Summary for white blemishes on cobalt chromium Discussion
White blemishes/ spots on nitric passivated cobalt chromium alloy were discussed as a likely surface-preparation or etching issue rather than a mysterious defect. Darren Harris asked about small white blemishes appearing after nitric passivation of highly polished cobalt chromium parts. James Watts said that, since the process details and exact alloy were unknown, the best guess was polish that had “burned” into the surface; he asked whether the parts were solvent cleaned or electrocleaned to remove all traces of polish. In a later thread, Hubert McGuire described similar white/foggy spots on ASTM F75 CoCr parts after phosphoric acid, potassium hydroxide, and nitric acid processing, noting they sometimes subside over time. The reply suggested the first suspect was localized etching, which could be checked under 30x or 40x magnification, and recommended considering citric-acid-based passivation. It also noted that for a non-ferrous alloy, passivation may be unnecessary unless iron contamination is actually a concern.
Q. Hello,
I am passivating highly polished cobalt chromium components in nitric acid. After the passivation process, the highly polishes surfaces have small white blemishes and spots which need to be sent back for rework. Can anybody tell me as to what these white spots might be and how I might solve this problem.
All responses are welcome.
Darren Harris
Engineer – Ireland
February 11, 2009
A. Since we know nothing of your make up and process conditions, or the exact metal, the best most can do is take wild guesses.
Since some turn out OK, I would look to polish that has “burned” into the surface. Do you solvent clean or electroclean or neither, to remove all traces of polish on/in the surface?
James Watts
– Navarre, Florida
Q. Hello,
I am degreasing/passivating highly polished ASTM F75 CoCr Alloy components in phosphoric acid, potassium hydroxide & nitric acid. After passivation the highly polished surfaces sometimes have small white/foggy blemishes and spots and they have to be re-polished.
They will show up for a period of time and then subside.Can anybody tell me as to what these white spots might be and how I might solve this problem.
All responses are welcome.
Hubert McGuire
student – New York
February 18, 2015
A. Hubert,
The first suspect would be that it’s just etching your surface in places. This can likely be checked with 30x or 40x magnification. I would look into changing over to citric acid based passivation. Though since this is a non-ferrous alloy, you could also check to see if iron contamination is genuinely an issue on these parts. If it is not, there is no need to passivate at all.
Ray Kremer
Stellar Solutions, Inc.
McHenry, Illinois
Q. Thanks for your reply,
What would cause the part to be etched in certain places and not in others? also if it helps the spots are sometimes blurred while other times very bright and form a very defined circle.
My initial guess was that it was to do with adhesive particles from polishing reacting with something in one of the baths, is this a possibility? Also the alloy is cast in house, could inclusions be a factor?
Hubert McGuire [returning]
UCD – New York
February 19, 2015