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smut after passivation

smut after passivation

smut after passivation

(OP)
Hello,

I have processed one part to Type 7 (20-25% nitric acid) along with a test slug of same material, 410SS. I processed 14 parts of the same material, and all parts smutted heavily. What caused this? Why did one part process normally but 14 did not? Temperature was within 5 degrees of each other. We have 2 theories. 1) The larger quantity caused a gas off and parts smutted due to some unknown impurity in the part that wouldn't gas off with the smaller quantity. 2) There is a rogue electrical current (from tank heaters or crane) that wasn't present in first instance but was present in second instance.

Thanks,

Joe

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