Letter 27043

What is the difference between Chromate and Chromium coatings? [Massachusetts] 

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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, JIS H 8625) and others that refer to Chromium (MIL-DTL-14538D, ASTM D 6492, SAE AMS-QQ-C-320 [link is to info about 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, MA, USA

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 you are looking at metallic chromium. 

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), not +3 (trivalent), but we talk constantly of trivalent chromate, which would seem to me 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 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 zinc plated parts, even if that chrome-based coating is not actually 'chromate' according to a chemist. Because of the carcinogeneity of hexavalent chromium, the use of trivalent chromates is increasing rapidly.

 
Ted Mooney, P.E.
finishing.com Inc. - Brick, NJ

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