Powder Coating Q&A: Adhesion on Aluminum

Q. I have recently had an issue with powder coating adhesion to aluminum processed through my single-stage pretreatment system. After performing energy dispersive spectroscopy testing on the failed areas, it was determined that a layer of magnesium was on the surface of the aluminum, which became brittle after powder coating and separated so that the coating released from the substrate. I have made chemical changes to the single stage and have solved the issue, but am still trying to determine where the magnesium is coming from on the aluminum. Do you know how the magnesium got there?

A. Pure aluminum, or more accurately, near-pure aluminum is rarely used. Most aluminum materials include a number of alloying elements to provide certain mechanical properties. The 6000 series of aluminum, often used for extrusions, contains magnesium in small amounts. So the most likely source of the magnesium is the aluminum alloy.

My guess is that you have some small areas of concentrated magnesium that your one-step material could not remove or convert. Magnesium can be very low in reactivity. Small deposits of loosely adhered elements are possible. Powder does not adhere very well to the oxide layer. In the presence of moisture, the surface can develop a low-pressure zone and pull moisture through the powder film, separating the powder from the base substrate and causing adhesion failure in the affected area. You may also have gotten the magnesium in the pretreatment material. You will want to check that out with the supplier.

Originally published in the December 2015 issue.