Topic: Help with passivating stainless steel (200/300 series)

We're a made-to-order job shop with an in-house nitric acid passivation system. We follow ASTM A967, yet when we test the parts, we always have some that are non-passive. Parts are vapor degreased first. Our nitric acid is the correct concentration and temp, parts are typically in the bath for 30 mins, we do two DI water rinses and a final ethyl alcohol rinse. We don't agitate any of the solutions nor do we rack the parts, so there is metal-to-metal contact in the baths. We don't use sodium dichromate either. Perhaps we're overlooking something? For a capable process, should we be able to passivate 100% of the parts 100% of the time?


Re: Help with passivating stainless steel (200/300 series)

Anytime you have metal-to-metal contact within your basket of parts, you are going to have significant variation in the effectiveness of your process.  Simply put, any area of your parts that do not touch the solution will not be passivated as much as an area that has constant solution exposure. 

I don't know much about passivation specifically, but usually agitation is helpful to make sure your solution (and therefore your batches) are uniform.  And as I understand it, sodium dichromate passivation baths are easier to use, but there may be some issue with using chrome in your facility or with your customer.

-Dustin Gebhardt, CEF

Advanced Manufacturing/Finishing Engineer


Sanford, NC

Ray Kremer

Re: Help with passivating stainless steel (200/300 series)

Mochs, what grade(s) of stainless are you dealing with here?