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Black stains on passivated stainless steel

We have passivated 6 stainless steel tanks 6 months ago. We have scrubbed them with a white cloth and a black stain came. Is this normal on stainless steel? Could this part of the passivation itself ?

William Gut
engineer - Singapore
November 16, 2010

Hi, William.

Fine metal particles usually look black, so I suppose it could be a normal tarnish. But you are making us take wild guesses unnecessarily :-)

Please tell us what spec they were passivated to (nitric or citric), what grade of stainless steel they are, and what environment they have seen for the last 6 months. Thanks!

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
November 16, 2010

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