cross-linked and
Powder coatings
non cross-linked coatings are possible.
Epoxies
The film is formed by a polymerization
Polyurethanes
between the resin and a curing agent. A
Co-reactive binders
three-dimensional network is formed.
Inorganic binders
Coalescent binders
Post-cured silicates
The binders are usually used in zinc-dust
Self-curing water silicates
pigmented primers where a reaction
Self-curing solvent based
between zinc and binder takes place
silicates
forming a very hard film.
Latex
Film formation by coalescence of binder
particles dispersed in water.
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2.3.3 Pigments
Pigments constitute a broad group of dry powder materials which are added to the paint in
order to provide functionality and appearance to the paint system. The pigments are insoluble in
the paint system and range from naturally occurring minerals to synthetic organic materials.
Besides the obvious purpose of providing colour and opacity, pigments are also an important
additive for corrosion protection, film reinforcement, coverage and adhesion. In anti-corrosive
paint systems, pigments mainly provide protection by one or more of the following mechanisms;
1) Inhibition of corrosion; 2) Passivation of substrate metal; 3) Barrier against water permeability
and 4) Cathodic protection. Certain pigments also enhance heat, abrasion, acid or alkali resistance
to the final dry film. Important properties for all pigments are particle size and shape, wet ability
by the binder and bulking. Some important pigments, their grouping and functionality are listed in
table 3. [25]
Table 3. Classification scheme of pigments according to their functionality. [25]
Classification
Examples
Functionality
Colour pigments
Titanium dioxide, iron
Provide colour to the paint. Titanium
oxides, organic azo
dioxide is the most popular white pigment
pigments.
because of its high refractive index.
Inhibitive
Zinc phosphate, aluminium
Provide active corrosion inhibition to the
pigments
phosphate, zinc molybdate.
metal substrate. The pigments are slightly
water soluble. Dissolved ion species thus
react with the metal to form passivating
reaction products.
Barrier pigments
Sacrificial
Aluminium flake,
Increase the permeation path length to the
micaceous iron oxide.
substrate for incoming moisture.
High purity zinc dust.
Function as sacrificial anodes which provide
pigments
Hiding pigments
cathodic protection of the substrate metal.
Rutile titanium dioxide,
Pigments with a high light refractive index to
zinc oxide.
provide good hiding.
Extender
Carbonates, silicates,
Acts as reinforcement and flow control
pigments
sulphates, barytes and mica.
pigments. They are relatively inexpensive.
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2.3.4 Solvents
Solvents are volatile liquid substances with the purpose to dissolve solid paint constituents,
reduce the viscosity and make the paint fluid for satisfactory appliance. After application, the
solvent must evaporate to allow the coating to cure and achieve hardness. If a solvent has low
volatility it can cause runs and sags in the drying coating film. In contrast, solvents can not be too
volatile and cause solvent pops, loss of gloss, dry spray, poor surface wetting and penetration,
poor film flow and inhibit cure. A blend of different solvents is therefore normally used in order
to achieve optimum properties. Solvents are usually categorized according to their chemical
composition. Table 4 lists some important groups of solvents, their advantages and disadvantages.
[25], [26]
Table 4. Classification scheme of solvents with a description of their application range. [25], [26]
Classification
Examples
Solvent description
Aliphatic
Naphtha, mineral
Used with asphalt, oil and vinyl based coatings. Poor
hydrocarbons
spirits, hexane,
to moderate solvency and wide range of evaporating
heptane.
rates. Least expensive of all solvents.
Toluene, xylene.
Used with chlorinated rubbers, coal tars and certain
Aromatic
hydrocarbons
Ketones
alkyds. Greater solvent power than the aliphatics.
Acetone, methyl ethyl
Effectively used with vinyls and some epoxies.
ketone, methyl isobutyl
Exhibit varying evaporation rates and relatively
ketone.
strong solubility parameters. Strong hydrogen
bonding and high polarity.
Esters
Ethyl acetate, isobutyl
Used as latent solvents with epoxy and polyurethane.
acetate, ethylene glycol.
Solvency power between aromatic hydrocarbons and
ketones. Strong hydrogen bonding and a relatively
high polarity.
Alcohols
Ethanol, isopropanol,
Good solvents for highly polar binders such as
n-butanol.
phenolics. Alcohols are highly polar with a strong
affinity for water.
Ether and
Ethyl ether.
alcohol ethers
Water
Excellent solvents for some of the natural resins, oils
and fats.
Water
Used in latex paint.
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25
2.3.5 Additives
The final component in a paint system is the additives which normally make less than 1%
of the entire paint formulation. Additives are used to stabilize the paint fluid and affect different
physical and chemical properties such as viscosity, surface and interfacial tensions, gloss and drying
time. Addition of additives is only done when necessary since unwanted effects on the paint
properties are likely to take place. The most common additives are listed in table 5 with an
explanation of their functionality. [27]
Table 5. Classification scheme of additives according to functionality. [27]
Classification Examples
Functionality
Antifoam
Mineral oils, silicone
Prevent the formation of foam e.g. by coalescence of
additives
oil, wax dispersions
small bubbles to larger bubbles which increases the
buoyancy of the bubbles.
Thickeners
Bentonite, cellulose
Increase the viscosity of the paint by creating a
derivates, polyacrylates
network between hydrophobic and hydrophilic parts
of the paint.
Dispersion
Tensides
additives
Siccatives
Increase the wetability of pigments to the binder
phase by formation of micelles.
Metal salts of organic
Used in paints with oxygen reactive binders.
acids
Improves the curing process.
Cold
Ethylene glycol,
Used in water based paint to improve the stability to
stabilizers
propylene glycol
freezing
2 THEORY
26
2.4 Corrosion protection by conductive polymers
The application of conducting polymers for corrosion protection has been examined
during the last decade. This section introduces the reader to the different protection mechanisms
which are proposed by a number of scientists. The chosen articles examine the anti-corrosive
behaviour of polyaniline and polythiophene on both steel and aluminium.
2.4.1 Hypotheses of anti-corrosive mechanisms
The aim of this project is to uncover the different anti-corrosive mechanisms of
conducting polymers. The following five hypotheses are believed to be the most important and are
the most documented mechanisms for corrosion protection of metals by means of conducting
polymers.
1. Anodic protection and passivation of the substrate metal.
2. Formation of a protective metal/polymer complex.
3. Absorption of OH- and inhibiting of cathodic disbondment.
4. Inhibition of the cathodic reactions.
5. Improve the conduction between zinc particles in zinc pigmented paints.
Hypothesis 1: According to several studies, conducting polymers are able to raise the surface
potential and provide an anodic protection of the substrate material. Tallman [28] has reported
that the redox potential of polyaniline is 0,4 to 1,0 V (vs. SHE at pH 7) and 0,8 to 1,2 V for
polythiophene. Both values are higher when compared to the corrosion potential of steel and
aluminium. This indicates that both polyaniline and polythiophene are able to ennoble the surface
of both steel and aluminium.
Anodic protection alone can be fatal if the coating is damaged. A local damage in the coating will
accelerate the corrosion of the underlying metal since the metal appears as an anode due to its
lower potential. An effective corrosion protection can therefore only be provided if passivation of
the metal takes place simultaneously. According to theory, the metal potential is stabilized in the
passive potential range and a protective barrier of metal oxides is formed on the surface which
prevents the active corrosion. The passivation takes place with an oxidation of the metal (M) and a
reduction of the electrically conducting polymer (ECP) as illustrated in reaction 2.21. [29]
2 THEORY
27
y
y
1
1
1
1
M + ECP m + + H 2 O → M (OH ) (yn − y ) + + ECP 0 + H +
n
m
n
n
m
n
(2.21)
Hypothesis 2: Another theory claims that a protective complex between the metal and the
conducting polymer is formed in the metal/polymer interface. Kinlen [30] found by electron
spectroscopy chemical analysis (ESCA) that an iron-polyaniline complex in the intermediate layer
between the steel surface and the polymer coating is formed. By isolating the complex, Kinlen
found that the complex has an oxidation potential 250 mV more positive than polyaniline.
According to Kinlen, this complex more readily reduces oxygen and produces a more efficient
electrocatalyst.
Hypothesis 3: In addition to the electrocatalytic character of polyaniline, some studies claim that
polyaniline has the ability to absorb OH- from the O2 reduction. The absorption of OH- causes a
reduction of polyaniline (PANI) from emeraldine salt (ES) to emeraldine base (EB), as illustrated
in reaction 2.22, where A describes the dopant ion.
PANI-ES + OH- → PANI-EB + H2O + A-
(2.22)
The reaction between polyaniline and the hydroxide ions limits the increase in pH at the
polymer/metal interface and creates a buffering effect on the electrolyte pH. This is a very
important quality for inhibition of cathodic disbondment which strongly depends on the
concentration of OH- in the interface. [31]
Hypothesis 4: Another approach to the protection mechanisms of conducting polymer is a
theory which claims that conducting polymers inhibit the cathodic reactions. By limiting the
cathodic reactions, one also limits the anodic reactions since both reactions are in balance. This
relation reduces the corrosion rate of the metal.
Seegmiller [32] used SECM (Scanning Electrochemical Microscopy) to analyze the hydrogen
production across a scratch in a polyaniline coated aluminium specimen. The specimen was
exposed to a 10 mM H2SO4 solution, and a redox mediator in the SECM tip electrode was used to
locate the surface during approach curves. The SECM data indicated that hydrogen production
was inhibited by the polyaniline coating. Seegmiller therefore suggested that polyaniline works as
an inhibitor towards cathodic reactions and reduces the decomposition of a protective oxide layer.
2 THEORY
28
Hypothesis 5: The final hypothesis is only present in the case of zinc pigmented paint systems,
which provide cathodic protection of steel. In cathodic protection the substrate material is placed
in galvanic coupling with a low potential metal, such as zinc. In this way, zinc provides the anode
and corrodes in favour of the substrate material. The zinc is added as a fine powder to the paint
system. In order for the paint to reach a high efficiency, the zinc particles must be in close contact
to each other. Different aids have been tested to raise the conductance between the zinc particles.
One theory adds conductive additives to the paint. The addition of carbon black has shown good
results but no research is done on the basis of conducting polymers. This project will try to
examine the efficiency of polyaniline and polythiophene as a conductive additive in zinc-rich paint
systems.
2.4.2 Article review of conducting polymers
This section gives the reader a review of the most comparable articles which have studied
the corrosion behaviour of conducting polymers. It is important to note that a complete
comparison between these articles and this project is not to be recommended. The results of the
following articles are based on very different coating formulations and different film thicknesses.
However, the results in these articles are good indicators for understanding the anti-corrosive
mechanisms of conductive polymers.
The results based on polyaniline coatings on aluminium substrates are listed in table 6 on the
following page. Both articles state that a ennobling of aluminium takes place but Seegmiller [32]
measures a much lower potential than Tallman [33]. This difference is probably caused by the
difference in the electrolyte and the coating formulation. Seegmiller uses sulphuric acid as en
electrolyte, which is a reducing acid. Polyaniline is mixed with polymethylmethacrylate (PMMA),
and this can possibly lead to a decrease in the corrosion potential. Both articles claim that
polyaniline is reduced in a corrosive environment which supports the presence of a redox reaction
between polyaniline and aluminium. According to Seegmiller, polyaniline also acts as an inhibitor
towards hydrogen production. This final statement is not to be found elsewhere.
2 THEORY
29
Table 6. Review of articles related to corrosion protection of aluminium by polyaniline.
