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Scratches after passivation of polished 316 steel

2005

We have pistons that reqire a .4 finish and citric passivation per ASTM A967. We polish the surface on a lathe and then send it out for passivation. They always come back with scratches. Could this be a result of the passivation removing the "polish" and revealing the existing scratches beneath? Or is that impossible. What would provide a "deeper" polished surface that wouldn't be completely removed by the passivation?
These parts are a part of a new hydrogen fuel system. Since hydrogen is the smallest element it needs a tightly connected surface to stop it; hence the .4 finish.

Thanks for any advice,

Frank Blakely
L & M Machining Corp - Anaheim, California, USA

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2005

Passivation is actually the deposition of a very fine and uniform layer of oxygen atoms bonded to the metal atoms on the material surface. It can actually cover up existing scratches if thick enough, rather than reveal them.

You should probably have the part electro-polished after the lathe polish to ensure there are no surface defects prior to passivation.

I would still suspect that the scratches are most likely the result of handling. It may sound obvious, but make sure that the part is immediately wrapped in fine, soft cloth after passivation and up to your delivery.

Bob McReynolds
- San Jose, California, USA