"Snowflake" Corrosion on Anodized Aluminum Parts

Q. I work in an aluminum extrusion facility. One of our customers sends their extrusions to an outside anodizer near their plant. They sent us some samples of their anodized parts that were exhibiting a condition that looked like small snowflakes all over the part. The customer is asking us if this is a metal condition, or did something go wrong in the anodizing process? What could cause such a condition? M.B.

 

A. Without actually seeing the parts or a photo of the condition, it sounds like electrolytic corrosion occurred during the anodizing process. I would have to see a sample piece to be certain. This type of corrosion is usually caused by “stray electrical currents” in the anodizing line. This happens as a result of the metal tanks not being properly grounded. If the tanks are metal it can occur in any tank except the anodizing tank, but primarily this happens in rinse tanks.

One reason why this type of corrosion is often found in rinse tanks is because rinse tanks are sometimes used as holding tanks for loads awaiting the next process step. The longer a load of parts dwells in an improperly grounded rinse tank the better are the chances for a pulse of stray current to occur. In nearly every industrial plant it is possible to have these random, and usually very weak, stray electrical currents that course through the steel building structure from time to time.

It can be very difficult to track down the source of these fugitive currents, but suffice to say that it is pretty difficult to electrically isolate every possible electrical device, or wire in the plant. There are many ways that these stray currents can find their way into the metal tanks in the anodizing line. Some of these ways are through steam lines and metal water lines.

Electrolytic corrosion is also caused by stray currents coming through the overhead hoist system. Plastic tanks don’t have to be grounded, but this type of corrosion can still occur if the hoist provides a path for the stray currents. To prevent this, whether the tanks are metal or plastic, the hoist pick-up bar or hook device must be electrically isolated (insulated) from the hoist itself, including the hoist cable. Another way to do this is to insulate the rack or load bar pick-up point from the hoist hook. It takes a good design and diligent maintenance to prevent this sort of thing from happening. Once this happens, the only way to remove it from the part is by mechanical finishing.