“Snowflake” Corrosion on Anodized Aluminum Parts

Q. Our customer sent us some of its anodized aluminum extrusion samples that were exhibiting a condition that looked like small snowflakes all over the part. Is this is a metal condition, or something wrong in the anodizing process?

A. This sounds like electrolytic corrosion. If it is, this type of corrosion is often caused by “stray electrical currents” in the anodizing line, resulting from metal tanks not being properly grounded. If tanks are metal, it can occur in any tank except the anodizing tank, but most often occurs in rinse tanks because they 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 the chances are 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 occasionally course through the building structure. It is difficult to isolate every possible electrical device in the plant. There are many ways that these stray currents can find their way into both metal tanks and plastic tanks in the anodizing line. 

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One common way is through steam or hot water lines if a boiler is the source of heating the baths. Installing electrical isolators in the heating lines will eliminate the stray currents through this path. 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 tanks are metal or plastic, the hoist pickup bar or hook device must be electrically isolated (insulated) from the hoist itself, including the hoist cable. You can also insulate the rack or load/flight bar pickup point from the hoist hook. 

Good design and diligent maintenance can prevent electrolytic corrosion. But when it happens, the only way to remove it from the part is by mechanical finishing.

If there are metal tanks in the processing line, all metal tanks must be solidly grounded. The ground can be made from tank-to-tank by welding a “strap” from the tanks to a buried metal water line or to a solid grounding rod. A steel column anchor bolt can also become a good ground.

Originally published in the July 2017 issue.