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What is the difference between Chromate and Chromium coatings?


I am confused about the difference between CHROMATE and CHROMIUM as it relates to conversion coatings (hexavalent or trivalent) on zinc. There are standards that refer to Chromate (ASTM B 201, ISO 3613 [link by ed. to spec at TechStreet], JIS H 8625) and others that refer to Chromium (MIL-DTL-14538D, ASTM D 6492, SAE AMSQQC320 [link by ed. to spec at TechStreet] ). My research in this subject has not yielded any clear understanding on the differences between them. Some articles I've read seem to jump back and forth using the terms chromate and chromium almost interchangeably. Any information to help me understand the difference or proper context of those terms would be greatly appreciated.

Stewart Owen
mechanical engineer - Springfield, Massachusetts


You make a very valid point, Stewart. Those of us who have been in the metal finishing industry for a long time may see specs and articles and 'read into' them based on what we already know. And we tend to accept, without much complaint, the use of terms that are screwy or in error.

Chromium, sometimes called 'chrome' in slang, is an element. When you see a bright chrome plated finish like an auto grill or truck bumper you are looking at metallic chromium. This metallic chromium is usually plated onto the part from a bath of strong and hexavalent chromic acid (H2CrO4), although chrome metal can also be plated from a trivalent plating bath.

But there is another very common, probably even more common, use for chromium-based process solutions in a plating shop. Aluminum parts and zinc plated parts receive a "chromate conversion coating". This is not a metallic chrome plating but instead is a complex mixture of reaction products of the metal with the chromate.

If I'm remembering my high school chemistry correctly, 'chromate' should mean Cr04--. This means the chromium has to be at a valance of +6 (hexavalent), but still we talk constantly of trivalent (+3) chromate, which seems to be an oxymoron. Some platers also use the slang 'chromate' to mean a clear finish and 'dichromate' (Cr2O7--) to mean a yellow finish, but that isn't actually correct either.

What it really comes down to is that the word 'chromate' is widely used to refer to any chrome-based corrosion-retarding conversion coating that is put onto aluminum or zinc plated parts, even if that chrome-based coating is not actually 'chromate' according to a chemist. Because of the carcinogenicity of hexavalent chromium, the use of trivalent chromates is increasing rapidly.

Ted Mooney, Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

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