Q. The aluminum cathode bars in our Type II anodizing tank have become completely encrusted with white “salts” at the solution/air interface. I would normally think that this was a build-up of aluminum sulfate, but I thought this only happened when the concentration of dissolved aluminum in the bath was higher than normal. The aluminum in this bath is less than 2 g/L. What do you think is going on? J.M.
A. I wonder how often this anodizing tank is used. Or how vigorous the air agitation is. Even at low aluminum concentrations, if the solution evaporates, it will leave a deposit of aluminum sulfate salts around the tops of the cathode bars. It is perhaps the same as when drag-out acid drips on the sides of the anodizing tank and the ventilation hood. As the water in the acid evaporates it leaves behind a white deposit of aluminum sulfate.
An additional possibility, especially if the tank is idle for periods of time, is that the aluminum concentration very near the aluminum cathodes could become higher than in the bath as a whole. Then as the solution evaporates, the salts are more readily formed at the interface.
This may not be the correct answer, but it’s the best possibility I can think of.