The authoritative public forum
  for Metal Finishing since 1989
-----

Tests for nitric acid re-use as passivation bath

I have been asked by one of the metallurgists in my company the following question: Does anyone know how long you can use nitric acid as a passivation bath? i.e., what are tests for its effectiveness? I refer to passivation of stainless steel - we currently use this - and we are wondering how long we can re-use the same solution and if there is a test for the "potency" of the acid? Any references to standards, articles or published material would be greatly appreciated. - Amanda

Amanda Rose
- Austin, Texas
1999

Amanda,

Do a titration with 1.0N NaOH [1N NaOH on Amazon [affil links] , 1 mL sample, 50 mL DI H20, and Phenolphlalein as the indicator. A change from clear to pink marks the endpoint. Mols of acid = mols of base. do the calculation to calculate percent acid. We use a 45% HNO3 bath that is made from a concentrate Nitric plus water. When the solution becomes grayish black its time to dump and use bath as metal stripper now.

good luck

Doug Nash
- Alpine, California
1999

Hi Amanda,

Your customers may have spec's for the passivation. The spec may limit the impurities in the solution and so limit the useful life of the bath. (Boeing) Check it out!

Regards,

Fred Mueller, CEF
- Royersford, Pennsylvania
1999

Amanda when we passivated we used iron as a measure as well as titrating the strength of the nitric. I believe is was shown in the old MIL-SPEC for passivate.

Bill Hemp
tech svc. w/ chemical supplier - Grand Rapids, Michigan
1999

Could anyone please send me a method to test for the presence of nitric acid?

Matthew Coleman
AS-Level Student - Swansea
2003