Passivation / Electropolish -- Proper Sequence?

A. Hi Lyndon. Compliance with specs is probably the only reason to both passivate and electropolish, since electropolishing is believed to offer the best known passivation.

silly :-) So I'd do the best finish as the final, and the 'just to comply with the spec' process first :-)

A. Hello Lyndon!

I can't see the point of passivation before electropolishing. Regarding the passivating effect of electropolishing, take a look at specification ASTM B912, on "Passivation of Stainless Steels Using Electropolishing". It notes that a film from electropolishing should be removed before you run your passivation verification test. If you do a post electropolish film removal dip, you may be fine stopping there.

Good luck!

A. Lyndon:

There is no logic to passivation before electropolishing, since you would remove the passive layer in the EP process. Electropolishing normally provides a good corrosion resistant surface. However, if you want the BEST corrosion resistance and the highest chrome oxide layer on the surface, you should passivate after electropolishing. We have data showing considerable improvement in chrome oxide content on the surface using this process with our citric acid formulations.

A. The reason for passivating either before or after electro-polishing is that the spec. says so. There is no engineering reason for this. Electro-polishing will provide the best passivation possible.

The film left by electropolishing is the passive film and this should NOT be removed.

Proof of the passivating effects of the electro-polish have been confirmed by ESCA and plasma stripping of the surface.

This mistake is not uncommon. I receive parts to be passivated and pickled or passivated and electro-polished every week. Usually a brief discussion with the engineer responsible for the spec clears things up and the expectations of the customer are satisfied without going through a dual step process.

A. Hi Mike. Have you done any examination of those "white marks" to see what they might be? Might they be areas of etching caused by problems like insufficient current density or excess water in the solution?

Regards,

A. Hi Mike. I think I'd be amazed if passivation fixed that problem. To me, that surface doesn't look even vaguely like an electropolished stainless steel surface should. Please tell us what the part is, what grade and form of stainless steel it's made from, and why it is supposed to be electropolished.

And if you could put a coin or pencil or tape measure or something into the picture for a sense of scale and resolution, it would probably help because I'm not sure whether this is a 'visual' scale photo of a rough sparkly surface, or an extreme closeup where you are concerned with the ghost-like drippy areas. Thanks.

Regards,

A. Mike,
Any other surface treatment before electropolishing? Castings don't have a great surface to begin with, it's usually good to sandblast/beadblast, or maybe for parts your size something more like tumbling.