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Burn spots on 304 SS after passivating

I recently received some passivated half round 304SS wires back from our vendor that had spots which appeared as rust. The spots looked like rust to the naked eye but under a microscope they were actually brown burn marks. The wires have been passivated using the same process for years. Not all of the wires had the spots in each processed lot and some of the lots didn't have any spots. This would lead me to believe that the problem was with the material. We checked our material certs and the same heat lot of wire material had been used for the previous two shipments which had no problem with the passivating. I'm not sure what process is being used except that it is a nitric and I don't feel that it has to be changed. I am wondering if anyone has seen this before and if the cause was identified. I'm also wondering what the best way to get rid of the spots would be. It would have to be some type of a bulk process to make it economically worth while.

Mike Olson
Process Engineer - Anoka, MN, USA
2007

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Hello MIKE,
It seem that the pasivate parts were:
not precleaned up properly (maybe some organic was leaved on),
Or some pure acid dropped on the parts and attacked them.
In conclusion: a mistake was done and nobody want to recognize it. Passivation is a controlled process aimed to protect parts agains oxidation and not to attack them.
I would like to analyze the spots to evaluate IGO.

Jose Castellanos
- Minneapolis, MN, USA
2007