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When I was overhauling deep ocean transponders, we used an anti-seize compound we called "baby shit" since it looked like the final result of feeding an infant spinach. I was told the stuff had some nasty compounds in it (These were the days before MSDS) and the vendors (Channel Industries and Amatek-Straza for the most part) used a zinc grease to get around the hazmat problem. Either worked quite well for equipment we were into often. On larger equipment, Helicoils were used for strength when working with aluminum assemblies. Zincing the crap out of the gear and using the baby shit was relied on to fight galvanic corrosion. I didn't know double Helicoils existed until I got that job. If double Helicoils couldn't fix it, it was time to get the guys with the TIG torches involved. One of the more creative ways around the bimetal problem was to use Delrin bushings to keep SS nuts, bolts, and washers completely isolated from the aluminum. It was a pity that technique wasn't practical very often. It worked like a charm. Perhaps someone on the list has a line on "baby shit." Zinc grease shouldn't be too hard to come by. The adhesive idea has some problems since it takes very little contact area to create a lot of corrosion. Don ----- Original Message ----- From: Doug Chapman <dougch@at*.ne*> To: <techdiver@aquanaut.com> Sent: 26 August, 2001 17:54 Subject: Those pesky stainless screws in aluminum > 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'. > ---------------------------------------------------- NetZero Platinum Sign Up Today - Only $9.95 per month! http://my.netzero.net/s/signup?r=platinum&refcd=PT97 -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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