Pretreatment of Aluminum

Recently, we had a job to paint several pieces that were too long to fit into our chromate conversion coating tanks, so our customer arranged with another shop to anodize them. These pieces were painted some months ago and we now find there is an adhesion problem after only three months in the field. In fact, the finish is peeling off in large sections. Can you give us some suggestions as to why we are running into adhesion problems using the anodizing pretreatment, and why you feel anodizing is preferable to a chromate conversion coating as a pretreatment? Also, is it possible that initially there may be good adhesion of the coating to the anodized surface, but later there is a reaction which causes peeling problems?

Q. I’m the president of a company that finishes aluminum products. We need your advice on problems we experience from time to time in painting our products. Some time ago in one of your answers, you pointed out that you recommended anodizing first and then chromate conversion coating as a pretreatment for adhesives and painting of aluminum.

From time to time, we have painted over anodized surfaces and have had some problems. We normally apply baking enamels including polyvinylidine chloride-based finishes. Recently, we had a job to paint several pieces that were too long to fit into our chromate conversion coating tanks, so our customer arranged with another shop to anodize them. We then applied an air drying acrylic urethane enamel.

These pieces were painted some months ago and we now find there is an adhesion problem after only three months in the field. In fact, the finish is peeling off in large sections. It is possible that these products were stored in closed containers, outdoors in the Southwest, for the last three months.

We also note that the specifications for our polyvinylidine chloride-based finishes always state that chromate conversion coating systems should be used as a pretreatment rather than anodizing.

Can you give us some suggestions as to why we are running into adhesion problems using the anodizing pretreatment, and why you feel anodizing is preferable to a chromate conversion coating as a pretreatment? Also, is it possible that initially there may be good adhesion of the coating to the anodized surface, but later there is a reaction which causes peeling problems?
M. C.

A. The answers to your questions are:

Some of the acrylic urethanes require a primer for adhesion. I suggest you use an epoxy primer over the anodized surface before applying the acrylic urethanes.
When anodized surfaces are heated to the curing temperature of polyvinylidine chloride based finishes, the crystalline surface is altered. This can cause a loss of adhesion. This alteration of the anodized surface is the reason chromate conversion coating systems are specified for these coatings.