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George, It is my business (and I'm damn good at it) building systems that are used in ocean service that in all reality dwarf the technical challenge of your scooters. You probably interpret that as a slam, but none is intented. I don't build dive scooters, but I have designed and built many remote tethered and untethered deep ocean marine vehicles including ROVs, AUVs, manned submersibles, towed acoustic sound sources, submarine-launched probe systems, submarine systems, various oceanographic instrumentations, to name a few blah blah blah. 'Don't think for a second that a dive scooter is technically challenging to everyone. Perhaps you think your scooter is the most technically challenging ocean system in the whole wide world? I hope not; if you do then you are a fool, which I don't think you are. I've seen your scooters in their smallest detail. They are OK, simple, reliable, low cost - great for their application. 'But you could still learn a thing or two. The corrosion system I described works and has been working for years; perhaps yours does too. Fine! Let's hear it. There are many ways to solve this as well as other problems. Mine is a simple way to correct existing installations. You should really learn about people before you assume too much and get over your head, technically speaking. Take care, Doug -----Original Message----- From: George Irvine [mailto:girvine@be*.ne*] Sent: Monday, August 27, 2001 7:12 AM To: Doug Chapman; techdiver@aquanaut.com Subject: RE: Those pesky stainless screws in aluminum Bullshit - how many have YOU built? Anyone out there who has a problem with one of mine speak up. There is a far better solution and I use it. You are really too much. -----Original Message----- From: Doug Chapman [mailto:dougch@at*.ne*] Sent: Sunday, August 26, 2001 5:55 PM To: techdiver@aquanaut.com 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'. -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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