Powder Coating Chipping on Aluminum Extrusion

Q: We currently powder coat a variety of parts, mostly steel. We have had some opportunities to coat aluminum extrusions. We clean parts with a blast process or three-stage hand wash. We have done a few jobs, but we have seen a few field failures where the coating is easily chipped off the extrusion. Do you have any thoughts on what might be causing this?

A: Aluminum has a tight oxide layer that can be removed, but oxidation occurs rapidly after cleaning or blasting. Initial powder adhesion might appear to be good, but any exposure to heat and moisture can lead to delamination and failure. Blasting might wor, but there is no certainty. The only way to get reliable results with powder on aluminum that will be used outdoors is to use an aluminum conversion coating that neutralizes the surface so the oxide cannot rapidly reform. Traditionally, chromic acid was used to provide good adhesion and long-term corrosion resistance. Non-chrome products are now available that work very well.

I suggest you contact a chemical supplier regarding options that will work in your shop, and consider the operational issues and the cost. You may find a good product that fits. You can test these products with a wet adhesion test as described in AAMA 2604 or 2605. The surface is cross-hatched and subjected to boiling demineralized water for 20 minutes. After removal from the water, a tape pull test can be used to confirm adhesion. If this test is successful, the part should be good. If it consistently fails, the product is not adequate for your parts. The wet adhesion test is also a good quality control measure.