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Platinum aluminide plating

I am a Process Engineer at a R&O station for aerospace engine parts. One of the coatings we do is platinum aluminide. Recently the platinum has small white spots appearing in the plate in the last 15 minutes of plating. The plating is very thick but we have never observed this phenomena before. After diffusion the white spots spread to a star like appearance. If the are allowed to remain (usually we blend and brush plate) after the aluminide portion of the coating, the coating will delaminate in those areas. We only get the spots on a certain part that is close to MAR-M 247 material. The manufacturer of the platinum solutions have suggested several corrections but nothing seems to improve this problem. Luckily it does not appear on any other parts. Any ideas? Analyzing the spots under SEM show only platinum and aluminum. Thank you in advance for any assistance you may give.

Cheryl Smith
aerospace overhaul
1996

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Cheryl, are you using the black (sulfuric/nitric) Pt plating solution or the Clear (Chloride) one? I have found that the black one is inherently unstable and the pH will dive below zero due to excessive acid generation at the anode. When this occurs to excess, I noticed microscopic white spots, along with a reduction in Cathode efficiency even though Pt content was constant. Correcting pH with KOH solved this pbm but the acid generation was so excessive that I soon reached the solubility limit of the potash! Since Plating temperature is extremely hot, I was able to control pH by freezing out the spent potash overnight and replacing new KOH in the morning. The Pt in the spent potash salts was recoverable. This was economically preferable to a feed & bleed system on the bath replenishing with fresh solution, which was the only other solution to this pbm, as it generated more gms of waste Pt per amp-hr.
Hope this helps.
Dave

Dave Kinghorn
Chemical Engineer
SUNNYvale, California
1998

I have experienced problems like this before. We paid a university to run XPS studies underneath some blisters that appeared after diffusion heat treating. They discovered aluminum which is a component in Nickel based superalloys. We found the best solution was an aggressive grit blast but on some turbine parts the sidewall thickness is important which limits the use of grit blast. Good luck.

Dave Fairbourn
- Sandy, Utah
1998