At 11:11 AM 11/11/04 -0500, Dennis (BarrMark262@aol.com) 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 x 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 1600dF, 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.
Barney Gaylord
1958 MGA with an attitude
http://MGAguru.com
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