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Passivation turns black

We've been passivating 303 by using type II passivation. Sometimes, the parts would turn greyish in a matter of 10 minutes in the type II bath. If we leave it even longer, it would turn very dark and dirty. In both case, the parts smell like sulfur. We use alkaline cleaner and reverse electro clean. We haven't been able to find the root cause of the problem.

Please help

Emilina Bautista
plating shop employee - Carson, California
August 30, 2008

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All grades of stainless steel vary considerably melt to melt, and depending on the manufacturer, but if you smell H2S on the surface after passivation then you know that you did not adequately clean the surface of the sulfides. This will cause the black color.
We do not passivate with nitric acid baths here, but with citric systems we get excellent results by treating the steel with a HOT NaOH bath prior to (and sometimes after) the passivation. This gives us excellent results. by HOT I mean 70-80 deg.C. for 15-30 minutes.
Depending on the source and the melt this can vary greatly, and some lots of steel get by with NO pretreatment. However, we always recommend hot alkaline treatment of 303. Let us know if we can help.

Lee Kremer
Stellar Solutions, Inc.

McHenry, Illinois

September 8, 2008