Trivalent Chromates

I normally do not write about recent papers in the technical literature. However, a paper that appeared in the October issue of Plating & Surface Finishing was of great interest and requires wider distribution.

 

The paper, T. Rochester & Z.W. Kennedy’s “Unexpected Results from Corrosion Testing of Trivalent Passivates” [Plating & Surface Finishing, 94 (10), 14 (2007)], discusses results using one of the standard tests for the presence of hexavalent chromium.

 

One of the tried and true testing methods for hexavalent chromium is the use of 1,5-diphenylcarbazide to form a colored complex with hexavalent chromium. The intensity of the color is measured colorimetrically. The colored complex does not form with trivalent chromium. (I personally performed this test many, many times early in my career.)

 

Why has the test taken on added significance?

 

Everybody should know the answer to this question! The main “drivers” are the End of Life Vehicle Directive (ELV) as well as the Restriction of Hazardous Substances Directive (RoHS) promulgated by the European Union. These directives limit the amount of hexavalent chromium that can be present in or on a component or part. Surprisingly, neither of these directives spells out a procedure for testing for hexavalent chromium in parts.

 

So much for the background. What did the authors report? Salt spray tests were performed using the standard ASTM B 117 salt spray test procedure. The 1,5-diphenylcarbohydroxide test solution was freshly prepared. The test pieces, primarily M10 machine screws and washers, were evaluated after 24 hr in the salt spray chamber. Test results were as follows:

 

Test Conditions	Test Results
1) Trivalent passivated part with a drop of test solution on surface place in salt spray chamber in a horizontal position for 24 hr	Drop of test solution turned reddish brown
2) Same test as 1, but part not passivated	Drop of test solution did not turn reddishbrown
3) Trivalent passivated part placed above a crystallizing dish and introduced into the salt spray chamber. The condensate after 24 hr was diluted and tested with the test solution	The solution turned red violet, indicating the presence of hexavalent chrome
4) Same test as 3 but part was not passivated	No color development observed

 

What do these test results mean? They seem to indicate that trivalent chromium can be oxidized to hexavalent chromium in the salt spray chamber. The implication is that parts that are indeed free of hexavalent chromium when plated may later test positive and be rejected.

 

The authors of the paper postulate what a possible mechanism might be for this behavior. The readers of this column need not worry as to what the possible mechanism might be (at least not at this moment), but they should be aware that the dash to trivalent chromium passivation may not be as simple as initially thought. Another case of “unintended consequences?”
This may also explain some of the anomalous behavior that some vendors of trivalent chromium passivation products have experienced.

 

For those readers who are members of the National Association of Metal Finishers (NASF) pick up your copy of the October issue of Plating & Surface Finishing and read the article. I’m sure we are going to have a lot more discussion on this subject in the next few months.