Zinc Phosphate of Brazed Steel Parts

Q. We often use a zinc phosphate treatment on our steel parts (corrosion protection for oilfield parts) and have recently tried doing so on a 4140 steel part that has tungsten carbide inserts brazed to the steel with a copper-based braze alloy. Both the tungsten carbide material and the braze material surfaces are badly etched after this process. Can I assume the copper (braze) and cobalt in the tungsten carbide are being attacked?—M.B.

A. The zinc phosphate process is primarily only applicable to ferrous substrates. There are a few exceptions, but this is generally the case. I know that the zinc phosphate process is not applicable to copper-based alloys. I do not have experience with the tungsten carbide material that you mentioned but would also expect that it would not be compatible with a zinc phosphate formulation. The acidic nature of the typical bath would probably tend to attack the carbide in this alloy.