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topic 9234

Trivalent chromate coatings


I've heard a lot of positive things about using trivalent chromium coatings and trivalent plating techniques. I want to know, what are some of the negative aspects of using trivalent chromate coatings (instead of hexavalent)? What are some negative aspects of using trivalent chromium in plating instead of hexavalent? i.e. cost,performance,regulatory problems, etc.? Will trivalent chromium continue to be used in the future, or are there better alternatives?

Bruce Wayne Flint
- Columbia, Missouri, USA


There are many disadvantages to trivalent chrome conversion coatings. (I am assuming you are speaking about conversion coatings on zinc or zinc-alloy platings and not actual chrome plating) Often, tri chrome systems require an additional topcoat/sealer layer to achieve equivalent corrosion resistance. This added step adds complexity and cost. Mixing a new tank of tri chrome is a LOT more expensive than a hex chrome bath, although the tri chrome baths seem to last a lot longer. One other thing to note is that tri chrome systems do a lot better in high temperatures than hex chrome will.

Hexavalent chrome is supposed to be eliminated from the auto industry as of July 1st 2003. Some people claim that trivalent chrome will be the next to be banned in the auto industry after hexavalent chrome, but I am not entirely convinced. There are three three main valence states of chrome: hexavalent (+6), trivalent (+3), and metallic (+0). Some people do not make much of a distinction between these valence states and declare that all chrome should eventually go. However, most people don't realize that trivalent chrome is used a lot in tanning and is all over leather goods, and that trivalent chrome is even a necessary nutrient found in vitamins. It may or may not be eventually restricted.

tim neveau
Tim Neveau
Rochester Hills, Michigan

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