Summary for zinc plate and passivate spec Discussion
zinc plate and passivate spec: The questioner reports inconsistent zinc-plated steel finishes with a white powdery bloom in fingerprint-shaped areas and asks how to specify the coating for humid indoor service and cosmetic appearance, noting that color is not important. John Wayman also asks whether passivation and chromate conversion are the same. Ted Mooney answers that, in this zinc context, passivation does mean chromate conversion coating, though the term can mean other things and is best avoided for zinc chromates. He says preventing white blush depends heavily on proper rinsing and drying, glove use to avoid fingerprints, and minimizing time in the plating shop where humid, acidic air can attack the parts. Mooney adds that white, clear, and blue chromates have generally seemed similar to him in corrosion resistance, and if color is unimportant, the shop should be given latitude to choose the chromate they believe performs best. He notes a differing opinion exists elsewhere in the archive.
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
Need quick confidential answers? $25
Need project assistance? $100/hr.
2002
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
Need quick confidential answers? $25
Need project assistance? $100/hr.
November 2013