Summary for stainless passivation temperature ranges Discussion
Stainless passivation temperature ranges in ASTM A967 are questioned because no nitric method covers 90-120 °F, even though ranges exist above and below it. PT Omega asks whether the gap has a thermodynamic reason. McHenry, Illinois replies that the omission seems arbitrary and likely traces back to older QQ-P-35C requirements, whose original reasoning is probably lost. He notes that QQ-P-35B once included a Type IV range of 70-120 °F, but it was removed in the C revision; AMS2700 later restored those treatment types. He also cautions that the nitric bath types are not interchangeable, since they are intended for different stainless steel families. Finally, he points out that A967 allows users to develop their own process parameters as long as the results are acceptable.
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Q. Does anyone know the reasoning why there is no nitric method that covers between 90-120 °F as there are methods above and below this temperature range (ASTM A967)?
I have not yet dived deep into the thermodynamics of this situation, but I would expect that this temperature range would also result in a sufficiently oxidizing environment?
PT Omega
– Roanoke, Virginia
June 1, 2022
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A. PT,
It does seem rather arbitrary, doesn’t it.
One would expect that for a given bath recipe, there should actually be a range of time/temperature combinations, with less time needed at higher temperatures. You do see this given for the citric acid option.
The nitric acid options in ASTM A967 were copied over from QQ-P-35C, and the reasonings behind the specific temp/temperature choices used are, I’m sure, long forgotten.
The older revision QQ-P-35B did have a “Type IV” with a temperature range from 70-120 °F, but that was removed for the C revision. AMS2700 does have it though, having restored those treatment types that were removed in QQ-P-35’s C revision.
Though please note that the various nitric bath types are not equivalent to each other. They are each designated for different families of stainless steel grades. You can’t pick one arbitrarily because you like the temperature it uses.
Please also additionally note that A967 specifically allows the user to create a process with any parameters that produce acceptable results.
Ray Kremer
Stellar Solutions, Inc.
McHenry, Illinois
Hi “PT Omega”,
You can use that temperature range, but the temperatures mentioned in those ASTM standards are the optimal ones for each different composition of stainless steel.
There are, moreover, another different methods for passivating stainless steels with proprietary products that exceed those ASTM results, with different operation temperatures.
Best regards!
Daniel Montañés
Quality Manager – Cañuelas, Buenos Aires, Argentina