Anodizing of Diecast Aluminum

Question:

Our company uses a lot of diecast aluminum parts now for agricultural water application equipment such as big gun sprinklers and valves. We generally anodize parts that are going to be in constant contact with the water in order to help protect the aluminum from fast rate corrosion. The dilemma that we run into is two-fold in that with the sulfuric acid anodizing that our vendor uses, the parts come out dark gray and a charcoal residue, which wipes off on everything that it comes into contact, is left behind on the surface. Do you have any suggestions for solving this problem? R.K.

Answer:

I will make an attempt to answer your question, although I am no expert in anodizing and your question would probably be better addressed to our aluminum finishing expert, Larry Chesterfield. A couple things to look at may be the anodizing temperature and sealing. If the anodizing is done at too high a temperature, the resultant coating may be too soft, although I am not sure if it will rub off like you describe. I would also check the seal step. That process step should “fill” and seal the porous aluminum oxide film that is formed in the anodizing tank. You should also talk to the shop doing this work and ask them to address this. They have much more experience than either of us do on this subject.