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topic 9387



I am a student and needs some basic information or definition on the following processes:

(1) Conversion Coating

(2) Black Oxide

(3)Zinc Whiskers

What is the difference between chromate and dichromate? What is the purpose of both? Why do we use black oxide? From what I heard it does not offer an excellent corrosion protection.Why can't chrome be barrel plated.

Hey this is all I will ask from all you experts and pro for now(<;)

Thanks you,

Rishie Jhapps


Metal finishing, like any other field, has its lingo and shorthand. But sometimes a piece of lingo or shorthand only has a specific meaning within a specific context.

To a zinc plater, conversion coating means the process where chromates are applied to the zinc plated surface. To an aluminum processor, conversion coating means the process where chromates are applied to the aluminum. To a painter or powder coater, conversion coating means phosphatizing. But in general, it is a non-electrified process wherein the substrate takes part in the reaction. But, as I say, it's contextual: some people consider aluminum anodization (which is an electrolytic process) to be a conversion coating process.

Black oxide usually means an oxidation process applied to steel to give it a shiny black finish and a modicum of corrosion protection. It is performed in hot, concentrated caustic solutions containing an oxidizing agent.To circuit board platers, though, it's something totally different and involves, I believe, oxidation of the copper for better interlayer bond strength.

It is believed that metal coatings attempt to relieve internal stresses by reconstituting themselves as shoots or whiskers. This has long been a problem with tin plating. It has likewise been a known phenomena with zinc plating, but in the old days when things were bigger, zinc whiskers were not much of a problem. With miniaturization, these tiny whiskers are far bigger in proportion and are causing problems. Ted Mooney, Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey


I am also interested in Zinc Whiskers. I am having trouble with zinc possibly coming off of a raised floor in a computer room and bridging gaps in the power supply. This causes the power supply to fail. Is there any way to accelerate the growth of these zinc whiskers, or come up with a way to find them?

Paul Hilgeman
- Waukesha, Wisconsin, USA

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