How to Determine the Cause of Adhesion Failure

Q. We recently powder coated an aluminum structure prepped with a medium grit aluminum oxide media. We used an epoxy primer and applied polyester topcoat. The total film build for the two coats was around 6 mils. After a few weeks in a warehouse, a small area was discovered where the coating had chipped to bare metal along a weld seam. What caused the adhesion loss?

A. Adhesion failure occurs when the bond between the coating and the surface is broken by the presence of some contamination or oxide. When a chip breaks away (probably from being hit by something) it indicates that the entire surface was not treated correctly or that a small spot has a problem. A blasted aluminum surface will usually give you good initial bond. With a good primer coat, you should get a tight mechanical bond and the two coats should stop moisture penetration.

The fact that it is adjacent to a seam may be a critical factor. It is likely that the seam contained some moisture that bled out during cure and that caused the small area of adhesion loss. A couple of simple tests should help to show if the failure is wide spread or just in that small area. First, remove more coating from the edge of the failure using the tip of a sharp knife. If it is fairly easy to get under the film and lift more coating, the affected area is larger than the chip. If a large area can be easily removed you may have primary failure and the entire surface could be affected. Try some spots away from the seam and use an adhesion test to confirm if it is isolated or wholesale adhesion loss.

If the area is limited to a small space you should be able to clean the affected area thoroughly, sand the rough edges and apply a high quality liquid primer and topcoat to patch the area. You can color match the liquid paint to the powder although they may not fade at the same rate. You will also be able to see the affected area, so if the cosmetic appearance is critical, this may not be satisfactory. If it is wide-spread adhesion loss, the only good remedy is to remove the coating and start over. If you do recoat the structure, you should consider options for adding a conversion coating that will protect the aluminum from oxidation. That is one excellent way to improve the bond and ensure longevity of the coating.

Originally published in the May 2017 issue.