Coating
Results and observations
Reference
Polyaniline coating
- An oxidation of aluminium and a reduction of
Tallmann et
on aluminium
polyaniline are found by EIS.
al 2000
(40-50 µm)
- An OCP potential of 0,2 V (vs. SCE) is measured at the
[33]
polyaniline coated aluminium specimen in a solution of
0,35% ammonium sulphate and 5% sodium chloride.
- The pH in the immersion electrolyte decreases,
indicating that an interaction between polyaniline and
aluminium takes place.
Polyaniline
- An OCP potential of -0,12 V (vs. Ag/AgCl) is
Seegmiller et
/PMMA blend
measured on polyaniline coated aluminium in 10 mM
al 2005
on aluminium
H2SO4.
[32]
(≈10µm)
- Raman spectroscopy indicates an increase of reduced
benzenoid groups and a decrease of oxidative cation
radicals in the polymer chain.
- SECM indicates that the hydrogen production is
inhibited by the polyaniline coating.
Several studies have also examined the anti-corrosive behaviour of polyaniline on steel substrates.
The different articles show no remarkable difference in the protection mechanisms regardless of
whether the substrate material is steel or aluminium.
Table 7 on the following page lists the results of three articles which examine the corrosion
protection of steel by different polyaniline blends. These articles give the best comparable
measurements in relation to this project which are based on paint formulations and not pure
polyaniline coatings. An interesting discovery is made by Samui [34], who found that the best
corrosion protection is provided with a low loaded polyaniline blend. At higher loadings of
polyaniline, the compatibility of polyaniline and the styrene-butyl acrylate copolymer matrix is very
low and the film becomes porous. This allows water and other contaminants to permeate the
coating to a higher degree and decrease the corrosion protection.
2 THEORY
30
Table 7. Review of articles related to corrosion protection of steel by polyaniline.
Coating
Results and observations
Reference
Polyaniline
- The potential increases from 0,2 to 0,56 V (vs. SHE) as
Holness et
/PVB blend
the polyaniline content is raised from 0 to 0,2. Measured
al 2005
on steel
by a Scanning Kelvin Probe in 96% RH and 20°C.
[31]
(30±5 µm)
- The potential in the defect zone is equal to iron.
- Cathodic disbondment is inhibited by polyaniline but
not prevented.
- As the alkalizing of the coating proceeds polyaniline is
converted into the emeraldine base.
Polyaniline
- More porous film and higher water permeability with
Samui et al
/St-BuA blend
increasing polyaniline content.
2003
on steel
- Better corrosion performance in low loaded polyaniline
[34]
(80±5 µm)
paint.
- Potentiodynamic measurements in 3,5 wt% NaCl shows
a higher corrosion potential and a lower corrosion rate at
low loadings.
- Best performance is obtained in a 0,1 part polyaniline
loaded paint.
Polyaniline primer
- An iron/polyaniline complex is found by ESCA. The
Kinlen et al
blends + topcoat on
complex is 250 mV more noble compared to pure
1997
steel
polyaniline.
[30]
(0,5 -3,4 mm primer +
- An epoxy topcoated PANI/thermoplastic primer has
1,7-7,2 mm topcoat)
equal performance to inorganic zinc in a salt fog
exposure.
Only a few articles deal with the action of polythiophene as an anti-corrosive additive. Table 8 lists
the results of two articles which examine the corrosion protection of steel by polythiophene. As
indicated in table 8, the same protection mechanisms are observed for both polyaniline and
polythiophene.
2 THEORY
31
Table 8. Review of articles related to corrosion protection of steel by polythiophene.
Coating
Results and observations
Reference
Paints modified with
- Problems with homogeneity in the paint blends.
Ocampo et
a polythiophene
- FTIR indicates that polythiophene enhances the
al 2005
deviate on steel
protecting role of the resin by delaying resin degradation.
[35]
(100-200 µm)
- Colour chance from yellowish grey to violet after
exposure in a cyclic test.
Polythiophene
- Potentiodynamic measurements in 3,5 wt% NaCl
Kousik et al
coatings on steel
indicate an ennobling of steel by the polythiophene
2001
(10-24 µm)
coating.
[36]
- The best corrosion resistance are obtained with a 22 µm
coating at a corrosion potential of -346 mV (vs. SCE).
- EIS indicates the presence of a passive oxide layer.
2.4.3 Research from Ormecon
Ormecon, the German manufacturer of the CORRPASSIV paint systems, has performed a
number of tests in order to prove the anti-corrosive effect of their paint systems. The following
results and claims are recited by Ormecon on their homepage and in their publications. [37]
Ormecon states that a primer containing their organic metal (red. doped polyaniline) will enhance
the surface potential of steel by up to 800 mV. The internal specifications of the company demand
a potential shift of at least +100mV, which is stable over time. Ormecon also claims that the
CORRPASSIV paint system produces a passive layer of Fe2O3 about 1 µm thick, between the steel
surface and the primer. The formation of stable iron oxides is according to Ormecon, possible due
to the following reaction mechanism (see figure 16). As seen in the reaction scheme, polyaniline
oxidizes the iron to Fe2+ and is reduced to leucoemeraldine (LE) in return. The iron ions are
oxidized further to Fe3+ which is able to form Fe2O3 in the presence of OH-. The leucoemeraldine
form of polyaniline will by means of oxygen undergo an oxidation to the emeraldine base (EB)
form, which in the presence of H+ will oxidize polyaniline back to its original emeraldine salt (ES)
form. According to the reaction scheme, polyaniline is thus a catalyst for the production of Fe2O3.
[38]
2 THEORY
32
Figure 16. Reaction mechanism for passivation of iron using polyaniline according to
Ormecon. [38]
A closer look at the reaction mechanism proposed by Ormecon reveals that the reactions are not
in balance. As recalled on figure 3, the same amount of hydrogen is produced and used in the
catalytic circle of polyaniline. The release of two hydrogen ions for reduction of oxygen (cf. figure
16) will thus put an end to the catalytic circle. A supply of hydrogen ions from the environment is
therefore necessary if polyaniline has to act as a catalyst.
According to Ormecon, a topcoat on the polyaniline primer is needed for obtaining an effective
corrosion protection. Without a topcoat, H+ ions penetrate the primer layer to the surrounding
media and the catalytic circle in figure 16 will stop. An optimal utilization of the anti-corrosive
character of polyaniline can therefore only be obtained if an acidic pH is maintained in the primer.
According to Ormecon, a low pH can be retained within the primer by a powerful barrier coating
which forces the H+ ions to stay in the primer.
Ormecon also claims that their paint systems are capable of protecting even non-painted areas, for
instance scratches in the paint film. According to their specifications, the formation of rust in a 1-2
mm broad score is inhibited. [39]
3 EXPERIMENTAL PROCEDURE
33
3 EXPERIMENTAL PROCEDURE
A number of experiments are performed in order to uncover the applications and
efficiency of anti-corrosive paint systems based on conducting polymers. Another purpose of
these experiments is to disprove or confirm the mechanisms which have been proposed in section
2.4.1.
3.1 Applied paint systems
Different paint systems are formulated and applied to steel and aluminium substrates. For
protection of steel, the paint systems are divided into three sections of applications: shopprimers,
paint for ballast tanks and paint for off-shore environments. A brief introduction to each of these
application areas and their characteristics are given in the following.
3.1.1 Test panels and surface preparation
Two types of test panels, made of steel or aluminium, are used in this project. The steel
panels, which are standardized test panels from Hempel, are made of normal carbon steel (St37)
and blast cleaned to a designation SA3 according to ISO8501-1. A grit blasting provides the panels
with a surface roughness of designation N9 or N10 according to Rugotest 3. Designation N9 and
N10 corresponds to a Ra-value of 6,3µm and 12,5µm respectively. The Ra value describes the
arithmetical mean deviation of the profile. Only the shopprimer uses a designation N9 while the
paint systems for ballast tanks and off-shore protection uses a N10 profile. The size and thickness
of the steel panels depends on the test procedure.
The aluminium panels are made of the alloy AA 6016 and provided by Chemetall. This type of
alloy contains magnesium and silicon as the main alloying elements (0,25-0,6 wt% Mg and 1-1,5
wt% Si), but smaller amounts of copper, chromium or manganese can also be present. Magnesium
and silicon are added in order to increase the strength of aluminium by precipitation of Mg2Si
particles in the alloy. The test panels have the dimensions 105 x 190mm and a thickness of 0,75
mm.
3 EXPERIMENTAL PROCEDURE
34
3.1.2 Shopprimer
A shopprimer is a thin organic coating that provides temporary corrosion protection of
steel plates and profiles during storage, transport and production. The application of shopprimers
is related to marine and industrial use, where a protective coating is needed in order to retain a
smooth and viable surface workable for further machining. Applying a shopprimer pre-treatment
to steel plates makes additional surface treatment, cutting and welding operations less complicated.
The requirements to a shopprimer and its specifications are listed in table 9.
Table 9. Requirements and specifications of shopprimers.
Requirements
Specifications
Coating thickness
- Max. 20 µm and no pinholes.
Corrosion protection
- Protection against atmospheric corrosion for 3-12 months
depending on the environment. Temporary corrosion protection
against salt and fresh water during building period.
Weldability
- High temperature resistance and less than 5 mm burn damage along
the cutting edge.
- Low gas and fume production with low toxicity.
- No influence of arc stability and low splatter.
This project examines the corrosion resistance of shopprimers based on three different binder
systems with and without the addition of polayniline as described in table 10. Polyaniline is added
as a powder with the name Panipol F, which is produced by the Finnish company Panipol. A
detailed product description of the standardized shopprimers, which are stated in table 10, is
found in the respective data sheets in appendix H, J, K and L.
3 EXPERIMENTAL PROCEDURE
35
Table 10. Overview of applied shopprimers.
Classification
Product description
Polyvinyl butyral Polyaniline containing PVB paint from Ormecon (CORRPASSIV 4000)
(PVB)
Standard PVB shopprimer from Hempel
Zinc phosphate
pigmented
epoxy polyamide
Low-zinc ethyl
silicate
Standard zinc phosphate shopprimer from Hempel
Standard zinc phosphate shopprimer with 10vol% Panipol F
Standard zinc silicate shopprimer with 50w% zinc
Standard zinc silicate shopprimer with 40w% zinc and 5vol% Panipol F
Standard zinc silicate shopprimer with 30w% zinc and 5vol% Panipol F
3.1.3 Paint for ballast tank
A ballast tank is a tank that is filled with sea water to provide stability when the ship is
unloaded. The corrosion of ballast tanks is not evenly distributed since the environment in the
tank contains areas with different electrochemical loadings. When the tank is full, the upper
regions are anodic and the lower regions are cathodic. The upper regions therefore tend to
corrode, and the lower regions tend to blister and delaminate. In order to control this problem,
ballast tanks are provided with sacrificial anodes or impressed current in order to establish
cathodic protection of the tank. To fulfil the application requirements, the paint specifications in
table 11 must be fulfilled.
Table 11. Requirements and specifications of paints for ballast tanks.
Requirements
Specifications
Film thickness
Min. 2 x 150 µm
Corrosion protection
In accordance with ISO15711.
No disbonding with an equivalent diameter <20mm.
3 EXPERIMENTAL PROCEDURE
36
The paint system used within this project is a polyamide-adduct cured epoxy paint from Hempel
especially designed for protection of ballast tanks. Three versions are prepared according to table
12. A detailed product description of the conventional paint system is stated in appendix N.
Table 12. Overview of applied paints for ballast tank protection.
