On Sun, 07 Dec 2008 15:39:36 -0700, Joe Pfeiffer
wrote:

Ned Simmons writes:

On Sun, 7 Dec 2008 13:18:55 +0100, "Steve Lusardi"
wrote:

Karl,
Saving the threads, even if it were possible is a waste of time, because the
treads have become aluminum oxide and have no residual strength anyway. Next
time use anti-sieze at assembly. Most likely the fault was electrolysis, as
they were probably diecast and zinc is often alloyed with the aluminum to
promote flow in the mold. In the presence of water, the unit becomes
thousands of little batteries.

Is there something unique about diecasting alloys that make them prone
to becoming "thousands of little batteries?" Leaded red brass
(85-5-5-5; copper-tin-lead-zinc) is very durable in sea water, and is
composed of metals much more widely separated on the galvanic series
than aluminum and zinc, which are quite close to one another.

It isn't the zinc in the aluminum alloy, it's that the zinc in the
alloy is in contact with the steel bolts.

Maybe I'm misunderstanding what Steve is saying, but I don't see how a
lump of aluminum alloy and and a bolt would become "thousands of
little batteries." I took that to mean that the zinc and aluminum are
finely segregated, and not truly alloyed.

But if you've got the right interpretation, I doubt there's enough
difference between the galvanic potentials of aluminum, zinc, and
aluminum die casting alloy to make much difference in the case at
hand.

--
Ned Simmons