Ask the Expert Question-and-Answer Archive

by Ron Joseph

February, 2005

Corrosion of Dissimilar Metals

Q. We have a heatsink made out of copper that has a bright dip chromate on it. It is being mounted into a steel chassis that is zinc chromate, and secured with zinc plated steel screws. The steel and copper are both showing corrosion. How can this be stopped? Should the two parts be mounted directly to each other, or should thin gasket be used with stainless steel screws?

A. Since I haven't seen your assembly, I can't give you a definitive answer. However, in general if two dissimilar metals are in contact with each other, you are almost always advised to separate them with a gasket or other dielectric to prevent the flow of an electric corrosion current between them. Stainless steel screws might be fine, provided that they do not make electrical contact with the steel or copper. Remember, stainless steel behaves much like a noble metal (similar to copper) and while its contact with copper might be OK, contact with steel can be expected to accelerate corrosion.

Before you take this advice, I strongly suggest that you conduct some basic experiments in which you subject the assembly to salt spray (salt fog) testing per ASTM B117 to determine if the problem has been solved.

Best Wishes,

Ron Joseph