What are the effects of anodizing sharp edges or corners?

During the anodizing process, the aluminum oxide film grows perpendicular to the surface of the part. Because of this there will be less coverage on the sharp corners of parts leaving a thin spot in the anodic coating. That thin spot will then be a site of low dielectric strength and reduced corrosion resistance. In the case of parts being hard anodized, because it is a site of low electrical resistance, an increased amount of current will flow at that spot, frequently causing the part to overheat and produce a discoloration or “burn”. In some cases the temperature becomes so hot that the anodic coating and the underlying aluminum are actually dissolved in the sulfuric acid electrolyte and large amounts of material are removed from the part.

This phenomenon very rarely causes a problem in conventional anodizing. However, in the hardcoat anodizing process where power densities can approach 3000 watts per square foot, sharp corners or points can initiate a “burn” which can destroy portions of the part. Being aware of this potential problem, Alpha Metal Finishing has developed procedures and computer control technology that will minimize this possibility. However it is much better practice to design parts without sharp points or edges.

A different but equally undesirable phenomenon occurs with sharp inside corners. The two surfaces growing perpendicular to each other impinge upon each other at the corner creating a seam. The seam is not contiguous like normal anodizing and can expand and contract with temperature allowing the environment access to the base aluminum at its root. MIL-A-8625 recommends a minimum radius of .030 in. on both inside and outside corners.