Black, Pitted Areas on Anodized Parts

Q. We anodize aluminum cookware, and have recently been getting parts with black pits scattered all over them. Can you tell me what this is and how it might be happening? H.R.

A. From the photos you sent, it looks like electrolytic corrosion. It may be taking place in the anodizing tank, but I cannot be certain of that. You can pinpoint where it is occurring by inspecting the parts after each tank.

Electrolytic corrosion can occur in any metal tank that is not well grounded. If your tanks are metal, they should all be welded together with a metal strap, bar or angle of some type and then welded to a metal water line that goes into the ground. If there is no buried metal water line close by, the ground strap can be welded to a steel building column or any other positive ground. If the anodizing tank is carbon steel lined with lead it is definitely grounded since the cathode is the ground. Check to make sure that all other steel or stainless steel tanks are well-grounded.

Another very common source for electrolytic corrosion is stray currents in your building that could be conducted through the overhead crane system. This can happen even if the building is made of wood, in which case the stray current could be isolated to just the crane system. The crane/hoist pick-up point(s) must always be electrically isolated from the anodizing rack or load bar so that these stray currents cannot reach the parts you are processing. Insulating the crane from the loads can be done in a number of ways, but it is not always easy to do. Possible electrical isolation points could be between the hoist cable and the load pick-up bar, on the pick-up bar itself, at the load bar pick-up point, or other locations. Typical insulating materials can be polypropylene, UHMW polyethylene, PVC or neoprene rubber. Wood is usually not a good choice.

Try all of the above and evaluate the effect after each step to find the root cause of the electrolytic pitting/corrosion.

However, another possible cause of “pitting” could be chlorides or fluorides contaminating the anodizing bath. You may be able to detect the presence of chlorides by taking a sample of the bath and filtering it through filter paper in the lab. Add a few drops of barium sulfate to the sample, and a faint white precipitate (barium chloride) will indicate the presence of chlorides. Any level of chloride close to 200 mg/L or higher could cause a pitting problem.

Chlorides could come from tap water, or, if you use DI water and regenerate the resin beds in-house, chlorides could come from regenerating the cation resin with HCl and not getting a good rinse cycle in the regeneration. If you use hydrochloric acid anywhere in your facility, there is a chance that the fumes will settle in the anodizing tank, which would also cause problems.

Fluoride levels of even less than 200 mg/L in the anodizing bath also can be troublesome. The most likely source would be from a fluoride etch, if you have one, but they could also be present in some deox solutions or other sources. All possible sources must be checked out.