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Zinc Plate and Passivate spec

Q. We are having some difficulty in achieving consistent finish on our Zinc plated steel parts. Some of the parts exhibit a white powdery bloom, which appears to be located in 'fingerprint shaped' areas. Our supplier insists the parts are zinc plated and 'white passivated' using a NaCl and CrO₃ solution. He says he gets a superior corrosion resistance using a 'blue passivation' with a BaCO₄ and CrO₃ solution.

Our parts are used inside a factory environment, but sometimes in a humid atmosphere. How should I specify the plating I want to achieve, in order to prevent this bloom? I am only concerned with corrosion resistance in humid indoor environments and cosmetic appearance. The precise colour of the finish is relatively unimportant.

One other dumb question: Am I right in thinking that passivation and chromate conversion are one and the same process?

John Wayman
printing machines - Weymouth, England
2002

A. Yes, 'passivation' means the same thing as chromate conversion coating in this context. However, passivation can mean several other things, so most metal finishers would probably prefer that you not use 'passivation' as a synonym for chromate conversion coating of zinc plated articles.

Proper rinsing and drying at the time of plating is an important component in preventing this white blush. Also the operators should wear gloves to avoid finger printing. Avoiding prolonged storage at the plating shop is vital because of the humid and acidic air in many plating shops.

White, clear, and blue chromates have meant pretty much the same thing to me and in my experience were always considered one and the same in terms of corrosion resistance. However, my very knowledgeable friend Khozem Vahaanwala expresses a different opinion in letter 46866.

If color is unimportant to you then don't specify it; if the plating shop feels one chromate is better than the other, allow them the latitude to help you decide.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2002

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Q. What is the difference between white & blue passivation

Surender kumar
- Gurgaon ,Haryana, India

A. Hi Surender. White, blue, and clear aren't specs, they're opinions and judgement calls. If you can explain your situation (attempting to specify, involved in contract dispute, attempting to formulate), we may be able to help.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
November 2013