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Eroded Tank Walls
Recently our chromic acid anodizing process has shown signs of pitting on the tank. The system that we use is the tank (mild steel) serves as the cathode. We have been using this process for 40 years and not till recently have we seen pitting on the tank. We have plastic bumpers around the inside of the tank to prevent the parts (cathode) from touching. Our tank is about 750 gallons; the makeup is 31/52 g/l free acid about .3 to 1.0 ph. Any insights would be appreciated. Thanks
George Hemmingerpropellers - Kent, WA, USA
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What is the chloride content of the make-up water?
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Robert H Probert Robert H Probert Technical Services  Garner, North Carolina
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I have discovered that the problem that you mentioned is actually pretty common for chromic anodizing baths where you use the tank as the cathode. Our shop did this for many years also without failure, then all of the sudden we went through 3 tanks in 3 years. We did not have chloride contamination. I would be willing to bet that the pitting you see is near the solution level and within 6 inches on either side of the corners. I think that the problem has something to do with how the tank distributes current and imperfections in the steel. Your tank will eventually fail. The ultimate solution is to install a PVC or polypropylene liner. You will then need to use steel sheet as the cathodes.
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Jon Barrows, MSF, EHSSC Chemical Process Engineer Polyfab Plastics & Supply Springfield, Missouri |
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George, Jon, or anyone else who is familiar with chromic acid anodizing.
A large facility that I am familiar with has had people burned on two occasions now while doing maintenance on their drained chromic acid tank because the cathode "flared up" after a spark or a mechanical scrape.
Perhaps the presence of aluminum fines of some sort, and/or the chromic acid acting as an oxidizer, makes this surface very ready to ignite at some particular dryness range after draining (probably the drier, the more prone, but that's supposition so far).
The most important part is trying to figure out a protocol for prevention in the future. Have you ever heard of it? Do you exercise any particular protocol to prevent it? If you haven't heard of it, let this be a heads up, it's real.
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 Ted Mooney, P.E. finishing.com Brick, New Jersey |
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It it unthinkable that the flammable metal is plated-out chromium? Although, even porous iron and chromic oxide could flare. I 'm surprized it full-density iron could do this, regardless of surface condition. I'm curious how much visible crusty deposits were on surfaces nearby. Was it near the bottom, middle, or top?
James Swenson- Los Angeles, California, USA
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"Crusty" may be as good a word as any. I would say that the surface of the cathodes had a relatively thin "crust" of foreign material, that looked about like medium to course sandpaper, with the coating perhaps no thicker than the largest "grain". I don't think there was a particular top to bottom distribution, but it was all below solution level.
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 Ted Mooney, P.E. finishing.com Brick, New Jersey |
November 29, 2008
Hi, folks. The subject of cathode material for chromic acid anodizing has come up repeatedly. So, for more insights see letters 524, 19316, 26783 and 39329.
Regards,
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Ted Mooney, P.E. finishing.com Brick, New Jersey |