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Re: [EVDL] Brass & corrosion: how much is "normal"

None of the corrosion you refer to is "normal" and it is almost certainly
not brass. For sure the bolts in the transmission are not brass. It is unlikely
that the vacuum pump housing is made of brass either. You don't say what
type of pump you have. It may be made from electroplated ferrous metal, or from
aluminum that has been chromated to a golden/brassy color.

The bolts would be made from ferrous metal which would then electroplated
with either zinc, cadmium, or an alloy of tin-zinc. After electroplating the
bolts would be given a chromate coating which could look either bright and
silvery, gold in color (often mistaken for brass) or black. There are others
colors too, but those are the ones used most commonly in the automotive industry
for electroplated parts. Olive drab is sometimes used, but more seldom.

The electroplated coatings are used to keep the ferrous metal substrate from
corroding. The secondary chromate coating is used to prevent the
electroplated coating from corroding. It's a two step anti-corrosion process that works
very well under normal conditions.

If a bolt is going to be used to fasten a piece of aluminum, it HAS to be
coated with either cadmium, or tin-zinc alloy. Very often cadmium or tin-zinc
is color coded with the gold chromate. Zinc is not at all compatible with
aluminum, and for that matter neither is brass.

The chromate coatings are very thin. Polishing them would only remove the
chromate coating and maybe even the electroplated coating. Then the parts would
look bad again, but even sooner. Your best solution would be to replace the
corroded bolts and try to prevent the sulfuric acid "mist" from occurring
again.

Those are just the basics, but it should help you decide on a course of
action.

Lawson Huntley







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