System
Primer
1
---
2
3
---
Barrier coating
Barrier coating
Polyamide-adduct cured epoxy
Polyamide-adduct
paint
cured epoxy paint
Polyamide-adduct cured epoxy
Polyamide-adduct
paint with 10vol% Panipol F
cured epoxy paint
Polyaniline/PVB paint Polyamide-adduct cured epoxy
Polyamide-adduct
(CORRPASSIV 4000)
cured epoxy paint
paint with 10vol% Panipol F
3.1.4 Paint for off-shore environment
An off-shore environment is a very broad term, and includes many different corrosive
scenarios. This project only examines the effect of an off-shore environment above the waterline.
Examples of application areas are drilling rigs and marine windmills. Both are placed in an
environment which is highly corrosive due to continuous exposure to salt spray and high relative
humidity. The protective coating also needs to withstand thermal stress due to varying temperature
conditions. The paint must therefore be able to withstand the requirements and paint
specifications listed in table 13.
Table 13. Requirements and specifications of paint systems for off-shore protection.
Requirements
Specifications
Film thickness
Min. 60 µm primer and min. 280 µm of complete paint system
Corrosion protection
Requirements after qualification testing (according to ISO 20340):
Undercutting corrosion: M < 3mm. M = (C-2)/2, where C is the
average corrosion from the scribe.
Adhesion: Min. 5 MPa and max. 50% reduction from original value.
The paint system applied in this project for protection in the off-shore environment is a
commercial paint system from Hempel. This paint system is composed of three layers: 1) a zinc-
3 EXPERIMENTAL PROCEDURE
37
rich epoxy primer, 2) an epoxy polyamide barrier coating and 3) an acrylic polyurethane topcoat.
Two versions of the paint system are prepared according to table 14. A detailed description of the
paint system is found in the respective data sheets in appendix M, Q and R.
Table 14. Overview of applied paint systems for off-shore protection.
System
Primer
Barrier coating
Topcoat
1
Zinc-rich epoxy primer
Epoxy polyamide barrier
Acrylic polyurethane
coating
topcoat
Zinc-rich epoxy primer with
Epoxy polyamide barrier
Acrylic polyurethane
10vol% Panipol F
coating
topcoat
2
A water based paint system is also formulated and tested in off-shore conditions. The paint system
is a commercial paint system from Hempel composed of two layers: 1) a zinc-rich epoxy primer
and 2) a polyamide cured epoxy barrier coating. According to table 15, four versions of the paint
system are formulated with varying zinc content and addition of Baytron P (red. polythiophene). A
detailed description of the paint system is given in the respective data sheets in appendix O and P.
Table 15. Overview of water based paint systems for off-shore applications.
System
Primer
Barrier coating
Topcoat
1
Zinc epoxy primer with 85w% Zn
Polyamide cured epoxy paint
---
2
Zinc epoxy primer with 85w% Zn
Polyamide cured epoxy paint
---
and 2vol% Baytron P.
3
Zinc epoxy primer with 60w% Zn
Polyamide cured epoxy paint
---
4
Zinc epoxy primer with 60w% Zn
Polyamide cured epoxy paint
---
and 2vol% BAYTRON P
3.1.5 Paint for aluminium substrates
Corrosion of aluminium depends mainly on the type of aluminium alloy and the character
of the corrosive environment. The aim of this project is to compare and examine the efficiency of
four different protective coating systems (see table 15). Anodizing and chromatizing converts the
surface of aluminium to Al2O3 and Cr2O3 while altizizing covers the surface with MnO2. Each of
these three coatings has a very high potential and provides anodic protection to aluminium. All of
3 EXPERIMENTAL PROCEDURE
38
the protective coating systems have a two layer paint system applied on top. This paint system is
based on an epoxy polyamide barrier coating and an acrylic polyurethane topcoat. A more detailed
product description is stated in the respective data sheets in appendix I, Q and R
Table 16. Overview of applied coating systems for protection of aluminium.
System
Pre-treatment
1
2
3
---
Primer
Barrier coating
Topcoat
Polyaniline/PVB primer
Epoxy polyamide
Acrylic poly-
(CORRPASSIV)
barrier coating
urethane topcoat
Epoxy polyamide
barrier coating
Acrylic poly-
Epoxy polyamide
barrier coating
Acrylic poly-
Epoxy polyamide
barrier coating
Acrylic poly-
Anodizing
Chromatizing
-----
(yellow)
4
Altizizing
---
urethane topcoat
urethane topcoat
urethane topcoat
3.2 Paint film characterization
The different paint systems have been characterized according to their film thickness,
structure and adhesion. Electromagnetic induction and the eddy current test are both used to
determine the film thickness. The structure of the dry paint and its composition is examined by
scanning electron microscopy and the adhesion is evaluated by a pull-off test.
3.2.1 Electromagnetic induction
Electromagnetic induction is a non-destructive method used to determine the dry film
thickness (DFT) of paint on ferrous substrates. Standard test methods are described in ASTM B
499 and ISO 2178.
Measurements are performed by a probe placed on the surface of the specimen. In the probe, a
magnetic field is generated by a magnetizing coil. The magnetic substrate material influences the
magnetic inductivity and the change in magnetic flux is registered by a measuring coil. The signal
from the coil is transformed to an electric signal that indicates the distance from the probe to the
magnetic substrate material, which is equal to the coating thickness.
3 EXPERIMENTAL PROCEDURE
39
Thickness measurements by electromagnetic induction are a simple method with a good accuracy
and a tolerance of ±1%. [24]
3.2.2 Eddy Current Test
Measurements of the dry film thickness on a non-magnetic substrate material, such as
aluminium, are performed by the non-destructive Eddy Current Test (ECT). Application and
performance skills are described in ASTM B 244 and ISO 2360.
In the ECT-technique, an AC-magnetic field is generated by a probe that induces an eddy current
in the base material, which is proportional to the frequency and the resistance. When a nonconductive coating is present on the substrate material, the magnitude of the induced eddy current
is reduced. This loss in permeability of the eddy current can be used to measure the coating
thickness. The eddy current generates further an opposite electromagnetic field. This field reduces
the reactant of the coil in the probe and thus the generated voltage output from the probe. The
change in voltage from the probe is therefore also a basis for measuring the thickness of the
coating.
An ECT-instrument retains normally a tolerance of ±1%, but the measurements are sensitive to
different factors such as surface roughness, curvature, substrate thickness and material. Modern
instruments are normally designed to incorporate both magnetic and eddy current principles in
one unit. [24], [40]
3.2.3 Scanning Electron Microscopy
Scanning electron microscopy (SEM) is used to determine the microstructural topography
and the composition of the different paint systems and surface treatments. A description of the
method is presented in ASTM E-1508.
In the SEM-technique, an electron beam is used to scan the sample surface. The electrons are
produced in a beam gun by heating a filament made of either tungsten or lanthanum hexaboride.
A column containing electromagnetic lenses is used to focus and direct the beam towards the
sample. The surface scanning is performed at high vacuum and with an intensity of the electron
beam of up to 30 keV. Once the beam hits the sample, secondary and backscattered electrons are
ejected. A detector collects these electrons and converts them into a signal that can be displayed as
a picture showing the surface topography with high resolution and depth. Interactions between the
atoms in the sample and the primary electrons from the beam can furthermore result in an
3 EXPERIMENTAL PROCEDURE
40
ionization of the atoms and emission of X-rays. Analysis of the wavelength or the energy of
emitted X-rays can be used to identify the elements in the sample. The relative intensities of these
X-rays are also a useful tool to make a qualitative analysis. [41], [42]
3.2.4 Pull-off test
Adhesion of the paint can be determined by means of a pull-off test. A standard method
for application and performance of the pull-off test is described in ASTM-standard D45541.
The pull-off test is performed by gluing a dolly to the surface of a paint film. In order to obtain
reliable measurements, it is very important that the surfaces of both the paint and the dolly are
clean and contain no grease or contaminants. Both surfaces are therefore slightly grinded and
rinsed carefully with both water and spirit. A portable adhesion tester is subsequently fixed on top
of the dolly, and the load is gradually increased until the dolly is detached. The force needed to
detach the dolly is therefore an expression of the adhesion of paint to the underlying substrate
material. Normally, the force is measured in psi or MPa and the load can be applied either by
mechanical, hydraulic or pneumatic pressure. The most reliable results are obtained on test panels
with a thickness of at least 5 mm. A pull-off test performed on a very thin test panel can cause
deformation of the panel and thus a smaller adhesion value. [40]
3.3 Corrosion resistance
Examination of the corrosion resistance is performed by electrochemical measurements
and different accelerated climatic tests. Electrochemical measurements are used to establish the
corrosion rate and corrosion potential of the painted steel and aluminium substrates. The
accelerated climatic tests are performed in order to reflect some of the worse climatic scenarios
which are fatal for the paint performance. The chosen climatic test methods depend on the
application area of the paint system. Table 17 on the following page lists the earlier mentioned
application areas and their test methods. A description of the performance of the different tests is
given in the following sub-sections.
3 EXPERIMENTAL PROCEDURE
41
Table 17. Test methods used for characterization of the corrosive behaviour in
the different application areas.
Application area
Test method
Shopprimer
Natural Weathering Test
Mebon Prohesion Test
Ballast tanks
Immersion Test
Cathodic Protection Test
Off-shore environment
Salt Spray Test
Cyclic Test
Aluminium
Salt Spray Test
Cyclic Test
Filiform Corrosion Test
3.3.1 Open Circuit Potential
The Open Circuit Potential (OCP) is the potential at which there is no current, and can be
used as an expression for the corrosion potential of the material in a given electrolyte. By
determining the OCP potential of both painted and non-painted substrates, an evaluation of the
galvanic coupling between the steel substrate and the paint can be made.
The experiment is performed in an electrochemical cell containing a solution of 3,5wt% NaCl. The
electrical potential of the test panel is measured against a reference electrode, when there is no
current flowing through the test panel. A reference electrode of saturated calomel is used in this
case and the experiment is performed at room temperature with access to air. The two electrodes
are connected to a potentiostat which measures the potential of the test panel in proportion to the
calomel electrode. The OCP potential is finally mapped as a function of time.
3.3.2 Anodic polarization
Anodic polarization is used to determine the corrosion rate and potential in a 3,5wt%
NaCl solution. The experiment is performed according to ASTM standard G5-94.
The polarization measurements are performed in an electrochemical cell containing a 3,5wt%
NaCl solution at room temperature and with access to air. A reference electrode of saturated
calomel, a counter electrode of graphite and a working electrode of the test material are placed in
3 EXPERIMENTAL PROCEDURE
42
the electrochemical cell. In an anodic polarization experiment, the test material is polarized and the
current is continuously measured as the potential is increased. The potential scan is performed
from an initial potential around 300 mV beneath the open circuit potential. A potential around 800
mV above the initial potential is chosen as the final potential and the scanning is performed at a
scanning rate of 10mV/min in a current interval from -300 to 300 mA. The potential is finally
mapped as a function of the current density, illustrating the anodic and cathodic polarization
curves of the test material.
3.3.3 Natural Weathering Test
The Natural Weathering Test is an atmospheric corrosion test performed on the roof of
Hempel in Lundtofte, Denmark. The test is part of the test procedure for shopprimers and is
comparable with standardized norms in ASTM D 1014.
The painted steel panels are prepared with a 50 x 1 mm score in a 45° angle to the horizontal and
placed in an exposure rack turning 45° to horizontal and facing south. Inspections are performed
regularly and are evaluated according to rust formation and blistering as specified in ASTM D 61095 and ASTM 714-02. By the end of the test, the panels are evaluated with regards to rust creep
and adhesion.
