Passivating stainless steel causes loss of mirror polish

A. Are you passivating (pickling) with HCl or HF? An HF pickling would etch the surface and ruin any polished appearance. But, I don't know if it is needed. Passivation generally calls for Nitric Acid (ASTM A380 ) or citric acid ( ASTM A967 ). Neither should significantly change the appearance of the stainless.

Nitric/HF is generally considered pickling rather than passivating. It is useful in removing the discoloration from the weld or heat affected zone. However, I prefer a mechanical cleaning of the weld zone followed by citric passivation as a safer alternative to using Nitric/HF. I have never heard of any one using HCl to passivate Stainless Steel. The Chlorides would cause more damage than good.

Good luck on your treatment of the metal.

A. When you passivate parts you should not change the surface condition. If material and process is correct, you stated using HCl for passivating, you might want to check you spec's. Most often Nitric acid or citric acid is used and citric acid only should be used when the verifying that there is no Iron on surface.

A. To the best of my knowledge Passivation of Stainless steel is performed with a nitric or citric acid solution.HCL will definitely "etch" most Stainless and result in a duller appearance at best.

A. You definitely cannot use HCl to passivate stainless steel. As stated above, this will harm the surface more than help it. You must use either nitric acid or citric acid formulations according to ASTM A967.

A. Sounds like you need to clean or degrease your parts thoroughly before passivation, as any surface contaminants will react badly with nitric acid, and leave a dull appearance.

Good luck.

A. Joseph,
This is usually one of two things.

Less common, it is dried residue of the acid due to poor rinsing or rinsing with water that is far from clean enough. It's generally not very hard to get residue off, though.

More commonly, the surface has been etched. Can you tell me what grade of stainless these parts were, and the pH of your citric bath?

A. Joseph,
I would not expect surface etching under those conditions, unless the steel is some out-of-spec garbage from China, etc.

I would start a fresh bath, just to make sure there isn't something in there that shouldn't be, check that your rinse tanks are good clean water, double check that what you're dealing with is not residue after all, and monitor things to see if was an isolated incident or if it happens again.

A. Hi Allan. Nitric acid can certainly etch and pit stainless steel. The components in question may have originally been electropolished; theoretically, at least, they can be electropolished again. This is an electrochemical process which dissolves the asperities and high spots, turning the surface smooth again for minimum adhesion of the silver.

Regards,