Chromic Acid Anodizing and Salt Spray Testing
We do Type III (hardcoat) anodizing. Occasionally, we get parts to anodize that are made from 7075-T6 alloy. When these parts are anodized they sometimes exhibit what looks like blisters in the anodic coating. What causes this to occur and how can the condition be eliminated? Our bath is 175-g/liter sulfuric, 6-8-g/liter aluminum. We use a proprietary anodizing additive and operate the bath at 40F. M.H.
This is an interesting phenomenon. I know anodizers who have lots of experience with 7075-T6 alloy but have never run into this blistering condition. I, in fact, had never seen it first hand until I received the sample from you, and I've anodized a lot of 7075. You can be somewhat relieved that the problem lies not in your process, but is a metallurgical condition.
Various descriptions of this condition that I have read go something like this. It's all about dissolved gases found in aluminum billet. Since hydrogen gas is soluble in aluminum, it sometimes precipitates out of solution in the process of being converted from the ingot to heat treated billet. The hydrogen may come from excess humidity present during heat treatment, the presence of moisture in the casting furnace or in the aluminum “charge” used in billet casting. The precipitated hydrogen forms pores in the aluminum billet. When extruded, these pores collapse, but instead of “healing” they form an interface between the flattened pore or void and the metal around it. As the anodic coating grows inward during anodizing, these interface areas become potential sites for blistering. When the coating reaches an interface it can actually separate from the substrate, forming a blister.
If this problem occurs consistently with one or more part numbers or batches of metal, it could be worth the effort for the part manufacturer to change metal suppliers in hopes that it might eliminate the problem.