Rinsing Parts with Entrapped Chemical Residue

Question:

We have a small, aluminum tubular part about one inch in diameter that we fabricate by, among other methods, torch-brazing a threaded extension to one end. The parts are sent out to our anodizer for MIL-A-8625, Type II, Class 2 (conventional sulfuric anodize and dyed). The returned parts exhibit a white, crystalline, acidic (sulfate) residue on the brazed portion. We removed the residue with an extended soak in a hot deionized water rinse. We thought the addition of a neutralizing soak before the hot rinse might provide neutralization of any entrapped acid. Is this a good way to go? We need a recommended neutralization method, or other process for these parts. T.A.

Answer:

I would first recommend rinsing thoroughly in a cold water rinse after the parts are anodized. Then go back and forth between a hot rinse and a cold rinse (use DI water for at least the hot one, if possible). This will cause expansion and contraction of the “pores,” or surfaces, and will most likely rinse out all of the residue. This will probably preclude any necessity to use neutralizing chemicals of any kind and it almost always works whenever entrapment is an issue. This would include crimped sheet metal, welding/brazing, small diameter bores, extruded screw “bosses,” and so forth.

One other thought. If the parts are caustic soda etched prior to anodizing, you would probably need to use the above procedure after etching as well as after anodizing. If caustic soda gets trapped in the part and it is then immersed in the anodizing tank before the residue is thoroughly rinsed, the resulting “crud” is impossible to remove chemically.