Excuse me if my chemistry is a little out it's been a few years but...

When you join any to dissimilar metals together you are essentially
creating a battery. The rate of corrosion will be based on a number of
factors probably the most important of which a

1) The ion transfer potential of the electrolyte (e.g. what ever is
between your two bits of metal). If you get say salt water between them
the rate of corrosion will be much greater than if you get deionized
water between them
2) How good the electrical connection is between the two materials
3) The difference in the electronegativity of the two materials. This
gives you an idea of how readily they will transfer charge in an ideal
situation. http://en.wikipedia.org/wiki/Electronegativity

Assuming that you have the two metals bolted to one another you have
essentially a perfect electrical connection. If you put two metal faces
together you will have some sort of electrolyte between then no matter
how much you clean them (even if it looks clean to you there is probably
a mono-layer of water on the metal surface). Although after thorough
cleaning whatever is left will be a very poor electrolyte. Aluminium
has an electronegativity of 1.5 and Iron 1.8. So the difference, 0.3,
isn't that great (you couldn't realistically use it as a battery)
meaning that corrosion will probably be quite slow.

The ways to stop galvanic corrosion are therefore to interrupt one of
the three things listed above that cause corrosion. Probably the easiest
if you really want to have Al and Fe in the same piece is to separate
them with rubber washers.

Graham


Anna Kettle wrote:
On Fri, 25 Feb 2005 18:01:00 +0000, The Natural Philosopher
wrote:


No, corrosion mainly.


Can you explain a bit more? Corrosion as in rusting ? - so only if
there is water present - and aluminium doesn't rust just gets a
coating of protective aluminium oxide doesn't it?

I'm very hazy on all this for which I blame my Dad. He was a chemist
by profession so there was NO WAY I was going to study chemistry at
school with the result that I have no idea what I am talking about

Anna

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