Letter 36024

Resistivity of Hexavalent Vs Trivalent Chromate [Massachusetts] 

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We are a supplier of switches using Zinc Die cast housings to the automotive industry. Over the past year we have been changing from Yellow Hexevalent chromate conversion to a clear Trivalent Chromate conversion on the Zinc housings. I recently change a housing to the Trivalent Chromate on a switch which is grounded through the Zinc housing. Aside from the Trivalent Vs Hexevalent change, our supplier is now using a Zinc preplate under the Trivalent conversion to improve corrosion resistance. The switches using the new Trivalent conversion appear to have a higher resistivity than the old yellow Hexevalent conversion without the Zinc preplate. Is there a difference in the conductivity of the Hexevalent chromate Vs the Trivalent Chromate?

Thank you for your time and assistance in this matter.

Raymond V Dart Jr
Low voltage automotive switch and sensor supplier - Canton, Massachusetts, USA

You need to specify your needs carefully, Raymond. In general, trivalent chromate per se does not offer the corrosion resistance that hexavalent chromate offers. So there is a tendency for shops, if left to their own devices, to apply a silicate or other topcoat or a water soluble lacquer on top of the chromate. In many applications, but not all, this has no downside.

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

Bingo Ted! Sealers are becoming too widespread without OEM's and specification organizations taking this into account. While the auto world loves them, they create fits for anyone in the electronics field that believes they're getting the same old surface conductivity. Platers could use common sense too, but then how is the plater supposed to know that increased resistivity might be a concern - platers have trouble even getting steel hardness out of manufacturers despite baking guidelines that clearly require them. It all comes down to communications and clearly defined expectations.

Milt Stevenson, Jr. Anoplate Corp Syracuse, NY, USA

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