3.3.4 Mebon Prohesion Test
The Mebon Prohesion Test is a cyclic wet/dry corrosion test designed to match the
conditions that exist in the atmosphere in industrial areas. The test is part of the standardized test
procedure of shopprimers and follows the specifications in ASTM G5 – ANNEX 5.
Each painted steel panel is applied with a 50 x 1 mm score in a 45° angle to the horizontal and
is placed in a special chamber with the conditions described in table 18.
Table 18. Test conditions of the Mebon Prohesion Test.
Cycle description
2 hours of dry-off and 4 hours of spray with 0,05
w% NaCl and 0,35 w% (NH4)2SO4
Temperature
Dry period: 35±2 °C
Spray period: 20±0,5 °C
Airflow
0,8 l/s ≈ 1-2 ml/hour pr. 80 cm2
Pressure
1,4 bar
pH
6,0 – 6,5
3 EXPERIMENTAL PROCEDURE
43
Inspections of the test panels are normally performed in an interval of 1, 3, 7, 14, 21 and 42 days
but the resistance depends on the applied paint system. Shopprimers will usually not last 42 days
of exposure. At each inspection, rust and blisters are evaluated according to standardized norms in
ASTM D 610-95 and ASTM 714-02. A final evaluation of the paint systems is given with regards
to rust creep and adhesion as specified in ASTM D1654.
3.3.5 Cathodic Protection Test
The Cathodic Protection Test is part of the test procedure for ballast tank paint systems
and determines the ability of the paint to withstand cathodic protection. The test procedure is
described in ISO 15711.
The test equipment consists of a rectifier to which a reference electrode of saturated calomel
(SCE) and a platinized titanium anode are attached. A test tank and a store tank are used for
cooling and circulation of the electrolyte. The electrolyte consists of artificial seawater as specified
in ASTM D 1141-98. During exposure the potential is stabilized at -1050 mV vs. SCE by
impressed current. Before exposure, the painted steel panels are provided with an Ø12 mm score
placed in the centre of the panel.
Inspections of blistering are regularly performed where blister size and density are reported
according to ASTM D 714-87. The disbonded area at the bare spot and the adhesion are evaluated
when the test is stopped by a knife test.
3.3.6 Immersion Test
Immersion in seawater is part of the test procedure that determines the corrosion
resistance of paint systems for ballast tanks. The immersion test is described and fulfils the
standardized norms in ISO 20340.
The painted steel panels are prepared with a 50 x 2 mm score placed across the panel and are then
placed in a test tank consisting of aerated artificial seawater at a temperature of 40±1 °C. The
standard recommends an exposure period of 4200 hours or until the paint system is degraded and
posseses no anti-corrosive effect. An evaluation according to rust formation, blistering and
delamination is reported frequently as specified in ASTM D 714-02 and ASTM D 610-01.
Delamination and adhesion are evaluated at the end of the test by a knife test.
3 EXPERIMENTAL PROCEDURE
44
3.3.7 Salt Spray Test
The Salt Spray Test is performed in order to determine the corrosion resistance of painted
steel and aluminium panels in the off-shore industry above the waterline. Guidelines and test
procedures are described in the international standard ASTM B117.
The salt spray test is performed in a special fog chamber where salt fog enters the chamber by
nozzles at a constant temperature and pressure. A salt fog with a concentration of 5% NaCl and a
pH of 6,5-7,2 is applied and the chamber is kept at a constant temperature at 35 °C. Before startup, all the test panels are provided with a 50 x 2 mm score across the panel and placed in a defined
angle in the salt spray chamber. The exposure time depends on the corrosion resistance of the
painted panels. Each panel is frequently inspected during the test period and is evaluated according
to rust formation, blistering and delamination. The same evaluation procedure is performed at the
end of the test together with a pull-off test in order to determine the adhesion. Evaluation
procedures are performed according to ASTM D 714-02 and ASTM D 610-01.
3.3.8 Cyclic Test
The Cyclic Test is a standardized test designed especially for applications in the paint
industry and determines the corrosion resistance of painted steel and aluminium. The test is
developed to reflect the harsh environment in off-shore applications above the waterline. Test
conditions and procedures are described in standard ISO 20340.
Before exposure, each test panel is applied with a 50 x 2 mm score across the panel. The cyclic test
consists of an alternating test procedure involving three different types of corrosive conditions.
Table 19 describes the test cycle and the coherent conditions.
Table 19. Test description and applied conditions for the cyclic test.
Period
Day 1
Day 2
Day 3
Day 4
Day 5
Description
Alternating exposure in 4
Salt spray exposure
Low tem-
hours UV-lighting and 4
according to ASTM B 117.
perature
hours condensation.
Conditions
UV-light: UVA 340 nm and
60±3 °C
Condensation: 50±3 °C
Day 6
Day 7
exposure.
5% NaCl and 35 °C
-20±2 °C
3 EXPERIMENTAL PROCEDURE
45
Between salt spray exposure and low-temperature exposure, the test panels must be rinsed with
de-ionized water in order to remove most of the salt from the surface. According to the standard,
the normal test period is 25 cycles or 4200 hours.
The test panels are inspected frequently and are evaluated with regards to rust formation, blistering
and delamination according to the standards ASTM D 714-02 and ASTM D 610-01. When the test
is finished, a pull-off test is performed in order to determine the adhesion of the painted panels.
3.3.9 Filiform Corrosion Test
The Filiform Corrosion Test determines the resistance of organic coated aluminium to
filiform corrosion. Test procedures and conditions are described in standard ISO 4623-2.
The painted aluminium panels are provided with 2 scores, with the dimensions 50x1 mm and 50x2
mm. The scores are placed on the panel at least 20 mm apart and perpendicular to each other. In
order to accelerate filiform corrosion, the test panels are exposed to concentrated 37% HCl
vapour for an hour at room temperature. After exposure in HCl vapour the test panels are placed
in a holder for 15-30 min to dry. Finally, the test panels are placed horizontally in a moist chamber
at 40 ºC and 82% relative humidity for about 1000 hours.
During exposure in the moist chamber, the test panels are inspected regularly and are evaluated
according to filiform corrosion as specified in ISO 4628-10. When the test is finished, a final
evaluation of the extension of filiform corrosion is reported and the paint is removed in order to
establish hidden corrosion products beneath the paint surface.
4 RESULTS AND DISCUSSSION
46
4 RESULTS AND DISCUSSION
This chapter contains a review of the experimental results which are obtained in light of
the described experiments in chapter 3. A thorough analyse is made in order to confirm or
disprove the hypotheses stated in section 2.4.1, and the efficiency is evaluated in relation to other
anti-corrosive paint systems or surface treatments. This chapter is divided in to four sections
which deal with the four application areas described in previous sections.
4.1 Shopprimer
The Mebon Prohesion Test and the Natural Weathering Test were used to examine the
protection properties of the three different shopprimer systems based on either PVB, zinc
phosphate epoxy polyamide or low-zinc ethyl silicate. The two tests were finished after an
exposure time of 92 days. Pictures taken after test exposure are presented in appendix A and B.
4.1.1 PVB shopprimers
A conventional PVB-shopprimer from Hempel and a polyaniline containing PVB paint
from Ormecon are examined and compared.
The two types of PVB paints do not distinguish significantly from each other after exposure in the
natural weathering test, and the degradation of the panels is poor. The applied score is corroded
but the rest of the panel is more or less intact. Two of the CORRPASSIV panels contain marks of
chalking and the film thickness is probably the reason for this difference. Measurements of the dry
film thickness showed that panel 1 and 2 only had a film thickness about 12 µm. The other panels
had a film thickness about 20 µm, which are the normal film thickness of shopprimers.
The degradation of the two PVB paints is more severe after exposure in the Mebon Prohesion
Test. The panels are corroded in the score and on the plain paint surface, but no significant
difference is seen between the two PVB paints. The degradation of the plain paint surface starts as
blisters, which subsequently burst and cause the underlying steel surface to corrode.
4 RESULTS AND DISCUSSSION
47
Electrochemical measurements are performed in a 3,5w% NaCl solution in order to establish the
corrosion potential and corrosion current for each of the PVB paints. The OCP potential and the
polarization curves are illustrated in figure 17 and 18.
0
-50
Potential vs SHE (mV)
-100
-150
Steel
CORRPASSIV
PVB shopprimer
-200
-250
-300
-350
-400
0
2
4
6
8
10
12
14
16
18
20
Time (min)
Figure 17. OCP potential of PVB based shopprimers in 3,5w% NaCl.
600
400
Potential vs SHE (mV)
200
0
Steel
CORRPASSIV
PVB shopprimer
-200
-400
-600
-800
0,0000001
0,000001
0,00001
0,0001
0,001
0,01
0,1
1
10
Current density (mA/cm2)
Figure 18. Polarization curves of PVB based shopprimers in 3,5w% NaCl.
100
4 RESULTS AND DISCUSSSION
48
The OCP measurements in figure 17 show an increase in corrosion potential for both PVB paints
when compared to uncoated steel after 20 min exposure to 3,5w% NaCl. The conventional PVB
primer stabilizes at a potential of -125 mV while CORRPASSIV reach a potential of -200 mV. A
greater ennobling is therefore present in the conventional PVB primer when compared to
CORRPASSIV. The reason for this difference is to be found in the paint composition of the
conventional PVB primer. It emerged that the conventional PVB primer contains additives such as
zinc phosphate (Zn3(PO4)2) and iron oxide (Fe2O3), which both contributes to an increase of the
potential. The same pigment additives were not found in the CORRPASSIV paint, which indicates
that polyaniline also has an ennobling effect on the steel substrate.
The polarization curves in figure 18 shows a decrease in the corrosion current of the painted steel
specimens when compared to uncoated steel. The corrosion current of the painted specimens is
approximately the same, and it is therefore likely that inhibition of the corrosion rate is due to a
barrier effect. The corrosion potential of the two PVB paints is in agreement with the OCP
measurements while the uncoated steel specimens has a corrosion potential about 100 mV more
noble when compared to the OCP potential. This raise in the corrosion potential of steel is
possibly caused by formation of noble iron-chloride complexes on the surface.
The conclusion is therefore that no significant difference in the anti-corrosive effect is present in
the two examined PVB paints. The content of inhibitive additives in the conventional PVB primer
complicates a comparison of the two paints, but electrochemical measurements indicate that
polyaniline has an ennobling effect of the steel substrate.
4.1.2 Zinc phosphate shopprimers
Two versions of a zinc phosphate pigmented epoxy polyamide shopprimer from Hempel
were examined and compared in relation to their anti-corrosive character.
The preparation of a polyaniline containing zinc phosphate primer was difficult and caused
problems with the homogeneity of the paint blend. The final dry paint film therefore contained
non-dispersed polyaniline particles. These difficulties are also stated in the literature and are
probably caused by the physical nature of polyaniline. Polyaniline possesses a very high surface
tension (69,4 mN/m), which makes it difficult to wet and corporate into a paint blend. [43]
The natural weathering test causes no serious damage to the two zinc phosphate shopprimers.
Only the applied score is exposed to a slight degree of rusting, while the plain paint surface is
intact on both of the shopprimer systems. An interesting observation was however made after
about 1 month of exposure on the polyaniline containing zinc phosphate primer. It appeared that
4 RESULTS AND DISCUSSSION
49
the surface of the paint film had started some kind of segregation and the dry paint film felt wet. A
possible explanation could be that polyaniline is increasing the absorption of water which causes
the paint film to segregate.
