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440C problem with passivation

We have passivated 440C in the past and have not had problems, but the last time we did it the parts turn black and look like they were etch. We did it to type II per QQ-P-35. Temp was 125 °F. time was 20 mins any reason that could make the parts turn out like this?

Ed Dunn
- Chatsworth, California
June 29, 2009

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June 30, 2009

Hi, Ed. This is an issue that has periodically confounded metal finishers for years. The two most likely scenarios seem to be, first, that the metal is not what was claimed or, at least, if it complies with the required analysis, it still isn't right (segregated gains or something along those lines). Or, second, that the metal was not truly 100 percent clean (one theory is that even a small amount of oil can react with the nitric acid locally to form an aggressive organic acid that etches the parts.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

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One other possibility is that the parts that turned black were not hardened. Did you check the hardness?

Lee Kremer
Stellar Solutions, Inc.

McHenry, Illinois

July 15, 2009

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It was because of problems like this (which we experienced more on 416, but occasionally on 440C) that we replaced our Type II bath with Type VIII. It has worked well, and we've not had etching problems since.

Lee Gearhart
metallurgist - E. Aurora, New York
July 22, 2009