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Black stain after passivation, brown spots after stress relief on stainless steel

Hello everybody,
Sometimes we have brownish spots on 15-5 stainless steel after stress relief process and sometimes the same part has dark/black stains after passivation process, in other words these stains/spots are located all over the part when we received them from Stress relieve/passivisation process.
I checked out our internal processes like cleaning process, mfg process, and packaging process and also I have checked the passivation process which has done outside the company. But nothing has been found?

I would appreciate if you guys could assist me with this problem.

Thank you

Ali Zarrin
product designer - Valencia, California, USA
September 28, 2011

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