The exposure in the Mebon Prohesion Test was more severe to the painted test panels, and
corrosion damages appear both in the score and on the plain panel. As it appears in the pictures in
appendix B, the polyaniline containing zinc phosphate primer is exposed to a greater extent of
corrosion in comparison to the conventional zinc phosphate primer. Earlier observations showed
that the degradation was initiated by small liquid containing blisters on the paint surface. It is
unclear if the blistering is accelerated by the presence of polyaniline, but it is possible that the nondispersed polyaniline particles act as small local cathodes. The formation of cathodic reaction
products (e.g. OH-) will therefore be concentrated around the polyaniline particles and blisters are
formed. Observations related to segregation of the dry paint film were also made in the Mebon
Prohesion Test and the polyaniline containing primer turned mat.
The conclusion is therefore that no improvement of the corrosion resistance is reached by adding
polyaniline to a zinc phosphate shopprimer. A very likely explanation for this is related to the
difficulties of creating a fully dispersed polyaniline paint blend.
4.1.3 Zinc silicate shopprimers
Three versions of a low-zinc ethyl silicate shopprimer from Hempel were formulated and
examined according to their anti-corrosive character. The aim is to examine the influence of zinc
content and polyaniline addition.
As it appears on the pictures in appendix A, the panels are exposed to corrosion in the natural
weathering test. A corrosive attack is present both in the score and on the plain paint film. The
corrosion present on the panel is named pin-point rusting due to the formation of pin holes in the
paint film. Pin-point rusting is normally formed if the shopprimer does not entirely cover the
profile of the steel surface. In this way, the corrosive attack is concentrated at small peaks on the
steel surface. As seen on the pictures, the corrosion is most severe on the paint systems with
reduced zinc content, i.e. respectively 30 and 40w% Zn. No significant differences are present
between the paint containing Panipol (panel 44) and the pure zinc paint (panel 42). The conclusion
is therefore that anti-corrosive effect is determined by the zinc content in these zinc-silicate
shopprimers.
The observations from the Mebon Prohesion Test are similar to the ones seen in the natural
weathering test. The only difference is the degree of corrosion which is higher due to a more
4 RESULTS AND DISCUSSSION
50
severe environment. As it appears from the pictures in appendix B, the conventional zinc silicate
shopprimer (panel 45) is exposed to white rusting. This corrosion phenomenon is caused by a
higher zinc content (i.e. 50w% Zn) and a more effective galvanic contact between zinc and steel.
The final conclusion is therefore that the best corrosion resistance is obtained by the conventional
zinc silicate shopprimer. The tests also show no improvement of the cathodic protection by
addition of polyaniline.
4.2 Paint for ballast tanks
A Cathodic Protection Test and an Immersion test were used to examine the protection
properties of three versions of a polyamide-adduct cured epoxy paint system, designed especially
for ballast tank protection. The two tests were stopped after 96 days of exposure, and the adhesion
was evaluated by a knife test. Pictures taken after the final evaluation are presented in appendix C
and D.
As seen in figure 19, a cathodic disbonding occurred on the painted steel panels after exposure in
the cathodic protection test. This cathodic disbonding is created in continuation of the applied
defect in the paint and forms a recognizable circle pattern.
Figure 19. Cathodic disbonding after exposure in CPT. Panel 106: Standard ballast tank paint; Panel 116:
Ballast tank paint w. Panipol; Panel 126: CORRPASSIV primer + ballast tank paint w. Panipol.
The cathodic disbonding is stated as an average of the distance from the defect to the point where
the adhesion is intact. Table 20 lists the measured values of the cathodic disbonding for each of
the three paint systems. The reference panels are not cathodic protected by an impressed current.
4 RESULTS AND DISCUSSSION
51
Table 20. Measurements of the cathodic disbonding after the cathodic protection test.
Panel nr.
Paint system
Cathodic
Comments
disbonding (cm)
105 (ref.)
Standard ballast tank
1,0
Bad adhesion in the area with
106
paint
1,8
cathodic disbonding.
107
1,8
115 (ref.)
Standard ballast tank
1,2
The paint is softer and the adhesion
116
paint w. Panipol
1,3
in the area with cathodic disbonding
0,8
is lower than panel 105 to 107.
2,4
Very bad adhesion in the area with
117
125 (ref.)
CORRPASSIV primer
126
+ standard ballast tank 2,7
cathodic disbonding and the steel
127 (ref.)
paint w. Panipol
surface shows signs of rusting.
1,8
As indicated in table 20, the best resistance to cathodic disbonding is seen in panels 115 to 117.
These panels are painted with a standard ballast tank paint containing Panipol (red. polyaniline) in
the first layer. The second best performance is seen in panels 105 to 107, which contain the
conventional ballast tank paint from Hempel. Both of these two paint systems keep the
requirement, i.e. less than 20 mm disbondment. The final paint system stated as panels 125 to 127
in table 20 is painted with a CORRPASSIV primer and a paint system similar to panel 115 to 117
on top. This paint system shows very bad adhesion properties and a high degree of disbonding.
The results therefore show that the addition of Panipol provides an inhibition of the cathodic
disbonding, but the type of binder is also important. The CORRPASSIV primer which is based on
a PVB binder is not a suitable paint for ballast tank application. The lower degree of cathodic
disbonding is in agreement with the results provided by Holness [31]. Therefore, the assumption is
made that polyaniline inhibits the disbonding by absorbing OH- and reacts to form the nonconductive emeraldine base form.
Figure 20 shows a section of the same three paint systems after seawater immersion at 40 °C. The
paint systems are presented in the same order as figure 19.
4 RESULTS AND DISCUSSSION
52
Figure 20. Cathodic disbonding after seawater immersion. Panel 109: Standard ballast tank paint;
Panel 118: Ballast tank paint w. Panipol; Panel 130: CORRPASSIV primer + ballast tank paint w.
Panipol.
As illustrated in figure 20, the best resistance to cathodic disbonding is seen in panel 118. This is
similar to the results obtained in the cathodic protection test. The width of the area with cathodic
disbonding is measured and listed in table 21 for each of the three paint systems.
Table 21. Measurements of the cathodic disbonding after the immersion test.
Panel
Paint system
nr.
Cathodic
Comments
disbonding (cm)
108
Standard ballast tank
3,3
Low adhesion in the area with
109
paint
3,2
cathodic disbonding.
110
3,3
118
Standard ballast tank
2,8
Low adhesion in the area with
119
paint w. Panipol
2,7
cathodic disbonding.
3,1
Small blisters on the panel.
120
128
CORRPASSIV primer
3,7
Very bad adhesion in the area with
129
+ standard ballast tank
5,0
cathodic disbonding. Rust formation
130
paint w. Panipol
5,0
at the steel surface around the score.
Some differences are observed between the immersion test and the cathodic protection test.
Despite the good resistance to cathodic disbondment, the paint films in panels 118 to 120 show
small blisters about 1 cm from the score and in the splash zone. Both areas are exposed to a high
degree of oxygen which increases the risk of cathodic blistering. The formation of cathodic blisters
is properly accelerated by a higher exposure temperature and by the presence of non-dispersed
4 RESULTS AND DISCUSSSION
53
polyaniline particles in the paint film. If the particles act as local cathodes at the steel surface, the
cathodic reactions are concentrated in these spots and cathodic blisters form more easily.
An interesting discovery was made in the case of the last paint system which contained the
CORRPASSIV primer. The primer had changed colour from green to blue after the exposure to
seawater, but only in the area with cathodic disbonding. This observations states that a chemical
reaction is taking place between the primer and hydroxide ions. A small experiment was
performed in order to confirm this statement. A dry film of CORRPASSIV was exposed to
ammonium hydroxide (NH4OH) and the same colour change was observed. Exposing the blue
paint film to hydrochloride acid (HCl) forced the opposite colour change. The primer is therefore
reacting to pH changes in the surrounding environment. As recalled in figure 3, the emeraldine salt
form is green while the emeraldine base form is blue. Without further analysis, the assumption is
made that polyaniline reacts with OH- to form the blue emeraldine base form.
The conclusion is therefore that polyaniline has an inhibiting effect on the cathodic disbonding.
This is due to absorption of hydroxide ions, but the efficiency is strongly dependant on the type of
binder and the homogeneity of dry paint film.
4.3 Paint for off-shore environments
A Salt Spray Test and a Cyclic Test were used to examine the protective properties of paint
systems for application in off-shore environment above the waterline. The examination was
performed on a solvent based paint system and a water based paint system. Pictures taken after
test exposure are presented in appendix E and F.
4.3.1 Solvent based paint system
The solvent based paint system is a conventional anti-corrosive paint system from Hempel
composed of three coatings: A zinc-rich epoxy primer, an epoxy polyamide barrier coating and an
acrylic polyurethane topcoat. Two versions of the paint systems are prepared, and deviate from
each other by the addition of Panipol F in the primer. The two tests were finished after an
exposure period of 90 days.
The salt spray exposure causes no serious degradation on either of the paint systems. Only the
applied score is corroded, while the plain paint surface remains intact and free of blistering. A final
evaluation is performed in order to examine the adhesion and the degree of undercutting
4 RESULTS AND DISCUSSSION
54
corrosion. The undercutting corrosion is insignificant and there is no difference between the two
paint systems. The adhesion is determined by a pull-off test and the results are listed in table 22.
Table 22. Measurements of adhesion by pull-off test after the salt spray test.
Panel
Paint system
nr.
POT measured
Description of fracture
(MPa)
Zinc-rich epoxy
52
53
5,7
primer
40% break in glue
+ epoxy polyamide
7,2
50% adhesive break, 50% cohesive break
barrier coat
5,6
90% cohesive break, 10% break in glue
+ acrylic
5,2
20% adhesive break, 80% cohesive break
polyurethane
5,8
25% adhesive break, 40% cohesive break,
topcoat
62
63
35% adhesive break, 25% cohesive break,
35% break in glue
5,4
90% cohesive break, 10% break in glue
Zinc-rich epoxy
5,8
90% cohesive break, 10% break in glue
primer w. Panipol F
4,8
45% adhesive break, 55% cohesive break
+ epoxy polyamide
6,2
100% cohesive break
barrier coat
5,9
95% cohesive break, 5% break in glue
+ acrylic
5,0
98% cohesive break, 2% break in glue
polyurethane
5,5
15% adhesive break, 70% cohesive break,
topcoat
15% break in glue
As indicated in table 22, the adhesion is very low and the requirement of at least 5MPa is barely
obtained. The most important reason for these low POT values is the panel thickness which is less
than 5 mm. This causes the panel to deform and a lower POT value is measured. The fracture is
based on three types of break in the paint: 1) an adhesive break between the steel surface and the
primer 2) a cohesive break in the topcoat and 3) a break in the glue. As described in table 22, the
break is primarily concentrated in the topcoat in both of the paint system. An incomplete curing of
the topcoat could explain this phenomenon.
The cyclic test contains a very aggressive test environment which causes a more severe degradation
of the paint film. Each of the test panels are exposed to score rusting and blistering around the
score. The degree of blistering is most severe on the panels coated with a paint system containing
Panipol in the primer. In the final evaluation, the paint was removed from the steel panel and the
4 RESULTS AND DISCUSSSION
55
degree of undercutting corrosion was determined. Figure 21 shows the undercutting corrosion in
four of the test panels. Panel 54 and 56 are coated with the conventional anti-corrosive paint
system from Hempel while panel 64 and 66 are coated with the version that contains Panipol in
the primer.
Figure 21. Undercutting corrosion. Panel 54 + 56: Normal anti-corrosive paint system;
Panel 64 + 66: Anti-corrosive paint system with addition of Panipol in primer.
