Some parts have a slightly yellow tint to them after running through out two-stage zinc phosphate line, even when we lower the free iron. Do you have any ideas what this could be?

Q. We currently use a two-stage zinc phosphate line. All of our parts are cleaned before coming to the tank line, and we measure our total acid and free acid, as well as our free iron. Some parts have a slightly yellow tint to them after running through the line. I was told that it was because our free iron was too high, but even when we lower the free iron, some areas of our parts sometimes have a yellow/golden hue to them. Do you have any ideas what this could be? J.S.

A. The color that appears on a part is partly related to the chemistry and partly related to the surface. Chemistry creates a reaction that changes appearance. The surface reaction is affected by the condition of the metal. For example, if you have machining or grinding on some areas of the surface, that area will have a different reaction and color than the rest of the surface that is not machined. Not seeing the part or the yellow appearance, it is hard for me to pinpoint the exact cause.

However, the first thing to consider is whether the appearance affects the performance. If not, do not worry about it. Pretreatment often results in color variation across the surface. If the yellow appearance does affect the performance, you will need to do some careful analysis of the operation. Check all chemical variables with special attention to pH, immersion or dwell time and temperature. A lower pH, longer exposure time or higher temperature will affect the reaction and color of the phosphate. Do you use a chrome rinse? If so, that also can make the surface turn yellow/gold. Check with your suppliers on what the parts should look like, and get their input.