48390

Citric Passivation of 440C stainless steel [UK] 

April 3, 2008

I have a very strange problem that I cannot solve and I would appreciate any comments or feedback :-

I but a 440C stainless steel ball that has been citric passivated.
The ball is boil tested , in boiling water for 4 hours. The water strength is 12 ohms. the balls are always 2" below surface of water to prevent any oxidisation. These parts never fail.

My customer then repeats the test using 18 ohms water (all else the same) , but we then have isolated marks on the surface.

These marks look like small rusts spots , by when SEM is done there is no Iron ???

Does anyone have any ideas or comments ?

I am really desperate for help or comments.

Nick Price
Customer of plated products - Pelsall , West Mids , UK

Of course, it is impossible to have "rust" without iron, so if the SEM shows no iron, it could possibly be a sulfide bloom. I have not seen this with 440C, but I do not know what else it would be IF in fact there is no iron in the spot.

My first thought, given all of the facts, is that it is an inclusion on the surface.

Lee Kremer Stellar Solutions, Inc. Algonquin, IL, USA