The degree of undercutting corrosion is measured at 9 points along the score. The final value of
undercutting corrosion (M) is calculated by means of equation 4.1, where C is the average of the 9
measurements.
M =
(C − 2)
2
(4.1)
The M-value is determined for each of the panels illustrated at figure 21, and the results are listed
in table 23.
4 RESULTS AND DISCUSSSION
56
Table 23. Measurements of undercutting corrosion after the cyclic test.
Panel 54
M (mm)
Panel 56
2,9
4,3
Panel 64
Panel 66
(w. Panipol)
(w. Panipol)
4,7
4,9
As seen in table 23, the best resistance to undercutting corrosion is provided by the conventional
anti-corrosive paint system from Hempel (panel 56), but the results has a low reproducibility. The
addition of Panipol in the primer does not seem to be a suitable inhibitor against undercutting
corrosion. It is possible that the anti-corrosive effect of polyaniline is not functional in an offshore environment in which undercutting corrosion and blistering are dominant. Blistering and
undercutting corrosion both produce an acidic environment beneath the paint film. Polyaniline is
possibly an efficient inhibitor in an alkaline environment (cf. ballast tank application), while an
acidic environment has the opposite effect.
The adhesion was finally determined by a pull-off test. Table 24 lists the measured POT values and
a description of the belonging fracture in the coating.
Table 24. Measurements of the adhesion by pull-off test after the cyclic test.
Panel
Paint system
nr.
POT measured
Description of fracture
(MPa)
54
56
64
66
Zinc-rich epoxy
10,5
100% cohesive break
primer
9,5
5% adhesive break, 95% cohesive break
+ epoxy polyamide
13,9
45% cohesive break, 55% break in glue
barrier coat
12,3
100% cohesive break
+ acrylic polyurethane
12,1
40% adhesive break, 60% cohesive break
topcoat
14,2
20% adhesive break, 80% cohesive break
Zinc-rich epoxy
10,9
20% adhesive break, 80% cohesive break
primer w. Panipol F
13,1
100% cohesive break
+ epoxy polyamide
18,8
100% cohesive break
barrier coat
15,0
40% adhesive break, 60% cohesive break
+ acrylic polyurethane
13,7
40% adhesive break, 60% cohesive break
topcoat
10,0
100% cohesive break
4 RESULTS AND DISCUSSSION
57
In comparison to the measured POT values after the salt spray test are the POT values in table 23
considerable higher and satisfactorily in relation to the requirements. However, no significant
difference is seen between the adhesion of the two paint systems, and the fracture is mainly
concentrated as a cohesive break in the topcoat.
The conclusion is therefore that no improvement of the corrosion resistance is reached by adding
polyaniline to a conventional anti-corrosive paint system from Hempel. The results indicate that
the anti-corrosive effect of polyaniline is inhibited by the acidic environment formed by blistering
and undercutting corrosion.
4.3.2 Water based paint system
Four versions of a water based zinc rich epoxy primer from Hempel were prepared and
examined according to their corrosion resistance. The primer was covered with a polyamide cured
epoxy barrier coating. The four paint systems were exposed to a salt spray test and a cyclic test for
60 days.
The panels to be tested by salt spray exposure were only applied with a 70µm primer coating and
contained no barrier coating. The barrier coating was excluded in order to examine the protective
character of the primer. Early inspections of the panels showed that white rusting was present to a
higher extent on the panels coated with a primer containing polythiophene. The white rust was
especially formed near the score, but no visual difference was present between primers with high
and low zinc content. White rust is formed when zinc corrodes due to the galvanic coupling with
steel. It is therefore clear that the content of polythiophene has an effect on the galvanic coupling
between zinc and steel. A final evaluation was performed after 60 days of exposure. This
evaluation indicated that the low-loaded zinc primer (60w% Zn) provided a better corrosion
protection than the high-loaded zinc primer (85w% Zn). This is a surprising discovery and no
explanation was to be found. Electrochemical measurements were performed in order to
understand the mechanisms of the four different zinc primers. Measurements of the OCP
potential and the polarization curves are illustrated at figure 22, 23 and 24.
4 RESULTS AND DISCUSSSION
58
200
100
Potential vs SHE (mV)
0
85% Zn
-100
85% Zn, BAYTRON
60% Zn
60% Zn, BAYTRON
-200
Steel
-300
-400
-500
0
2
4
6
8
10
12
14
16
18
20
Time (min)
Figure 22. OCP measurements of water based paint in 3,5w% NaCl.
600
400
Potential vs SHE (mV)
200
0
Steel
85% Zn
85% Zn, BAYTRON
-200
-400
-600
-800
0,00000001 0,0000001
0,000001
0,00001
0,0001
0,001
0,01
0,1
1
10
100
Current density (mA/cm2)
Figure 23. Polarization curves of water based paint with 85w% Zn in 3,5w% NaCl.
4 RESULTS AND DISCUSSSION
59
600
400
Potential vs SHE (mV)
200
0
Steel
60% Zn
60% Zn, BAYTRON
-200
-400
-600
-800
1E-09
1E-08
1E-07
0,000001
0,00001
0,0001
0,001
0,01
0,1
1
10
100
Current density (mA/cm2)
Figure 24. Polarization curves of water based paint with 60w% Zn in 3,5w% NaCl.
The OCP measurements in figure 22 clearly show that the addition of polythiophene increases the
corrosion potential by 100-200 mV independently of the zinc content. The OCP measurements
also state that a cathodic protection of the steel surface is not present in any of the four paint
systems. Is this possible or are the results incorrect? The polarization curves on figure 23 and 24
can maybe lead to an answer. Figure 23 illustrates the polarization curves of the high-loaded zinc
primers and no remarkable differences in the corrosion potential and current are present between
the two primers. The appearance of the two curves shows that it is difficult for a current to travel
through the paint film. Only when a small defect is introduced to the paint film can a transport of
electrons take place. The polarization curves of the low-loaded zinc primers are illustrated in figure
24 and show a significant difference between the two primers. The pure zinc primer is able to
suppress the corrosion potential of steel and establish a cathodic protection, while the primer
containing polythiophene has a corrosion potential about 200 mV higher and similar to steel. The
lowest corrosion current is also provided by the pure zinc primer. The polarization curves
therefore confirm that the low-loaded zinc primer provides the best corrosion protection, as
observed in the salt spray test. These observations change the basic idea of the protective
mechanism of a zinc-rich primer. Maybe the amount of zinc powder in the primer is not crucial
for the efficiency of corrosion protection, and is it possible that the cathodic protection only
provides a small part of the corrosion protection? These questions are interesting hypotheses but
remain out of the scope of this project.
4 RESULTS AND DISCUSSSION
60
The cyclic test was performed on the same four zinc primers covered with a polyamide cured
epoxy barrier coating. Earlier inspections indicated that the best cathodic protection was provided
by the paint system with high zinc content and no addition of BAYTRON. Only a slight degree of
rusting was present at the applied score which indicates that cathodic protection had occurred. No
significant difference was present at the other three paint systems on which the score was fully
corroded. The final evaluation was performed after 60 days of exposure and a difference was
noted between the paint systems with and without BAYTRON. A greater extent of blistering
around the score was present at the panels coated with a paint system without BAYTRON. A
great extent of blistering around the score is normally equal to a high degree of undercutting
corrosion, and the paint was therefore removed from the steel surface. Figure 25 shows pictures of
the undercutting corrosion on the four different paint systems.
Figure 25. Undercutting corrosion. Panel 73: 85w% Zn (in primer); Panel 77: 85w% Zn +
BAYTRON; Panel 83: 60w% Zn + BAYTRON; Panel 88: 60w% Zn.
4 RESULTS AND DISCUSSSION
61
The degree of undercutting corrosion is calculated from 9 points along the score. Table 25 lists the
calculated M-value for the panels illustrated at figure 25.
Table 25. Measurements of undercutting corrosion after the cyclic test.
M (mm)
Panel 73
Panel 77
Panel 83
Panel 88
(85% Zn)
(85% Zn + BAYTRON)
(60% Zn + BAYTRON)
(60% Zn)
1,7
2,1
2,4
3,4
The results in table 25 indicate that the addition of polythiophene when combined with a high zinc
content provides the best resistance to undercutting corrosion (cf. panel 77). These results are in
contrast to the observations of the solvent based paint systems. A reasonable explanation could be
that a better dispersion of the conducting polymer occurred within these water based paint
systems.
A SEM analysis was performed in order to examine the corrosion behaviour of the four water
based paint systems. The aim was to examine whether any differences in topography and
composition were present between the paint systems after 60 days of exposure in the cyclic test.
The SEM analyse revealed no significant differences between the four paint systems exposed in
the cyclic test, but a general model of the degradation of zinc particles was established. Figure 26
illustrates an example of the corrosive behaviour of the zinc particles which are distributed in the
primer. The picture on the left is produced by ejection of secondary electrons and shows the
topography of the primer. The picture on the right is produced by backscattered electrons and
indicates the composition of the primer. The light components in the primer appear dark in the
backscattered picture, while heavy components appear light.
Figure 26. SEM pictures of a zinc primer in secondary signal (left) and backscattered signal (right).
4 RESULTS AND DISCUSSSION
62
The SEM pictures in figure 26 indicate that a big part of the heavy zinc particles, which are in
contact to the steel surface, are degraded to lighter corrosion products. Analysis of the
composition shows that these corrosion products mainly contain zinc, iron, chloride and oxygen.
A mapping of the composition shows that chloride and oxygen is evenly distributed within the
primer. The mapping also indicates increased iron content and reduced zinc content up to 10 µm
away from the steel surface. Therefore, the suggestion is made that iron ions diffuse into the
primer and form a complex with zinc which are thermodynamic stable at a high potential range. It
is proposed that this complex contributes to the protective mechanisms of zinc paint. This could
explain why the low-loaded zinc paint provides the best performance in the salt spray test. Maybe
the formation of a protective iron/zinc complex is more important than the galvanic contact
between steel and zinc?
The conclusion is therefore that the addition of polythiophene provides some kind of positive
effect on the protective behaviour of water based zinc paints, but only if the primer is coated with
a barrier paint. Without the barrier coating, polythiophene seems to accelerate the degradation of
zinc within the primer.
4.4 Paint for aluminium substrates
Salt Spray Test, Cyclic Test and Filiform Corrosion Test were used to examine the
protective properties of four different coating systems designed for protection of aluminium. The
four different coating systems were painted with the same barrier coating and topcoat, but deviate
from one another in the application of a primer or a preceding surface treatment.
The test panels were exposed to a salt spray test and a cyclic test for a period of 96 days. The only
sign of degradation of the paint film was due to adhesion problems. A qualitative analyse of the
adhesion was performed by a knife test in the final evaluation. Tables 26 and 27 on the following
page list the results.
4 RESULTS AND DISCUSSSION
63
Table 26. Evaluation of the adhesion after the salt spray test.
Surface
Adhesion evaluation
treatment
by knife test
Comments
Corrpassiv 4001 Very bad
Disbonding about 1,5 cm from the score.
Anodizing
Very good
Nothing to note!
Chromatizing
Bad
Disbonding about 1,0 cm from the score.
Altizizing
Good
The adhesion is good but not to be compared with
the anodized aluminium panels.
Table 27. Evaluation of the adhesion after the cyclic test.
Surface
Adhesion evaluation
treatment
by knife test
Comments
Corrpassiv 4001 Good
Improved adhesion compared to the salt spray test.
Anodizing
Very good
Nothing to note!
Chromatizing
Bad
Disbonding about 0,5 cm from the score.
