Anodic Coating on Rough vs. Smooth Substrate

Question:

I am hard anodizing a 7075-T6 hot-forged part for an M-16 machine gun that has the internal cavity and some external features machined prior to anodizing. We have been sending the unit out for anodizing and recently brought the process in-house. In spite of burning some of the parts due to racking problems (lack of good contact), we are successful in meeting the abrasion and coating thickness requirements. Problem is I am not getting an even coating across the entire part. The threaded end and the drilled holes that happen to be very tightly controlled tolerances are anodizing more than the surface that is left in the as-forged condition. All of the outside surfaces except for the threads are grit blasted with 150-grit aluminum oxide to provide a matte finish. For example, I cross-sectioned the threaded portion and also the main body that has the as-forged surface. The thickness on the threads is approximately 2.3 mils while the as-forged surface is about 1.0 mil thick. The requirement is 1.0 mil + 0.2 mil. Our hunch is that the forged surface is less receptive to anodizing while the machined surfaces take to it like a duck to water. Can you tell me what is going on? B.S.

Answer:

You are correct is your assessment of the coating thickness difference. The smooth, machined surfaces will build more coating than the as-forged or roughened surfaces. I am surprised that the coating thickness difference is so great, however. One reason for this is that the rougher surfaces have more area to be coated than do the smooth surfaces. It could also have to do with the way you rack the parts and their position in the tank. Perhaps the threaded portion is closer to the cathode in the tank. If the threaded area is on the end of the part it might take more coating just because it is “sticking out” there. To be honest, I’m not sure why the internal threads are taking so much coating. Perhaps you might experiment with different racking positions and orientations in the anodizing tank. Without seeing a drawing of the part, or the part itself, it is difficult to be certain that this would work. Try different positions and see what happens.