Surface Roughness Before and After Anodizing

Question:

What change in surface finish, or roughness (Ra/Rz), can be expected when parts are anodized? S.D.

Answer:

Anodizing does affect the surface finish of the aluminum substrate. The extent of change in the surface roughness depends largely on the type of anodic process performed. Usually anodizing causes increased roughness. In preparation for anodizing, caustic etching, for example, roughens the surface to a degree depending on the etch bath conditions and time of etching. Chemical and electrolytic brightening smooth the surface. Generally speaking, the higher the anodizing voltage, the more roughening that occurs. For example, a hardcoated part anodized to a 1-2 mil anodic coating thickness and with little, or no, etching prior to anodizing will result in a Ra of 2-3 times that of the original bare metal finish. This means 2-3 times rougher after anodizing than before. In lower voltage processes such as Type II anodize with 0.3-0.4 mil coatings, the surface may be roughened by about one-third to one-half again of the original Ra value. A little used process of anodizing in oxalic acid produces parts that are much smoother than if they were anodized in sulfuric acid.

If surface roughness after anodizing is a primary concern for the parts being manufactured, you may want to design some testing to see what gives the best results for your product. The type and amount of mechanical finishing, the type of chemical cleanup prior to anodizing and the anodizing itself can be varied to give you what is most acceptable.