We (and anyone using Yakima/NW Designs foot pedals) has been screwing a stainless steel screw through threads into aluminum for well over 20 years. Maybe if it was constantly under salt water there would be a problem. I don’ t recall ever seeing any significant corrosion because of it. What corrosion that does occur in normal salt water use seems to weld the two materials together more tightly. In the early 1980’s NW Kayaks used to put a screw through 3 or 4 layers of 1/8” aluminum when mounting one type of footbrakes. Those ¼ inch screws could be a real bugger to remove if they were old. Sometime I’ve had to resort to a hacksaw to make a much deeper slot in the screw so I could use an impact wrench to bust those screws loose. We were replacing them with our system because of a broken plastic part that would get brittle with age (and was no longer available in that size) not because of anything to do with incompatibility between the metals or corrosion on them. I have seen some pretty corroded rudder fittings and parts on older kayaks used in salt water a lot. Most rudders use stainless steel screws into aluminum but that was not the areas I saw corrosion problems. They seemed more likely on the cable ends or places where aluminum fit in tightly against other aluminum. Stainless screws must be of a hard enough grade to allow cutting of the threads into them. They can get a little surface discoloration on them if kept constantly wet. The only place I usually see this surface discoloration is on the spare wing-nut we string onto the nylon hatch attachment cord. I assume the cord holds the salt and moisture in place a lot longer. This staining is not really corrosion because it doesn’t “eat” into the material but just stays on the surface in a thin layer. My understanding is that’s because the new oxide is slightly smaller than the material it was formed from. Rust (iron oxide) expands to bigger than the material it was formed from so it buckles up and flakes exposing new steel underneath to be corroded further. I thought crevice corrosion in stainless steel occurred because of electrolysis between the materials (even though they were the same material) touching each other. I’m not very sure of that though so please don’t quote me on it. I have noticed that the stainless buttons on two piece graphite paddles are much more likely to stain than the ones on fiberglass paddles. I have guessed that this is due to the carbons greater electrical conductivity but I’d be happy to hear other possibilities from those more informed on this subject. We quit using any nuts on the bow and stern U-bolt placements because some of the earliest ones we did that were used in salt water a lot were staining the boat a little below the U-bolts probably due to crevice corrosion. Matt Broze http://www.marinerkayaks.com <http://www.marinerkayaks.com/> *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************Received on Fri Nov 08 2002 - 21:51:44 PST
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