plating, anodizing, & finishing Q&As since 1989
I've read elsewhere on this website that di-valent chromium, is responsible for the enhanced corrosion resistance of chromated coatings. Could some one tell me where this information comes from? This sounds like news, and should definitely be included in my thesis.Ian Brooke
university - Glasgow, Scotland
As far as I know, Cr(II) or chromous ions are generally very unstable but make good reducing agents as they are easily oxidised to Cr(III). This would obviously make them good oxidising inhibitors and therefore potentially good corrosion inhibitors. Try looking at a Pourbaix Diagram for chromium and see how it will affect your system. Chromating systems are very strange and complex - there is some evidence to show metallic chromium can be deposited in the initial stages of chromate passivation, but the structure rapidly changes to a sequence of oxides and hydroxides of increasing Cr oxidation states that have a self healing effect when damaged, thereby making chromate a good protection system. Since Cr(0) has been identified (if not fully confirmed) in chromate systems, I would expect Cr(II) to also be present in very small quantities.
R&D practical scientist
Chesham, Bucks, UK
I was probably the person who started that piece of urban legend, which is a simplification of the actual work by people who know a lot more about it than me. But some mentions of that subject on this site include "Novel Chromate Coating from the University of Baltimore", and letter 10026 which references the eye-opening work on this subject by Dr. Klaus Peter Klos.
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
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