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How long for the passive film to form?

2007

How long does it take for the passive film to form on 300 series stainless steel after cleaning in accordance with ASTM A967 ? Some say eight to 24 hours. Recently, a well respected consultant indicated the film forms instantly. He surely knows more about metals than me but I can't help but believe more time would be better.

The equipment I passivate on site is located in a highly corrosive environment. It has been my reasoning that the stainless steel needs at least a little time to create the chrome oxide film before subjecting the freshly cleaned metal to corrosive materials.

Years ago a consultant argued against chelant passivation in that citric acid is not a passivator. I agreed. Chelants are great cleaners to remove free iron. Oxygen is the passivator. Still, could we encourage a stronger chrome oxide film by adding a secondary non-chlorine oxidizer rinse following chelant passivation?

Todd Turner
Industrial Cleaning - El Dorado, Ark.

Indeed, I have also seen 24 hours recommended in a couple places. It's fairly certain that the longer air exposure time the parts have, the better, but I suspect it tops out somewhere prior to 24 hours. And the cleaner the air, the better.

However, I take it the equipment in question will be passivated in place, in the corrosive environment. Sometimes these things are unavoidable, but passivation in those conditions is still better than no passivation at all.

Correct, citric acid just removes the iron, the chrome oxide is formed during the air exposure time. It's likely the same is true for most of the chrome oxide with nitric passivation as well. Exposing the freshly treated surface to an oxidizer/oxygen source (peroxide would be a fine choice) certainly couldn't hurt anything and would most likely be beneficial, yes.

Ray Kremer
Stellar Solutions, Inc.

McHenry, Illinois

2007

Hi,

Everything I've read has said 24 hours, but if you find out differently please let us know.

Thanks,

Micheline Forth
- Rincon, GA, USA

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