Does Trivalent Chromate really turn to Hexavalent Chrome during Salt Spray Testing?

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An article in the, October 2007 Plating and Surface Finishing magazine, by Tom Rochester, presented an interesting theory as to what happens to a trivalent chromate during the salt spray test ASTM B-117.

Testing with diphenyl carbizide (DC) prior to salt spray indicates that there is no hexavalent chrome on the zinc plated steel part that has a trivalent chromate on top of the zinc plate. After salt spray a positive result using the (DC) occurs. EPi tested some of its trivalent chromates and found this to be true as well. So what is causing this phenomenon? I am not sure, but I wanted to see if there was another test we could perform to check for the hexavalent chrome test after salt spray.

We had our three trivalent chromates E-Chrome Ultra Blue, Ultra Yellow, and Super Yellow tested and verified that there was not any hexavalent chrome in the coating. We used Technimet our metallurgical lab to conduct these tests and we found out that they utilized an ISO 3613:2000(E) section 5.6 in verifying that all of our trivalent chromates did not contain any hexavalent chrome. This test does not use the indicator test (DC).  To save time with this blog entry please contact us or Technimet if you want the exact test procedure. In fact there is a another ISO test you can perform for adhesion ISO 3613:2000 (E) section 5.9.

I will be discussing more about these facts during my paper in Trivalent Yellow Chromates at Sur/Fin 2008 in Indianapolis in June. To sign up for the conference go to