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Pitting of aluminum parts when stripping and rechroming bores

Q. My name is Marshal Rasmussen from Chicago Il. USA. At work we inspect forged aluminum parts and remove and rechrome bores and mounting faces which are hard chromed and ground to +- .002 in. tolerances. We inspect the bare aluminum surfaces of the parts for any physical damage or corrosion before sending the parts out to be stripped of the hard chrome to be replaced and then have the stripped area rechromed and ground. The parts have any pitting removed before stripping and if pits are present before rechroming and grinding. On several occasions we have received the finished parts back and some of them are pitted to such an extent the part must be scrapped. Is there an explanation for this occurrence? It seems that the removal and rechroming of structural aluminum parts is a rather tricky operation. We would like to avoid the loss of time and materials by identifying problem pieces as early as possible. This problem has also caused us difficulty in accepting a part as a core till all the work has been completed as the core value of these parts is very high. Any information someone could provide on this matter would be greatly appreciated. Right now we do not know if it is something with the parts themselves, 7075 forgings, poor quality control by the plater or simply an unavoidable risk of the process. Thank you for your help with this matter. Sincerely,

Marshal Rasmussen
- Chicago, Illinois, USA
January 2, 2012

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You left out two critical items. What is the alloy and how are you stripping it. Alloys with higher copper or zinc are hard to strip without pitting. They have to be monitored extremely closely.
Have you considered grinding the chrome out or regrinding after strip. Actually, for aluminum machining has less problems than grinding.

James Watts
- Navarre, Florida
January 5, 2012