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Importance of pH in Passivation

July 22, 2011

Dear sir,

1) Which is important in passivation, pH or Dipping time?

2) What is the role of pH in passivation?

3) If low pH (1.8-2.0) can we reduce dipping time (20 to 30 sec)?

Darshan Vernekar
plating shop employee - Bangalore, Karnataka, India

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Hi, Darshan.

Please try your best to describe your situation rather than posing your inquiry in the abstract. I'm not sure if you are talking about passivation of stainless steel (with straight nitric acid?) or not. Many people call chromate conversion coating of zinc plating 'passivation', and some people even call phosphatization of steel 'passivation'.

The pH must be within spec because you are not pickling the stainless steel (where a more aggressive acid working for a shorter time might remove the same amount of scale), you are trying to simultaneously oxidize it . Good luck.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
June 2011

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I'm going to take a stab at this, assuming that we are talking about passivation of stainless.

Check the industry standards (ASTM A967AMS2700, even the canceled AMSQQP35 [canceled]c) and you'll see that acid baths are assigned by concentration in vol% or weight%, not by pH. pH is a measure of acidity, so in many cases you can use pH to monitor the bath. I.e. if the pH increases from its initial value, you know that the acid content of the bath has been reduced due to the treatment of parts.

Which is more important, acid concentration or immersion time? Both! There's an old comedy bit where a first time baker doubles the oven temperature in order to bake the case twice as fast and only gets a charred cinder. Likewise if you halve the temperature that cake most likely isn't going to bake at all. Both the acid concentration and the immersion time must be within recommended boundaries to produce good results. You can't sacrifice one by boosting the other. (However, with stainless passivation you CAN reduce the time required by boosting temperature. That trade-off DOES work.)

Ray Kremer
Stellar Solutions, Inc.

McHenry, Illinois

August 16, 2011