Corrosion
Rust is perhaps the most familiar form of corrosion because so many of our tools and implements are made of iron
alloys. Rust is the common name for iron oxide. Iron oxide, the chemical Fe2O3, is common because iron combines very
readily with oxygen. Iron rusting is an example of corrosion -- an electrochemical process involving an anode (a piece of
metal that readily gives up electrons), an electrolyte (a liquid that helps electrons move) and a cathode (a piece of metal
that readily accepts electrons). When a piece of metal corrodes, the electrolyte helps provide oxygen to the anode. As
oxygen combines with the metal, electrons are liberated. When they flow through the electrolyte to the cathode, the
metal of the anode disappears, swept away by the electrical flow or converted into metal cations in a form such as rust.
For iron to become iron oxide, three things are required: iron, water and oxygen. When a drop of water hits an iron
object, two things begin to happen almost immediately. First, the water, a good electrolyte, combines with carbon
dioxide in the air to form a weak carbonic acid, an even better electrolyte. As the acid is formed and the iron dissolved,
some of the water will begin to break down into its component pieces -- hydrogen and oxygen. The free oxygen and
dissolved iron bond into iron oxide, in the process freeing electrons. The electrons liberated from the anode portion of
the iron flow to the cathode, which may be a piece of a metal less electrically reactive than iron, or another point on the
piece of iron itself.
The chemical compounds found in liquids like acid rain, seawater and the salt-loaded spray from snow-belt roads make
them better electrolytes than pure water, allowing their presence to speed the process of rusting on iron and other
forms of corrosion on other metals.
Corrosion, like rust, can form over large areas or be very localized. General attack corrosion is the most common type of
corrosion and is caused by a chemical or electrochemical reaction that results in the deterioration of the entire exposed
surface of a metal. General attack corrosion accounts for the greatest amount of metal destruction by corrosion, but is
considered as a safe form of corrosion, due to the fact that it is predictable, manageable and often preventable.
Unlike general attack corrosion, localized corrosion specifically targets one area of the metal structure. Localized
corrosion may cause pitting, attack crevices, or occur under painted or plated surfaces when water breaches the coating.
Whether general or localized, corrosion will eventually cause the metal to fail. Since iron oxide is larger in size than iron,
the oxidation process causes it to “puff up” and sometimes flake. When
a great amount of rust accumulates, the buildup may create a powerful
force that can actually separate or seize together adjacent parts.
When it comes to rust damage, there are contributing factors that cause
steel or iron to rust faster than others. For instance, water is a common
culprit responsible for the development of rust that attacks iron and
steel. Dissimilar metals will also rust faster than single metals because
of the electrochemical reactions that take place. Salt water also causes
rust to develop quicker than fresh water because salt is a better electrical
conductor. Heat also causes an increase in the rate of rust development.
The rate of oxidization can be reduced through a variety of treatments.
Corrosion resistant alloys like “stainless steel” have become more common
and cost effective. “Sacrificial Zincs” use the chemistry of oxidization to
eat away a less important part of an object rather than the structural or
valuable part. “Galvanized” metals are plated with zinc to protect the
iron or steel underneath. The most common method to resist corrosion
is to isolate the iron surfaces from contact with water. Protective coatings
like paint, oil, and wax are used to seal moisture away from the metal.
Through these methods metal objects can last for many years.
Items that have been submerged in water are at a high risk for corrosion, like this
anchor found in Lake Champlain, shot off the bow of the British frigate Confiance
on September 11, 1814. Note that the very bottom of the anchor is free of
corrosion: that part was sitting under the mud in an anaerobic environment,
eliminating one of the key ingredients needed for corrosion formation.
Image: Lake Champlain Maritime Museum
Name
Date
Corrosion, continued
Looking Around at Corrosion in Your Environment
It’s time to take a closer look at the objects and structures around you to see how they are affected by corrosion. This
can be done at school or home, inside or out. Look for objects that contain iron. Rust is easy to find because of its
distinctive reddish brown color and pitting or flaking. You may also find corrosion on other metals like copper and
aluminum. Find and describe at least ten items that show signs of corrosion. Record your findings below.
Object
Location
Description
Which object showed the greatest deterioration from corrosion?
Did any of the objects surprise you? Why?