Q. We currently powder coat over anodized extrusions for some of our products. Recently, we have seen a few field failures in which the coating is being easily chipped off of the extrusion. Do you have any thoughts on what may be causing this?—S.W.
A. Anodizing is a process for converting the aluminum surface to a thicker, more corrosion-resistant aluminum oxide. Anodizing uses an acid to create an anodic film on the part. The acid may be chromic, sulfuric or phosphoric. Chromic acid and phosphoric acid anodizing are more commonly used for aluminum that will subsequently be coated with an organic coating. Chromic acid offers superior corrosion resistance but also more environmental challenges.
Anodized surfaces usually are quite porous and receptive to bonding, but, like any process, anodizing needs good, consistent control to produce reliable and consistent results. If the part is not cleaned or rinsed correctly, it can lead to unusual hardness or other surface imperfections that interfere with adhesion of the organic coating. This is true during the anodizing process and also true for the powder coater.
Some anodized surfaces do not work well with powder because they have inorganic dyes or other issues that make the surface incompatible with powder. My suggestion with anodized surfaces is to perform routine wet adhesion testing as described in AAMA 2604 (220.127.116.11) and 2605. The surface is cross-hatched and subjected to boiling demineralized water for 20 minutes. After removal, a tape pull test can be used to confirm adhesion. If this test is successful, the part should be good. If it consistently fails, the anodizing process is not compatible with powder. If it fails periodically, the anodizing process needs more precise control or the coater needs a better pretreatment process.