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Stainless Steel "rust" marks on paint
Hi Keith,
Keith Hughes wrote: Don, Bruce, While nitric acid is the historical method of passivation, citric acid had been the standard for *well* over a decade - not new technology at all. Well, my use of the term "new" is relative. A decade takes us back to 1997, and I still think of that as relatively "new". For any given temperature and contact time, nitric works better. I'm not sure that this is true... I read an independent technical lab report on the quality of passivation films formed by nitric acid and citric acid when I was researching the Citrisurf. It compared the CrO2? films at a microscopic level for different passivation techniques, and concluded that the quality of passivation using citric acid was slightly better than nitric acid. IIRC the thickness of the passivation film using any method is limited to some maximum because eventually the reaction stops due to complete oxidation of the surface. The report is available on the web somewhere. I did not bookmark it because I was only trying to decide what technique to use for myself. Citric acid is used since it's far less dangerous and environmentally unfriendly. Phosphoric acid is also frequently used for ambient temp passivation. Hydrochloric acid is NOT used for passivation of stainless - ever. Neither is HF, unless you're pickling (i.e. removing significant material - etching). I use Phosphoric for pre-cleaning, rust removal and etch of regular (non-stainless) steel before painting. It works well. Hydrochloric acid (Battery acid) can be used with a DC power supply for surface passivation of aluminum. The process is called anodizing ;-) The Citrisurf material looks OK, but I have little faith in combination products that both clean and passivate. Far better to remove all oils *first* with a heavy duty surfactant (e.g. TSP), then passivate with citric (or other) acid. AFAIK Citrisurf does not claim to clean. You are supposed to do a cleaning step (cleaner/degreaser) before passivation. At least that is what I do. As has been previously discussed, mechanical polishing of non-orbital welds (prior to passivation) is still a pre-requisite for prevention of oxidation. Agreed. Don W. |
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