The level of sulfates and chlorides in our sodium dichromate sealing bath is pretty close to the highest recommended limit. The level of chlorides in our sulfuric acid anodizing bath is also approaching the limit. Is there a practical way to reduce these contaminants without dumping my tanks? L.C.
As you know, the recommended limit for sulfate in the chromic acid anodizing bath is 50 ppm (50 mg/L). The upper limit for chloride is 20 ppm. Excessive chlorides in the bath cause etching, and in extreme cases, pitting of the substrate during anodizing, which results in a somewhat more opaque film formation or a condition that is unacceptable (pitting). Excessive sulfates are not particularly harmful to the process, but will shorten the life of the bath. This is because reduction of hex chrome is increased at the cathode in the presence of the bisulfate anion, thus depleting the free chrome content of the bath resulting in more chromic acid having to be added.
There is a “formula” for precipitating sulfate in the bath. The addition of barium hydroxide or barium carbonate will cause barium sulfate to fall out of solution. It is said that this precipitate can be left to lie on the bottom of the tank, but I don’t know how this is possible with any air agitation of the bath. The approximate formula for this is to assume that one gram of barium hydroxide per liter of solution results in the reduction of sulfate concentration of 0.2 g/L. You might want to try this in the lab before you consider doing this to your tank. Chloride precipitates in the presence of silver nitrate to form silver chloride. This is a common way to test for the presence of chlorides in water, but I don’t recommend doing this to your bath.
So the practical answer to your question is “no.” The best way to reduce these contaminants is by decanting some, or all, of the bath and adding fresh chromic acid to the concentration you desire.