The MGA With An Attitude

At 11:11 AM 11/11/04 -0500, Dennis ( wrote:
>"We all know when moisture and steel mix, the result on our cars is rust.
>Question is, what causes this to happen?"

I'm surprised there wasn't a rash of off the cuff responses to this one. Brace yourself for more than you need to know, and I will try to condense a couple of chapters of RUST-101 into a single page.

Rusting is a chemical reaction of iron in the presence of oxygen.
Common sheet metal rusting in dry air works like this:
4 Fe + 3 O2 --> 2 Fe2O3
This reaction is relatively slow and produces a thin coating of stable iron oxide Fe2O3, which is (technically) rust, but is a fairly benign form of rust.

However, .... if it happens in the presence of water, or even moisture in the air, a second step works like this:
Fe2O3 + H2O --> Fe2O3 + H2O
This produces hydrated iron oxide, or RUST (the common kind the eats your sheet metal). This reaction happens much faster and is much more destructive. Add a little road salt to the mixture to ionize the process, and it goes even faster (please don't ask for that equation). This form of ferric oxide is bulky and porous (mechanically much different than Fe2O3), so it allows more oxygen access to the iron below, causing additional rusting. If allowed to continue, the oxygen and water will completely convert the remaining iron to ferric oxide (or solid rust), which is weak and flaky, hence big holes in metal (cancer).

For parts submerged in water, or filled with water (like the cooling jacket of an engine block), it works more like this:
3 Fe + 4 H2O --> Fe3O4 + 4 H2
The resulting stable oxide Fe3O4 is magnetite (ferrous ferrite or magnetic iron oxide).
It can also work like this:
3 Fe(OH)2 --> Fe3O4 + 2 H2O + H2
This is ferrous hydroxide producing the same magnetite.
Notice that both of these reactions release (small amounts of) hydrogen (into the water) when oxygen from the water is incorporated into the rust. This does not require the presence of free air. It also results in a thin and generally protective coating. This coating will be constantly breaking down and re-sealing itself, so it can eventually lead to more extensive corrosion (and accumulation of caked up crud in the bottom of the engine block water jacket). This is why you want to add corrosion inhibitors when running plain water for coolant.

Fe3O4 can also be formed into a MORE protective coating. This is an industrial process. Do not try this at home. An iron or steel part is placed in a closed chamber and heated to 1600°F, after which it is blasted with superheated steam. This results in rapid formation of both red oxide (Fe2O3) and magnetic oxide (Fe3O4). Carbon monoxide is then introduced to reduce the red oxide material to magnetic oxide, like this:
3 Fe2O3 + CO = 2 Fe3O4 + C
The whole process only takes about 20 minutes. This leaves a bit of carbon in the surface, making it very black. The resulting magnetic oxide is highly corrosion resistant. This is also known as black magnetic oxide (or just black oxide). You may recall that this was used extensively on hand tools before chrome plating was popular. It is still used fairly extensively in industry to apply a cheap and relatively corrosion resistant surface to steel parts.

The glossy black coating on "Russian Iron" is produced by laying up sheets of iron with powdered charcoal in between, and the whole mass then being heated and hammered. This is a lot of work and not very practical with high labor rates, but it does yield a nice appearance. This is not actually rust at all, but I mention it because it is similar in appearance to black oxide.

Iron and steel may also be oxide coated by electrolytic means, with the object to be coated connected as the anode in an alkali solution. This is primarily for appearance, as for black iron stove parts (hardly worth mentioning as rust).

Many rust neutralizing paint-on coatings work by chemically converting red oxide (RUST) into black oxide (if it's not too thick). Then you can paint over the stable black oxide. That may be one of the key points you like to know about restoration work.

Collect your diploma on the way out.

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