Metal finishing Q&As since 1989

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Should 420 grade passivated stainless rust in a 24 hour salt spray test?

We have some passivated 420 stainless steel shafts that exhibit a thin rust stain on them after 24 hours of humidity salt test at 40C. Is this normal?

Ron Buchanan
Cooperate Q.C. Engineering - Blackwood, N.J., USA
February 22, 2008

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Ron, I wouldn't count on any 400 series martensitic stainless steel- 410, 416, 420, 431, 440A, B, or C- passing a salt spray test. They might pass, sometimes, depending on how well you passivated them and how well they're heat treated, but it's not something I'd count on. You'll notice ASTM A967, paragraph X1.11 notes the salt spray test "may not be applicable to all martensitic or ferritic stainless steels".

We use a humidity test, 105F, 100% relative humidity, 24 hours, as a test for our 440C parts.

Lee Gearhart
metallurgist - E. Aurora, New York
March 5, 2008

Thank you.

Ron Buchanan
- Blackwood, New Jersey
April 14, 2008

We had better luck with citric passivation on 400 series steel than when we used nitric and dichromate. 410 still passed most of the time.

James Watts
- Navarre, Florida
April 15, 2008

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April 21, 2008

I used Citric on the shafts that were mentioned. It held up great compared to the examples that were posted. I believe that we are going to go with 316 stainless.
Thanks!
Ron

Ron Buchanan
- Blackwood, New Jersey

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Dear Ron,
I suggest you to go ahead with citric acid [on eBay or Amazon] or 20% HNO3 acid cleaning of the workpiece before you heat treat, because martensitic steels as such are not recommended for salt spray test, but if you at all require it, you can go ahead with precleaning before heat treatment, also see to it that you keep the hardening temperature above 1050 degrees C with a sufficient cycle time, normally dichromate and nitric acid passivation suits better with these steels, if surface lustre is not a matter of concern for you then try this too, after complete heat treatment including tempering passivate the part with dichromate and nitric acid solution and do the second time tempering at not more than 180 degreeC temperature, you will witness amazing results. This is my experience.

Budhisagar Naik
- India
May 14, 2008