I don't want to beat a dead horse, and this is just a follow-up. . . There has been an interesting exchange about stainless steel, corrosion, rust and maintenance issues. I learned a few things, one of the most interesting being that the salinity of ocean water increases closer to shore. That's fascinating for many reasons, not least of which is the relevant corrosion, but also the fact that desalinators may better be applied further out (for other reasons, as well). A couple of questions that came up seem to have partial answers in a source I have, which I finally got out and looked at. The source is the "Boatowners Mechanical and Electrical Manual" by Nigel Calder. Among sailing types, this book is a sort of bible. The edition I have is 1996. Calder dedicates two pages (136-37) to the question "Is Stainless Steel Really Stainless." Calder states that "there are more than 500 different grades of stainless steel. . . " The grades relate to the relative combinations of iron, chromium and nickel. In the presence of oxygen, the chromium helps create a "skin" around the iron, which greatly increases corrosion resistance. There must be at least 12% chromium for this to happen, and then the ss is referred to as "passivated." Most "better" ss has at least 20% chromium. The addition of nickel enhances corrosion resistance. Product without any (or very little) nickel is known as "ferritic" or "martensitic" ss. The addition of nickel results in what is called a "austenitic" ss. Austenitic is non-magnetic; the others react to a magnet (thus, easy testing can be done). According to Calder, only austenitic ss is proper for a marine environment. He explains that non-austenitic ss is "all too often found on boats, steadily rusting away" -- especially on smaller fasteners (he describes undoing a fastener and finding corrosion, and discusses crevice corrosion). Austenitic ss is usually categorized in the 300 numbers (eg, 304, 316); ferritic and martensitic are in the 400's. Interestingly, Calder says that 304 is not the best metal for marine use. I say interestingly, because I see it used all the time. (304 is sometimes called 18-8, because it is 18% chromium and 8% nickel.) He says 316 is better, as it has a higher level of nickel, *and* the addition of a 2% quantity of molybdenum. Calder concludes by explaining why/when corrosion might occur in ss. Without repeating the details here, the general idea is that the surface layer created mainly by the chromium becomes compromised. He suggests using only 316 (but doesn't actually say that 316 *will not* corrode). He also suggests polishing the surface of ss and ensuring that fasteners are properly bedded in a waterproof sealant. For this, he suggests 3M 5200. He expressly states that silicone should *not* be used, because it is "minutely porous." (Interestingly, I just checked the West Marine catalog blurb on sealants, and it says silicone is an "excellent insulation barrier between dissimilar metals." I think I'll stick with Calder.) One further note. Calder devotes a paragraph to the question of rusting of ss welds (someone raised this earlier). His explanation is that by welding the ss, you remove the "chromium from its passivating film-forming role." There are ways to avoid the problem, but they don't sound very practical. Anyway, I apologize if I bored you. I've been meaning to check Calder since this issue came up, and since I got myself a little education this morning I thought I'd share it with the group. No earth-shattering conclusions, just a bit more information. Mark Lane *************************************************************************** PaddleWise Paddling Mailing List - All postings copyright the author and not to be reproduced outside PaddleWise without author's permission Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.paddlewise.net/ ***************************************************************************Received on Thu Apr 13 2000 - 06:31:47 PDT
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