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Stainless Steel "rust" marks on paint
On Wed, 28 Mar 2007 13:29:59 +1200, Eric Stevens
wrote: On Tue, 27 Mar 2007 06:52:12 +1000, Peter Hendra wrote: On Tue, 27 Mar 2007 20:51:00 +1200, Eric Stevens wrote: On Mon, 26 Mar 2007 17:56:36 GMT, "Gordon Wedman" wrote: "Peter Hendra" wrote in message m... On Sat, 24 Mar 2007 12:44:13 GMT, Rich Hampel wrote: Probability is that the welds are not ground flat and smooth and crevice corrosion is underway in the 'intersticies' of the weldment (poor weld technique ... not a full penetration weld). To be 'corrosion resistant' (chlorides, etc. attack stainless) all the weld must be flat /smooth by grinding and then polishing/buffing to a high mechanical mirror polish. Any 'weld-lap' or pin-hole in the weld will be a source of 'rust' .... gotta be closed and then smoothed. . This has just made sense as only one of the bolt on "feet" for the ladder suffers from the problem. I'll try it. Thanks very much to all who responded. regards Peter Hendra A friend has gone thru this with his stainless. People at his workplace that know about his sort of thing told him he needed to passivate the welds. They gave him some solution to paint on the welds. I believe this is hydrofluoric acid, perhaps a mixture with other acids. Requires very carefull application. It took away the surface shine. Don't recall the final result but I can ask him. It won't be Hydrofluoric Acid, which is one of the nastiest nasty chemicals on the face of this earth. It's much more likely to be a 50~50 mixture of Hydrochloric and Nitric acids plus, sometimes, a little Phosphoric acid. You should somewhere find an expert and read http://www.assda.asn.au/asp/index.asp?pgid=17982 Whatever you do, you don't want a strong brew and you should let time be your friend. Eric Stevens Thanks Eric, A great site and I love your phrase "let time be your friend" - poetic. May I have your permission to use it in future - yes, I'll cite you as the originator? I don't want to get flamed. I take back most of what I have written. I have just found a little booklet about stainless steels I bought some years ago from New Zealand's Department of Scientific & Industrial Research. Here is their advice: _____________________________ Pickling and passivation Acid pickling effectively removes all surface blemishes of low corrosion resistance, such as iron contamination, oxide scale, and heat tints. The process involves using an etchant (hydrofluoric acid) in the presence of an oxidising agent (nitric acid) which inhibits the etching action. A typical formulation consists of 15% nitric acid and 1-3% hydrofluoric acid, which will remove most scales on immersion of the stainless steel component for 5-30 min at ambient temperature. As mentioned previously, passivation treatment involves the use of 15-30% nitric acid on its own, to restore a passive film which may have been damaged by heat treatment or contact with harmful chemicals. The disadvantages of using concentrated chemical etchants, apart from the handling of hazardous materials, include the risk of excessive pickling at grain boundaries and hydrogen embrittlement of certain grades of stainless steels. Weld dressing and descaling are essential to enhance the corrosion resistance of welds. If immersion of the welded component in an acid mixture is not practicable, a weld-descaling paste may be used. This is an inert base material containing nitric acid and hydrofluoric acid, or, if a passivation treatment only is required, just nitric acid. These weld-dressing pastes are applied over the weld in a thin layer and then left for about 1 hour before the residues are cleaned off. All pickling and passivation chemicals applied to welds must be thoroughly rinsed off with fresh water after the appropriate residence time for the chemical cleaning is completed. Electropolishing is another batch process for post-fabrication shop cleaning and passivation of stainless steel components and smaller equipment. This technique removes heat tints on welds and surface impurities such as iron contamination, and it also imparts a highly corrosion-resistant finish with a pleasant dull-grey appearance to the treated surface. Electropolishing must be carried out by trained staff under carefully controlled conditions, because, as with the anodising process on aluminium, it is possible to produce a defective finish if the process goes out of control. ______________________________ What I said previously about hydrofluoric acid stands. Its a very nast material. Treat it with very great respect, even in the diluted form. See http://ptcl.chem.ox.ac.uk/MSDS/HY/hy...oric_acid.html and http://adm.monash.edu/ohse/assets/do...acid-draft.pdf Eric Stevens Thanks for quote. I knew that stainless reacted with oxygen to form a non-corrosive surface and that unless you had a flow of water the lack of dissolved oxygen might result in crevice corrosion but somehow had never related it to pasivating. I'll now use at least come nitric acid as an oxidizing agent. Bruce in Bangkok (brucepaigeatgmaildotcom) -- Posted via a free Usenet account from http://www.teranews.com |
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