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Black Oxide versus Black Chromate Conversion Coating

Q. Hello. The company I work for is an OEM of machines for the packaging industry. Specifically, I do mechanical design for can labeling machines. Many of the parts I design and deal with on a daily basis receive a black oxide coating. Recently, some of the other product lines at my company (liquid filling machines, bottle capping machines, etc.) have begun to use a "black chromate conversion coating" on many of their parts that used to receive a black oxide coating. I have been hard pressed to find any sort of information on the characteristics of this new coating or some comparison between it and black oxide. Does anyone have any experience with this coating or know how it stacks up against black oxide? Any info would be helpful, as I am being urged by my purchasing department to make the switch myself.

Thanks!

Grant Alberts
OEM Packaging Machinery - Akron, Ohio, USA
2006

A. It is not possible to put a chromate conversion coating directly on steel, so the components in question must be zinc (or zinc-iron) plated first and then chromate conversion coated. The coating will be substantially more corrosion resistant, but more expensive too. And there is one more issue: some say that consistently good looking black parts are not yet available from trivalent chromates. So if you believe that assessment, then your good looking corrosion resistant zinc plated and black chromate conversion coated products will not be RoHS compatible.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2006

Q. Can you touch up Black Chromate with Black Oxide?

Don Keddie
Defense/Aerospace - Chesterfield, Michigan USA
June 25, 2014

A. Hi Don. Black oxide is a high temperature process which would destroy the zinc plating and chromate, so no. Cold blackening (a selenium based process) could perhaps be used although I personally have never heard of it. But using a real chromate touch up kit would be better and probably would comply with many specs, whereas the cold blackening would not. The cheap and dirty, non-compliant, decorative-only, repair is simply a black indelible marker.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
June 2014

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