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Zinc Plating with Black Passivation comes off during handling

June 17, 2010

Q. Dear All,
I am SQA in Auto Industry, I received from my Fastener supplier with some plating defects.
Let me explain in detail .
Zinc Plating, Black Passivation with 8~10 micron thickness.
When we handle the fasteners the black passivation is removing and the same colour was applied on hand during inspection. But not fully.


Need your support.

Praveen Kumar
Buyer - India

June 29, 2010

A. Praveen,
Please look out for a better plater.

t k mohan
T.K. Mohan
plating process supplier - Mumbai, India

November 24, 2020

thumbsdown I don't find it very helpful when someone writes "find another supplier". It would be more useful to give some suggestions specifically about the plating processes. If I wanted a simple answer I would have Googled it.

John Perilli
- Rome Italy

November 2020

thumbs up sign Hi John. You're certainly welcome to offer that opinion, but I suspect that the issue here is that you are a plater, whereas Praveen is a buyer who received from a fastener supplier some fasteners which the fastener supplier had had plated at a plating shop somewhere; and it may not be practical for a reader to suggest what questions a buyer should ask a fastener supplier to ask his plating shop to help the plating shop fix their problem.

T.K. replied to 223 questions on this site, sometimes following up very helpfully with the inquirer as many as 4 additional times. The causes of plating defects are usually quite complicated, such that we need a lot of data tto get anywhere. The chromate may have been exposed to too high a temperature too soon, but we don't know if it's a silver-based chromate or not, a trivalent chromate or not, or if that happened ... and that's only one possible idea if the chromate itself is actually the cause, whereas it could be the brighteners in the zinc bath being wrong ... and we don't know if the zinc plating was acid, cyanide, or alkaline non-cyanide, and whether there is a nitric bright dip between plating and chromating, etc.

Praveen asked for "support", and TK's response did offer some support by answering the implied question of whether the spec itself, i.e., "Zinc Plating, Black Passivation", was the problem. TK's answer was that the chromate is not supposed to easily rub off that way: the execution, not the spec, is the problem :-)

I'm not criticizing your posting, I'm recognizing & thanking Mr. Mohan for his efforts here. Please review thread 45598 for a good detailed discussion of why chromates rub off, or search the site with the term "chromate rubs off". Good luck!


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Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

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