Corrosion of aluminum and other metals

[OSBMW@xxxxxxx]

PVETRI wrote:

<<Aluminum is usually more RESISTANT to corrosion than other metals because
there are several different kinds of protective finishes (anodizing, painting,
enamels, electroplating, etc.)  that
can be applied to aluminum.

- - -MR. SUPER ANAL
98 M3 blue/grey>>

First of all, NICE car; I drool in envy!! 

Now, having said that, aluminum corrodes SO fast that you'd think it would
dissolve in a day if it wasn't for the protective properties of its own rust. 
Let me explain (read: let me see if I remember this from Materials class)...
In any ferrous metal, when rust occurs, the portion of metal that is exposed
gives up electrons and turns into iron oxide, which is the brown/orange mess
we are all so familiar with.  As the degree of oxidation (giving up of
electrons) progresses, some of the iron oxide FALLS OFF, thereby exposing
clean, "new" metal, which in turn oxidizes again until it falls off again and
the cycle is repeated over and over until there is no metal left (in a BMW
this happens until one sees the bubbles underneath the paint and then runs in
a panic to the body shop to excise the "cancer")
Anyway...  In aluminum, which is NOT a ferrous metal, the oxidation occurs at
a much faster pace BUT the critical difference lies in that its rust, rather
than flaking off, forms a milk-like protective coating that basically can be
taken off only by abrasion or very harsh chemicals.  In other words, and in
contrast with iron and steel, "new" metal is never exposed or "put up" for
corrosion.  In practical terms aluminum is indeed non corrodible, but that is
only because its rust doesn't look like anything we are familiar with and,
most importantly, because the effects of it's rust are limited to just
spoiling its looks.  Evidence of this is the dull finish in about 1/2 of the
bumpers found on any e21, e28 and e30 out there (the other half still has a
clear protective coating to prevent that dulling)

Wow, now I can write my teachers back and tell them that I actually used some
of the info from their class in real life.

You'll be cool

Julian Osante
 <A HREF="http://members.aol.com/osbmw/";>OS BMW</A> 
BMW CCA 77760

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