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What is Zinc Passivation

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Q. I have a customer that calls out zinc passivate as the finish on their cold rolled steel (1018 steel) on a small hat section bracket job. We do a lot of gold, clear and black zinc jobs (through outside process platers) and have never heard of zinc passivate. All of our platers are dumbfounded by this callout and have no idea what it is either. Can anyone help me to understand what this process is and possibly where I could get this done?

Thank you.

Ken Stevens
- Livermore, California


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A. Hi Ken. The process of chromate conversion coating of zinc plated parts is sometimes called "passivation". I'm quite confident that the customer is looking for zinc plating followed by chromate conversion coating; but when experienced fabricators and metal finishing shops are unfamiliar with a term, and that term is a very loose one like "zinc passivate" rather than a particular specification number, it can be a recipe for a contract dispute if you don't resolve with the customer exactly what they want. And, as you note, you do gold, clear, and black . . . whereas this call out doesn't specify the color.

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey

Zinc Plating


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A. Zinc plated steel is almost always a bright, silver colored finish. Chromate conversion coatings are applied for increased corrosion resistance. The chromate conversion coatings are available (usually) in blue, yellow, olive drab, black etc. Another name for the chromate conversion coating is passivate.

Vic Waldman
- Naugatuck, Connecticut


May 3, 2013

Q. How to specify the requirement of thickness in a drawing after passivation. Is there any standard based on the need or requirement?
Regards,

R.M.Joseph
Student - Bangalore, Karnataka, India


May 6, 2013

A. Hi, RM. Zinc plating is virtually always chromate conversion coated. In these days of RoHS requirements, you should specify trivalent chromate conversion coating and pick a color (clear, yellow, black). Chromate conversion coating "converts" the zinc to a complex mixture of zinc chromates, chromic oxides, zinc oxides, and related compounds; it's not so much a topcoat as a conversion, so it adds very little to the thickness. One easy way to look at it is that it will add less than the tolerance on your zinc plating thickness ... so you don't have to worry about specifying or making allowance for its thickness. Good luck.

Regards,

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


How thick and dense are chromate conversion coatings? How to measure or calculate?

December 5, 2014

Q. Thanks for the information about chrome-based conversion coatings.
As an end user we should be able to demonstrate analytically that the parts we use comply with legislation. Hexavalent chromium has proved difficult to quantify in terms of a %w/w content within the conversion layer. If we can measure the thickness and find the density we can use measurements according to ISO 3613.
Which brings me to ask: Is there a known value for the density of chromium passivation layers?

Any other guidance on how to measure w/w% hexavalent chromium within these layers would be appreciated too.

Thanks,
Matthew

Matthew Macariou
- Cheshire East, UK



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