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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 -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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