Sound technical content, curated with aloha by
Ted Mooney, P.E. RET
Pine Beach, NJ

citric acid Passivation Problems

I work for a company that designs and manufactures surgical instruments. We have recently switched to citric acid passivation for our stainless steel components and have had much success, however we have had cases of black, grainy surfaces forming on parts during the passivation process. We have tried reworking these parts, but every time they go into the acid they come out with the same black surface, even though other parts of the same grade (420SS) have gone through the same batch of acid with no problems.

We use plastic baskets with rubber matting to reduce scratching of parts, and often we use weights to keep smaller parts from moving too much. We passivate in an ultrasonic tank at approx. 55 degrees C.

Has anyone come across this problem before? What can be done to eliminate the problem?

From the description, it is probably carbon. 420 stainless is somewhat high in that element. An alkaline preclean usually alleviates the problem.

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