52508

440C problem with passivation  

June 29, 2009

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 degs F. time was 20 mins any reason that could make the parts turn out like this?

Ed Dunn
- Chatsworth, CA

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.  finishing.com Brick, New Jersey

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

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 East Aurora, NY