Altizizing
Very bad
The adhesion was really bad and the paint was
pealing of.
As described in tables 26 and 27, the adhesion of the test panels coated with the CORRPASSIV
primer is dependant on the test conditions. The assumption is made that epoxy resins in the
primer are cured when exposed to UV light at 60 °C in the cyclic test. As described earlier, the
CORRPASSIV primer is a PVB based paint but small amount of epoxy was also discovered by
chemical analysis. A colour change similar to the one observed in the immersion test was also
present in the primer after the salt spray test and cyclic test. It is therefore proven that polyaniline
is reduced to its non-conductive base form due to the formation of an alkaline environment
beneath the primer.
The opposite adhesion property is current at the altizized aluminium panels. No adhesion
problems are observed in the salt spray test while the cyclic test causes the paint to peal of in large
areas. It was especially the exposure to heat and condensation in the cyclic process that caused
serious adhesion problems. The chromatizing process is surprisingly not a suitable surface
treatment, as a low degree of adhesion is found in both tests. No remarks are made regarding the
anodized aluminium panels, which had a superior adhesion to the paint. An analysis of the surface
4 RESULTS AND DISCUSSSION
64
topography was performed by SEM in order to explain some of the adhesive behaviour of the
applied coatings. SEM-pictures of an altizized aluminium panel and an anodized aluminium panel
are shown in figure 27.
Figure 27. Surface topography of altizized (left) and anodized (right) aluminium panels.
It is clear from figure 27 that the anodized surface is much more porous than the altizized surface.
This can be a possible explanation for the observed differences in the adhesive properties. It is
easier for the paint to stick on a porous surface than a smooth.
The resistance to filiform corrosion was examined for each of the four coating systems after an
exposure of 50 days. Pictures taken after test exposure are illustrated in appendix G. The best
resistance to filiform corrosion is seen on the anodized and the altizized aluminium panels, which
contain no sign of filiform corrosion. The chromatized panel and the CORRPASSIV painted
panel are both attacked by filiform corrosion and about 3 mm filaments are formed beneath the
paint film.
Finally, electrochemical measurements were performed in order to understand the protective
mechanisms of each of the four coating systems. The panels to be analysed were only provided
with a surface treatment or a primer. Measurements of the OCP potential and the polarization
curves are illustrated in figures 28 and 29.
4 RESULTS AND DISCUSSSION
65
-350
-400
Potential vs SHE (mV)
-450
Ano Al
Chrom Al
MnO2 Al
CORRPASSIV
Uncoated Al
-500
-550
-600
-650
0
2
4
6
8
10
12
14
16
18
20
Time (min)
Figure 28. OCP potential of aluminium specimens in 3,5w% NaCl.
100
0
-100
Potential vs SHE (mV)
-200
-300
Ano Al
Chrom Al
MnO2 Al
CORRPASSIV
Uncoated Al
-400
-500
-600
-700
-800
-900
-1000
1,0E-08
1,0E-07
1,0E-06
1,0E-05
1,0E-04
1,0E-03
1,0E-02
1,0E-01
1,0E+00
1,0E+01
1,0E+02
Current density (mA/cm2)
Figure 29. Polarization curves of aluminium specimens in 3,5w% NaCl.
As illustrated in figure 28, the corrosion potential of each of the three different surface treatments
is more or less similar to the corrosion potential of uncoated aluminium, while the CORRPASSIV
coated aluminium specimen exhibits a much lower potential. These observations are in contrast to
4 RESULTS AND DISCUSSSION
66
the theory which claims that an ennobling of the aluminium surface should take place for each of
the applied coatings. The corrosion potential is also determined by the polarization curves
illustrated in figure 29. These curves show an increase of the corrosion potential for the
chromatized and anodized aluminium panels, while the corrosion potential of the altizized and
CORRPASSIV coated panels is similar to pure aluminium. A decrease of the corrosion current is
observed at anodized and CORRPASSIV painted specimens. This decrease is attributed to a
barrier effect due to a higher coating thickness.
The conclusion is therefore that no satisfactory corrosion protection is provided by the
CORRPASSIV primer, and an ennobling process is also not registered. The best adhesive
properties and the highest resistance to filiform corrosion are still provided by a conventional
anodizing surface treatment.
5 CONCLUSION
67
5 CONCLUSION
Anti-corrosive paint systems based on conducting polymers have been formulated and
tested in four different application areas. The aim was to establish if polyaniline and polythiophene
are effective corrosion inhibitors and evaluate the protective mechanisms behind.
For shopprimer application, the conclusion is made, that addition of polyaniline has no positive
effect on the corrosion resistance. In some of the applied shopprimers, polyaniline seems to
accelerate the degradation of the paint film instead. It is suggested that a low dispersibility of
polyaniline is an important issue and plays the crucial part of the protective behaviour of these
shopprimer systems.
In the case of ballast tank paints, addition of polyaniline seems to improve the resistance to
cathodic disbonding. The assumption is made, that polyaniline possess an amphoteric nature and
therefore reacts with the hydroxide ions produced in the cathodic reaction. In this reaction,
polyaniline is reduced to its non-conductive emeraldine base form.
The applied paint systems for protection in off-shore environments show no significant anticorrosive improvements by the addition of polyaniline to a zinc-rich primer. The hypothesis that
polyaniline acts as a conductive additive and improves the cathodic effect of the zinc particles is
therefore not true. Some positive effect is registered in the water based paint systems with the
addition of polythiophene. It is possible that a better degree of dispersibility is the main
explanation for this observation.
Finally, different coating systems for aluminium protection have been tested, and the efficiency
has been evaluated. The results show that the polyaniline containing primer is not suitable for
protection of aluminium due to bad adhesion and low resistance to filiform corrosion. A
conventional coating system with an anodizing pre-treatment provides the best corrosion
protection of aluminium substrates.
6 PERSPECTIVE
68
6 PERSPECTIVE
The development of an anti-corrosive paint system based on conducting polymers in this
project has been difficult, and the results show no significant improvement of the corrosion
resistance when compared to conventional paint systems from Hempel. Some suggestions are
therefore presented in this chapter in order to create a more effective paint system based on
conducting polymers
The formulation of a homogeneous paint film is vital in order to achieve an effective corrosion
protection. The preparation of a polyaniline containing paint was very difficult, and some of the
dry paint films therefore contained non-dispersed particles of polyaniline. Similar dispersion
problems have been described in the literature and the explanation seems to be related to the high
surface tension of polyaniline. Due to a very high surface tension, it becomes very difficult to wet
these polyaniline particles which tend to agglomerate instead. Some scientists have tried to change
the physical properties of polyaniline by preparing a double stranded polyaniline compound in
which polyaniline is grafted to another polymer chain. Yang [44] have described a way to adjust
the solubility parameter of polyaniline by the synthesis of complex between polyaniline and
poly(acrylic) acid. This complex has a more stable conductivity and can be dispersed in water.
According to Yang, it is possible to adjust the compatibility with the binder resins by changing the
choice of the second strand. This is a very interesting discovery, and a further examination of the
anti-corrosive behaviour of these double-stranded polyaniline compounds is therefore
recommended.
Another approach for the preparation of an effective anti-corrosive paint system is to examine the
influence of varying polyaniline content. As described in the previous theory section, a better
corrosion protection is present in paints with a low load of polyaniline. This connection between
the corrosion resistance and the polyaniline content is also explained in relation to the
compatibility aspect. A high content of polyaniline creates a more porous paint film with higher
permeability and lower corrosion resistance. The author therefore recommends an examination
that determines the optimum polyaniline content.
In the case of paint for ballast tank protection, the results in this report indicate that polyaniline
improves the resistance to cathodic protection. The assumption is made that polyaniline acts as an
amphoteric substance and absorbs the hydroxide ions produced in the cathodic reaction. An
amphoteric substance is characterized by the ability to react as either acid or base. Other materials,
6 PERSPECTIVE
69
such as aluminium hydroxide, also possess this amphoteric nature. It could be interesting to see if
these materials also improve the resistance to cathodic disbonding in ballast tank application.
Finally, the author recommends a further examination of the protective mechanisms of zinc-rich
paint systems. This project has revealed some surprising results in relation to the protective
behaviour of zinc paints with varying zinc content. According to theory, the best cathodic
protection is established in paints with a high load of zinc (i.e. 85w% Zn), but this project indicates
that en effective corrosion protection is also present in paints with a low zinc content (i.e. 60w%
Zn). A SEM analysis has revealed that a complex between iron and zinc is formed near the steel
surface. It is assumed that this complex contributes to the protective mechanisms of zinc paints.
An examination of the protective behaviour of this complex is therefore to be recommended in
the future.
i
ABBREVIATIONS
DFT:
Dry Film Thickness
EB:
Emeraldine Base
ECP:
Electrically Conducting Polymer
ECT:
Eddy Current Test
EIS:
Electrochemical Impedance Spectroscopy
ES:
Emeraldine Salt
ESCA:
Electron Spectroscopy Chemical Analysis
FTIR:
Fourier Transform Infrared
OCP:
Open Circuit Potential
LE:
Leucoemeraldine
PANI:
Polyaniline
PEDOT/PSS:
Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate)
PMMA:
Polymethylmethacrylate
POT:
Pull-Off Test
PVB:
Polyvinyl butyral
RH:
Relative Humidity
SCE:
Standard Calomel Electrode
SECM:
Scanning Electrochemical Microscopy
SEM:
Scanning Electron Microscopy
SHE:
Standard Hydrogen Electrode
St-BuA:
Styrene-butyl acrylate
ii
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vi
LIST OF APPENDIX
Pictures:
Appendix A:
Pictures from the Natural Weathering Test.
Appendix B:
Pictures from the Mebon Prohesion Test.
Appendix C:
Pictures from the Cathodic Protection Test.
Appendix D:
Pictures from the Immersion Test.
Appendix E:
Pictures from the Salt Spray Test
Appendix F:
Pictures from the Cyclic Test.
Appendix G:
Pictures from the Filiform Corrosion Test.
Data sheets:
Appendix H:
Data sheet of CORRPASSIV 4000
Appendix I:
Data sheet of CORRPASSIV 4001
Appendix J:
Data sheet of PVB shopprimer from Hempel*
Appendix K:
Data sheet of zinc phosphate shopprimer from Hempel*
Appendix L:
Data sheet of zinc-silicate shopprimer from Hempel*
Appendix M:
Data sheet of zinc epoxy primer from Hempel*
Appendix N:
Data sheet of polyamide/epoxy ballast tank paint from Hempel*
Appendix O:
Data sheet of water-borne epoxy barrier paint from Hempel*
Appendix P:
Data sheet of water-borne zinc epoxy primer from Hempel*
Appendix Q:
Data sheet of polyamide/epoxy barrier paint from Hempel*
Appendix R:
Datasheet of acrylic polyurethane topcoat from Hempel*
* Confidential information material
vii
Appendix A
Test:
Natural Weathering Test
Application area:
Shopprimer
Time of exposure:
92 days
Panel
Paint system
nr.
1
PVB/polyaniline primer
DFT*
Panel
(µm)
nr.
11,1
2
(CORRPASSIV 4000)
3
PVB/polyaniline primer
Paint system
DFT*
(µm)
PVB/polyaniline primer
12,2
(CORRPASSIV 4000)
17,9
11
PVB shopprimer
21,4
(CORRPASSIV 4000)
12
PVB shopprimer
20,6
13
PVB shopprimer
19,8
21
Zinc phosphate shopprimer
39,8
22
Zinc phosphate shopprimer
40,7
23
Zinc phosphate shopprimer
51,6
31
Zinc phosphate shopprimer w.
