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topic 5857

Stainless steel passivation: 316L and 440C

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Q. Can anyone tell me what concentration of Nitric Acid I need to passivate a 316L tank

Alison Hartopp
- Manchester, UK


A. Hello Alison,

Both ASTM A967 [link by ed. to spec at TechStreet] and AMS- AMS-QQ-P-35 [link by ed. to spec at TechStreet] (soon to be replaced by AMS2700 [link by ed. to spec at TechStreet]) list two formulations as right and proper for 316L. Nitric 2 (or Type VI) of the ASTM specification uses 25-45% nitric acid in water, at 70-90F, for 30 minutes minimum. Nitric 3 (or Type VII) uses 20-25% nitric acid at 120-150F, for 20 minutes minimum. Hope this helps!

lee gearhart
Lee Gearhart


A. Dear Alison: You can also use excellent formulations based upon citric acid to get improved chrome oxide ratios on the surface of 316L. The formulations are spelled out in ASTM A967 [link by ed. to spec at TechStreet] and AMS 2700.

Contact us if you are interested in this.

lee kremer
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Lee Kremer
Stellar Solutions, Inc.
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McHenry, Illinois
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Q. As we are doing Bright passivation in a heat treated stainless steel (440c material to a hardness of 54 to 58 HRc), our problem is on the discoloration after passivation. Can anyone help over this to find the root cause of the discoloration?

Senthil Kumar.m



There are some who say that you cannot passivate 440C. It is difficult for sure, but you can make it corrosion resistant if you do it right. We suggest that you soak the parts in warm 5% NaOH solution before passivation. This should help. It would be better to go to 60-65 Rockwell with 440C.

Let us know if we can help.

lee kremer
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Lee Kremer
Stellar Solutions, Inc.
McHenry, Illinois

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