[s-cars] galvanic corrosion resulting in loose lug nuts: BS or not?

[Fred Munro]

B.S., Dave.

Galvanic corrosion involves two dissimilar metals. The less noble metal
corrodes away while the more noble metal remains relatively intact. This is
the principle behind zinc galvanizing; the less noble zinc protects the more
noble steel by corroding away to zinc salts, leaving the steel intact. Steel
is more noble than aluminium or magnesium. If the lug nuts were aluminium or
zinc in a steel hub, the mechanic might have a case. The last time I checked
lug nuts were steel in a steel hub. The several micron thick layer of
plating on the lug nuts is neither here nor there in this issue. The only
thing corrosion will do in this case is weld the lug nuts into the hub. If
he's claiming the mag wheel corroded away and loosened the nuts, it would be
obvious looking at the nut seat - instead of a smooth conical surface you'd
have a ragged hole. You'd probably have to park your car in salt water for 6
months for this to happen. Give the mechanic a smack upside the head for
forgetting to tighten the nuts. Sheesh!

Fred Munro
'94 S4
'97 S6


-----Original Message-----
From: s-car-list-bounces at audifans.com
[mailto:s-car-list-bounces at audifans.com]On Behalf Of David Cahill
Sent: October 3, 2006 5:38 PM
To: s-car-list at audifans.com
Subject: [s-cars] galvanic corrosion resulting in loose lug nuts: BS or
not?


A buddy of mine (who is not fortunate enough to have an S-Car) has a B5 A4
2.8L. After a recent alignment and tire rotation, he was driving down the
highway and noticed some serious wheel vibration. Ten seconds later, voila,
the cause is identified -- there's only ONE lug nut left in the driver's
side front wheel.

He confronted the shop about the missing lug nuts, and they had something
interesting to say. They say that they properly installed the lug nuts and
torqued them to spec, but the lug nuts likely fell out due to galvanic
corrosion.

Here's my question: has anyone heard of this happening? Is this total BS? My
gut instinct is that galvanic corrosion of the wheel wouldn't cause the lug
nuts to unscrew themselves. That, and I've simply never heard of this
happening despite the fact that there are 100,000s of Avus wheels out on the
road.

Thoughts?

TIA,
Dave
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