H2SO4 as passivation solution for Stainless Steel

A. No, Vasilis. Sorry, phosphoric acid will not passivate stainless steel.

Regards,

A. Dear Vasilis,

First of all let me correct your question. You are asking in headline about H2SO4 which is Sulfuric Acid but at the end you are asking about Phosphoric Acid.

Refer to your question. Phosphoric Acid can be a passivator too but is not as efficient as Citric or Nitric acids. The test is how far can the acid get rid of Iron from the stainless steel surface Nitric and Citric can do it better but I know some lines which uses Phosphoric Acid as passivator to S.S.

A. For many years nitric acid has been used for this process, but now you can get better results with citric acid formulations that are available on the market; and they are safe and easy to use.

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A. Dear Vasilis,

Sulfuric acid is absolutely not usable for passivation because it attacks and dulls the Metal. The best that you can achieve with phosphoric acid on SS are the cleaning from mineral deposits. From your question a suggest you talk about an already installed line. To achieve good Passivation results either you use citric or nitric and the right treatment of the equipment before and after the Passivation is significant. We use in our company a Nitric formulation for the Passivation of SS but the most times we have to use pickling in one installed new production line because the equipment for some reasons already attacked from rust.

A. Dear Vasilis,

I want to contradict the other answers: If you use extreme high concentrated sulfuric acid, it is possible to passivate without dulling or etching. But you should use the other proposed passivation agents because they are more economical in use and easier in handling (especially rinsing).

Michael Hekli
Switzerland
2003

Good point, Michael, concentrated sulfuric acid is an oxidizing agent!

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2003

A. Concentrated H2SO4 (98%) is mainly oxidant as hot solution

George Mocanu
- Bucharest, Romania
April 16, 2020

A. I would personally much rather work around a properly exhausted 50% nitric tank than a 98% (Max strength) sulfuric tank. Parts would have to go in dry or there would be acid dilution and also a risk of etched parts from the water film. I also think that 50% nitric is easier to rinse than 98% sulfuric. It would be a "dealers choice".

A. Hi, Joseph. There are countless acids, and an infinite number of operating conditions, so asking what acids you could theoretically use under what operating conditions is probably pretty much an open question as you can see from this discussion of the potential use of sulfuric acid and phosphoric acid. But the way to get practical would probably be to get a couple of the specs and lists of recommended practices: ASTM A967 and AMS2700. Then you'll know what people actually use, and what you must use to comply with the specs you will be expected to comply with. Good luck.