The authoritative public forum
  for Metal Finishing since 1989
  

  -----

Tri and Hexavalent Chrome Plate

Please forgive my ignorance but I am new to electroplating. I have been reading in these letters about tri and hexavalent chrome plate. We are talking about the valency of the chrome species in the bath are we not? The last I knew chrome plated onto a substrate as metallic chrome with a valence state of zero.

Pat Reid
Consultant - Costa Mesa, California, USA
2005

Hello Pat. You are correct, as far as you take it, but the situation you are describing does not cover all of the possible applications of trivalent and hexavalent chromium in the metal finishing field.

You are right that electroplated chromium, whether used as a "hard chromium" wear surface or as a "decorative" nickel plus chrome plating, is deposited as a metal at a valence state of zero such that it is somewhat irrelevant from an environmental standpoint whether that item was chrome plated in a trivalent plating bath or a hexavalent plating bath.

But there is another application for chrome in the metal finishing industry that is NOT chrome plating but is even more common that chrome plating, and where the valence of the solution in the process does matter. After parts are zinc plated, the zinc is virtually always subsequently 'chromated' to prevent white rust from occurring. This involves a dip in a bath of trivalent or hexavalent chromium as a "chromate conversion process"; in this case the chrome creates a corrosion fighting gel which is largely comprised of what was in the bath, and which ships with the part and remains on the part in service. This gel will have a lot of hexavalent chromium in it if the bath was hexavalent chromium. Some other plated surfaces besides zinc are often also chromate conversion coated, and chromate conversion coating is a very common corrosion protection process for aluminum as well.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2005