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Yellow residue on tank surface after electropolishing it
March 19, 2013
in my company we use a lot on stainless steel tanks.
Previously we used mechanical polishing to resurface the tanks, but we had some problem with left over polishing compound (black or red residues leeching out of the tanks following autoclaving, the same color of the polishing compound) and we started electropolishing some of the tanks.
1- One electropolished tank came back from our supplier and we tested the surface with a white wipe + IPA 70% and the wipe was clean.
2- We autoclaved the tank and after we performed a second test with Wipe+70% IPA and then a dark yellow residues is visible on the wipe. There is not spot on the metal and no coloration, the metal is visibly clean and spotless.
Any idea what that could be ?
The tank was autoclaved with another that presented no residues. Could the residues come from the electropolishing process ? Could it be the stainless steel that is too ''old''(~8 years)?
- Montreal, Canada
July 13, 2013
A. It sounds like you're almost there and just some fine tuning is needed. One quick fix might be to do another cold rinse after the final hot rinse after electropolishing and to use clean fresh DI H2O and do one additional hot rinse. It seems like you check the parts after e-polishing and you don't find a problem but then after the parts heat up you do have a problem. That tells me that microscopic pores could be opening up at higher temps and contaminants released at that point which would explain why you don't find contaminants at first (they are hidden in the sealed pores).
One other thing to check is to be sure that the tanks don't need to be mechanically buffed. The color that you see could be electrolyte or it could be rust. If you discover it is rust I would try mechanically buffing off a top layer and then going through the usual cleaning processes but you might have through in the towel or try a coating or surface treatment. One other possibility is to check the weld areas. Assuming there are some weld seams, there can be pores there that are bigger than pores in other areas and they can trap electrolyte. Also check to be sure the parts are cleaned right away after e-polishing and that the electrolyte is not allowed to dry, this is very important.
Feather Hollow Eng. - Stockton, California