Anodizing: Black, Pitted Areas on Anodized Parts

Q. We are anodizing aluminum kitchenware and have recently been getting parts with scattered black pits all over them. Can you tell me what this is and how it might be happening?

A. Your problem seems to be electrolytic corrosion, or pitting. Verify in which tank(s) pitting is occurring by inspecting parts on selected loads as they proceed through the anodizing line.

Pitting almost always appears as a small crater with a blackened peak in the center. This condition is best seen under a 25× or 30× handheld, lighted magnifier. Pitting 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 bar of some type and then welded to a metal water line that is buried in the ground. If there is no buried water line close by, the ground strap can be welded to a steel building column, or any positive ground. Make sure that all steel or stainless steel tanks are well grounded.

Another very common source for pitting 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 and/or if the processing tanks are polypropylene. If this is the case, the stray current is most likely coming from the crane system. The crane or hoist pick-up point(s) must always be electrically isolated from the anodizing rack or load/flight bar so that the stray currents cannot reach the parts you are processing. Insulating the crane from the loads can be done in a number of ways, thought it's not always easy. Possible electrical isolation points could be between the hoist cable and the load pick-up bar, on the pick-up bar itself or at the load bar pick-up point. There may be other places that can be insulated, but you have to use your imagination. Typical insulating materials can be polypropylene, UHMW polyethylene, PVC and neoprene rubber.

Properly grounding all metal tanks and electrically isolating (insulating) the hoist pick-up points from the loads should virtually eliminate the electrolytic pitting. Check the tank grounds at each tank and the hoist isolation every three months, or so, to make sure they are in good shape.

High levels of chlorides or fluorides in the anodizing bath can be a source of pitting in that tank. Though this rarely occurs, it is a possibility. Look at all of the possible sources of chlorides and/or fluorides. You may be able to detect the presence of chlorides in the anodizing bath by taking a sample of the bath and filtering it through filter paper in the lab. Then, add a few drops of barium sulfate to the sample. The presence of chloride will be detected by a faint white precipitate, or barium chloride. Silver nitrate can also detect the presence of chlorides where the white precipitate is silver chloride. You may want to send a sample to a lab and ask them to analyze for chloride and fluoride. A chloride/fluoride level greater than 100 milligrams per liter could cause a pitting problem.

If you have deionized water in your shop and if you regenerate the resin beds in-house, chlorides could come from regenerating the cation resin with hydrochloric acid and not getting a thorough rinse during regeneration. If you use hydrochloric acid any place in your facility, there is a chance that the fumes could migrate to the anodizing tank. This could be enough to cause problems.

The most likely source of fluorides could be from a fluoride etch. Fluorides are also present in some deox solutions. There could be other sources as well.

 

Originally published in the May 2016 issue.