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From: "Doug Chapman" <dougch@at*.ne*>
To: <techdiver@aquanaut.com>
Subject: Those pesky stainless screws in aluminum
Date: Sun, 26 Aug 2001 17:54:52 -0400
For those with problems with SS screws in aluminum (e.g. shrouds on Gavin
scooters), here's a simple solution.

One reason why SS screws in aluminum base metal is a problem, especially in
seawater, is it forms a very nice electrochemical battery. The seawater is
the electrolyte and the SS and aluminum is the cathode and anode
respectively with differing thermodynamic potentials. Why do threaded
applications show marked corrosion and how do we reduce it? Simply eliminate
the galvanic "battery" activity.

One simple solution is to drill out the threaded aluminum and thread it for
a Helicoil insert. Use a hardened SS insert (standard Helicoil SS insert).
The secret is to use a hard epoxy adhesive with low hygroscopic properties
and glue the insert into the aluminum base metal. After the adhesive has
hardened, use a thread tap, sized for the nominal threaded fastener
dimension, and clean out the excess adhesive in the threads.

The adhesive displaces the seawater/electrolyte that would otherwise be
present between the insert and the aluminum base metal. A concentration cell
will therfore be eliminated in the thread region and the corrosion between
the insert and the base metal would be minimized. The SS fastener threaded
into the glued-in insert would be at the same thermodynamic potential as the
insert and therefore no corrosion (with the possible exception of
accelerated crevice corrosion which is minimized by the 316SS composition in
the insert) would be present in the threads as a result. The only corrosion
that may form in this configuration would be a general corrosion process
which could be eliminated by using a zinc anode attached to the aluminum
base metal which sacrifically corrodes in lieu of the aluminum (i.e. the
aluminum is protected). The general corrosion would be more distributed
around the threaded fastener.

Further corrosion can be avoided by eliminating the crevices between the
fastener and the metal it is holding. Use a soft epoxy or RTV silicone to
displace the seawater in the crevices. This way the fastener can be removed
(maybe with the help of a little heat) and the corrosion in the joint
minimized.

I know it is a little more trouble than just screwing everything together,
but it works.

Take care,
Doug


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