Letter 42010

Electrode corrosion in anodizing bath [Greece] 

August 25, 2006

After a period of two weeks off (as a service period), i found out that in one of the two anodizing baths, the cathode electrodes were oxidized and dissolved. The concentration was 120g/l H2SO4 and 4-4.5g/l Al. The working concendration is 180-190g/l H2SO4 and 5-5.5g/l Al. I'm thinking of a ÄE change or short-circuit at the rectifier or both. There was no positive work electrode (aluminium) attached to the bath, but there was a water leak into the bath from the positive head's cooling spiral. what do you think happened?

Thanks

Vasilis K. Makkas
EUROPA PROFIL ALUMINIUM - Athens, Greece

Hello Vasilas. Are you using aluminum cathodes or lead cathodes (or something else)? Sorry, I don't know what you mean by "AE change". Why do you call the cooling spiral, "the positive head's". Is this cooling coil metallic, and is it not electrically isolated?

 
Ted Mooney, P.E.
finishing.com Inc. - Brick, NJ

Hello Ted,
The electrodes are made from aluminum, and I did not wrote AE but potential difference(wrong language pack-my mistake!). At the point that the aluminum is grabed to the positive head-vise we have attached a cooling spiral because of the temperature that appears there, so I was not meant the bath's cooling suspension. Probably I had to make it clear from the beginning. I'm not sure but I thing that there is an electrical isolation for the cooling coil that you mention-at least THERE has to be!

Thank you very much for your time and I hope that I made you understand of the parameters

Vasilis K. Makkas
EUROPA PROFIL - Athens, Greece

I strongly suspect that an electrical leakage current was the problem. It is possible that although the cathode is negative when the line is running, when there is no work in the tank the cathode is actually positive to a cooling coil or some other item in the tank. I don't think I'd suspect any chemical leaking from the cooling system to corrode aluminum.

Ted Mooney, P.E.
finishing.com Inc. - Brick, NJ

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