44,3
Panipol
32
Zinc phosphate shopprimer w.
45,3
33
Panipol
41
Zinc silicate shopprimer w. 50%
Zinc silicate shopprimer w. 40%
Zn and Panipol
21,3
42
Zinc silicate shopprimer w. 30%
25,2
Zn
30,6
44
Zinc silicate shopprimer w. 30%
Zn and Panipol
* DFT = dry film thickness
Panel 1
39,1
Panipol
Zn
43
Zinc phosphate shopprimer w.
Panel 2
28,7
viii
Panel 3
Panel 11
Panel 12
Panel 13
Panel 21
Panel 22
ix
Panel 23
Panel 31
Panel 32
Panel 33
Panel 41
Panel 42
x
Panel 43
Panel 44
xi
Appendix B
Test:
Mebon Prohesion Test
Application area:
Shopprimer
Time of exposure:
92 days
Panel
Paint system
nr.
4
PVB/polyaniline primer
DFT*
Panel
(µm)
nr.
16,5
5
(CORRPASSIV 4000)
6
PVB/polyaniline primer
Paint system
DFT*
(µm)
PVB/polyaniline primer
17,9
(CORRPASSIV 4000)
18,2
14
PVB shopprimer
16,4
(CORRPASSIV 4000)
15
PVB shopprimer
19,2
16
PVB shopprimer
21,1
24
Zinc phosphate shopprimer
50,4
25
Zinc phosphate shopprimer
56,6
34
Zinc phosphate shopprimer
47,9
35
Zinc phosphate shopprimer
47,6
w. Panipol
45
Zinc silicate shopprimer
w. Panipol
21,3
w. 50% Zn
47
Zinc silicate shopprimer
46
Zinc silicate shopprimer
w. 40% Zn and Panipol
28,7
w. 30% Zn and Panipol
* DFT = dry film thickness
Panel 4
Panel 5
30,6
xii
Panel 6
Panel 14
Panel 15
Panel 16
Panel 24
Panel 25
xiii
Panel 34
Panel 35
Panel 45 (OBS: 66 days of exposure)
Panel 46 (OBS: 66 days of exposure)
Panel 47 (OBS: 66 days of exposure)
xiv
Appendix C
Test:
Cathodic Protection Test
Application area:
Ballast tank
Time of exposure:
96 days
Panel
Paint system
nr.
105
(2 x) Polyamide/epoxy ballast
DFT*
Panel
(µm)
nr.
245,4
106
(2 x) Polyamide/epoxy ballast
DFT*
(µm)
(2 x) Polyamide/epoxy ballast
255,5
tank paint
tank paint
107
Paint system
254,5
115
Polyamide/epoxy ballast tank
286,0
paint w. Panipol
tank paint
+ Polyamide/epoxy ballast tank
paint
116
125
127
Polyamide/epoxy ballast tank
260,5
117
Polyamide/epoxy ballast tank
paint w. Panipol
paint w. Panipol
+ Polyamide/epoxy ballast tank
+ Polyamide/epoxy ballast tank
paint
paint
CORRPASSIV 4000
358,0
126
CORRPASSIV 4000
+ Polyamide/epoxy ballast tank
+ Polyamide/epoxy ballast tank
paint w. Panipol
paint w. Panipol
+ Polyamide/epoxy ballast tank
+ Polyamide/epoxy ballast tank
paint
paint
CORRPASSIV 4000
+ Polyamide/epoxy ballast tank
paint w. Panipol
+ Polyamide/epoxy ballast tank
paint
* DFT = dry film thickness
331,0
257,0
341,0
xv
Panel 105 (reference)
Panel 106
Panel 107
Panel 115 (reference)
Panel 116
Panel 117
xvi
Panel 125 (reference)
Panel 127
Panel 126
xvii
Appendix D
Test:
Immersion test
Application area:
Ballast tank
Time of exposure:
96 days
Panel
Paint system
nr.
108
(2 x) Polyamide/epoxy ballast
DFT*
Panel
(µm)
nr.
268,0
109
(2 x) Polyamide/epoxy ballast
DFT*
(µm)
(2 x) Polyamide/epoxy ballast
280,5
tank paint
tank paint
110
Paint system
294,5
118
Polyamide/epoxy ballast tank
291,5
paint w. Panipol
tank paint
+ Polyamide/epoxy paint ballast
tank paint
119
128
130
Polyamide/epoxy ballast tank
313,5
120
Polyamide/epoxy ballast tank
paint w. Panipol
paint w. Panipol
+ Polyamide/epoxy paint ballast
+ Polyamide/epoxy paint ballast
tank paint
tank paint
CORRPASSIV 4000
303,5
129
CORRPASSIV 4000
+ Polyamide/epoxy ballast tank
+ Polyamide/epoxy ballast tank
paint w. Panipol
paint w. Panipol
+ Polyamide/epoxy paint ballast
+ Polyamide/epoxy paint ballast
tank paint
tank paint
CORRPASSIV 4000
+ Polyamide/epoxy ballast tank
paint w. Panipol
+ Polyamide/epoxy paint ballast
tank paint
* DFT = dry film thickness
336,0
283,5
307,0
xviii
Panel 108
Panel 109
Panel 110
Panel 118
Panel 119
Panel 120
xix
Panel 128
Panel 130
Panel 129
xx
Appendix E
Test:
Salt Spray Test
Application area:
Off-shore environment
Time of exposure:
90 days
Panel
Paint system
nr.
51
53
62
Zinc epoxy primer
DFT*
Panel
(µm)
nr.
267,0
52
Paint system
(µm)
Zinc epoxy primer
+ Polyamide/epoxy barrier coat
+ Polyamide/epoxy barrier coat
+ Acrylic/polyurethane topcoat
+ Acrylic/polyurethane topcoat
249,5
Zinc epoxy primer
61
Zinc epoxy primer w. Panipol
+ Polyamide/epoxy barrier coat
+ Polyamide/epoxy barrier coat
+ Acrylic/polyurethane topcoat
+ Acrylic/polyurethane topcoat
Zinc epoxy primer w. Panipol
251,5
63
DFT*
Zinc epoxy primer w. Panipol
+ Polyamide/epoxy barrier coat
+ Polyamide/epoxy barrier coat
+ Acrylic/polyurethane topcoat
+ Acrylic/polyurethane topcoat
271,5
274,0
251,5
* DFT = dry film thickness
Application area:
Off-shore environment (water based paint systems)
Time of exposure:
60 days
Panel
Paint system
nr.
71
Water-borne zinc epoxy primer
DFT*
Panel
(µm)
nr.
83,5
72
w. 85% Zn
75
Water-borne zinc epoxy primer
Water-borne zinc epoxy primer
70,0
76
Water-borne zinc epoxy primer
w. 60% Zn
* DFT = dry film thickness
(µm)
Water-borne zinc epoxy primer
81,7
Water-borne zinc epoxy primer
68,0
w. 85% Zn and Baytron
76,4
82
w. 60% Zn and Baytron
85
DFT*
w. 85% Zn
w. 85% Zn and Baytron
81
Paint system
Water-borne zinc epoxy primer
80,9
w. 60% Zn and Baytron
84,0
86
Water-borne zinc epoxy primer
w. 60% Zn
86,0
xxi
Panel 51
Panel 52
Panel 53
Panel 61
Panel 62
Panel 63
xxii
Panel 71
Panel 72
Panel 75
Panel 76
Panel 81
Panel 82
xxiii
Panel 85
Panel 86
xxiv
Appendix F
Test:
Cyclic Test
Application area:
Off-shore environment
Time of exposure:
90 days
Panel
Paint system
nr.
54
56
65
Zinc epoxy primer
DFT*
Panel
(µm)
nr.
277,0
55
Paint system
(µm)
Zinc epoxy primer
+ Polyamide/epoxy barrier coat
+ Polyamide/epoxy barrier coat
+ Acrylic/polyurethane topcoat
+ Acrylic/polyurethane topcoat
275,5
Zinc epoxy primer
64
Zinc epoxy primer w. Panipol
+ Polyamide/epoxy barrier coat
+ Polyamide/epoxy barrier coat
+ Acrylic/polyurethane topcoat
+ Acrylic/polyurethane topcoat
Zinc epoxy primer w. Panipol
246,0
66
DFT*
Zinc epoxy primer w. Panipol
+ Polyamide/epoxy barrier coat
+ Polyamide/epoxy barrier coat
+ Acrylic/polyurethane topcoat
+ Acrylic/polyurethane topcoat
275,5
262,5
244,5
* DFT = dry film thickness
Application area:
Off-shore environment (waterbased paint systems)
Time of exposure:
60 days
Panel
Paint system
nr.
73
Water-borne zinc epoxy primer
DFT*
Panel
(µm)
nr.
183,4
74
w. 85% Zn + Epoxy barrier paint
77
83
87
Water-borne zinc epoxy primer
Paint system
(µm)
Water-borne zinc epoxy primer
176,4
78
Water-borne zinc epoxy primer
w. 85% Zn and Baytron
+ Epoxy barrier paint
+ Epoxy barrier paint
177,4
84
Water-borne zinc epoxy primer
w. 60% Zn and Baytron
w. 60% Zn and Baytron
+ Epoxy barrier paint
+ Epoxy barrier paint
Water-borne zinc epoxy primer
w. 60% Zn + Epoxy barrier paint
* DFT = dry film thickness
193,9
w. 85% Zn + Epoxy barrier paint
w. 85% Zn and Baytron
Water-borne zinc epoxy primer
DFT*
198,2
88
Water-borne zinc epoxy primer
w. 60% Zn + Epoxy barrier paint
167,3
183,0
205,0
xxv
Panel 54
Panel 55
Panel 56
Panel 64
Panel 65
Panel 66
xxvi
Panel 73
Panel 74
Panel 77
Panel 78
Panel 83
Panel 84
xxvii
Panel 87
Panel 88
xxviii
Appendix G
Test:
Filiform Corrosion Test
Application area:
Aluminium
Time of exposure:
50 days
Panel
Paint system
nr.
91
101
103
CORRPASSIV 4001
DFT*
Panel
(µm)
nr.
208,0
92
Paint system
(µm)
CORRPASSIV 4001
+ Polyamide/epoxy barrier coat
+ Polyamide/epoxy barrier coat
+ Acrylic/polyurethane topcoat
+ Acrylic/polyurethane topcoat
Anodized aluminium
193,6
102
Chromatized aluminium
+ Polyamide/epoxy barrier coat
+ Polyamide/epoxy barrier coat
+ Acrylic/polyurethane topcoat
+ Acrylic/polyurethane topcoat
Altizized aluminium
183,6
+ Polyamide/epoxy barrier coat
+ Acrylic/polyurethane topcoat
* DFT = dry film thickness
Panel 91 (1mm score)
DFT*
Panel 91 (2mm score)
210,5
187,3
xxix
Panel 92 (1mm score)
Panel 92 (2mm score)
Panel 101 (1mm score)
Panel 101 (2mm score)
Panel 102 (1mm score)
Panel 102 (2mm score)
xxx
Panel 103 (1mm score)
Panel 103 (2mm score)
xxxi
Appendix H
xxxii
xxxiii
Appendix I
xxxiv
xxxv
Appendix J
xxxvi
xxxvii
Appendix K
xxxviii
xxxix
Appendix L
xl
xli
Appendix M
xlii
xliii
Appendix N
xliv
xlv
Appendix O
xlvi
xlvii
Appendix P
xlviii
xlix
Appendix Q
l
li
Appendix R
lii