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Name: Alex
Status: educator
Grade: 9-12
Country: Australia
Date: Summer 2011


Question:
what type of corrosion is the rusting of steel? i did an experiment where we had steel pieces in water and the rust did not cover all of the surface only some. The rust was in a layer and when it was cleaned off, the steel underneath was smooth (no cracks or pits or anything). Is it still called uniform corrosion but just in the beginning stages or is it another type of corrosion?



Replies:
Hi Alex,

The rusting of steel is a complex process that requires the presence of oxygen and water. The result is a growing layer of Iron Oxide (the red rust you saw) that does not adhere well to the steel. This is unlike the oxidation of many other metals such as aluminum, titanium, nickel, chromium, etc., that results in a tough, thin, protective layer of that metal's oxide that protects the underlying metal from further corrosion.

The layer of iron oxide that results from the rusting of steel is easily removed and is very porous. It does not protect the underlying steel surface. If left to corrode for long enough, the entire steel object will be consumed, and all that will be left is a large mass of rust.

The only reason you saw smooth steel when you removed the surface rust, is because you did not leave the nail in the water long enough. Rusting of steel is a slow process, but eventually under the right conditions (presence of moisture or liquid water, and oxygen), the entire steel object will be consumed.

It is interesting to note that the speed of rusting proceeds much faster when the supply of oxygen is very limited. This is why so-called "crevice crack corrosion" (corrosion in deep cracks, for example, where oxygen access is very limited) can be very destructive. In areas where there is plenty of oxygen available such as on the surface of a cast iron object like a manhole cover, rust does occur, but its attack is much slower.

Regards,

Bob Wilson.


Corrosion is a complicated chemical process. Usually involving a number of reactions. The type of steel (which isn't just iron), time, temperature, pH, dissolved gases, and other variables are all a factor.

Whether the corrosion is smooth or pitted could be a factor of several of these variables. It is hard to say without having much more information about the specific conditions.

Vince Calder


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