Phosphatizing Powdered Metal Parts

Q. My company makes powdered metal parts that are pressed to shape from metal powders to a density of 6.0 to 6.5 g/cc. The composition of these parts is iron, copper and carbon. Following the pressing operation, the parts are sintered in a furnace with a protective atmosphere to reduce oxidation and to retain the carbon. On a particular series of parts, the requirements are such that we machine, hardened then finish-machine to size. The parts are then manganese-phosphate coated, and it is this last operation that causes us considerable problems. The phosphate coating does not always have a dark, even appearance, but is mottled. This has been attributed to the high copper content of the powdered metal parts, but some at the plant disagree. What do you think?  S.J.

A. I agree with your assessment of the problem. The fact that manganese phosphate reacts differently with various metals accounts for the irregular appearance on the surface of your parts. Although powdered metal technology is outside my area of expertise, I believe the mottled appearance indicates a lack of homogeneity on the surface of your parts. The manganese phosphate coating should have an even, dark appearance all over the surface.­