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Powder Coating: Enhancing Powder Adhesion on Aluminum

Enhancing Powder Adhesion on Aluminum

We apply powder coatings to HRS steel and extruded aluminum parts that are used outdoors. We use a five-stage washer with a cleaner, rinse, phosphate, rinse and rinse seal. We also use a blast system for the heavy steel parts. The steel product is working out fine, as long as we have good coverage and the correct film build. But we are having trouble with adhesion in the field on the aluminum. We do a crosshatch adhesion test on a regular basis and we always pass. But after the product has been used for a while the adhesion fails. Are there any chemical additives that will enhance adhesion on aluminum?

The challenge in preparing aluminum for powder is the oxide layer that is normally present on the surface. You did not say it, but most spray washers that are set up for multimetals use a fluoride additive in the phosphate solution to etch the aluminum surface and remove the oxide layer. This can be effective for products that are used indoors, but it is usually not sufficient for products that will be subjected to an outdoor environment.

Since there is no conversion coating applied, the surface can begin to oxidize very quickly after it is cleaned and dried, and the powder will not adhere if the part is exposed to moisture and sunlight. Aluminum that is treated in its own process without concern for running steel can be effectively prepared using a chrome seal rinse, a chromate or a dry-in-place nonchrome sealer designed for aluminum. Any of these processes will prevent the reformation of oxide on the aluminum and provide long-term adhesion.

Here are three suggestions for you:

Start coating the hot-rolled steel parts with the blast system and no additional cleaning step. Make sure that the dust from the blast process is blown off, and make sure that your film build is at least three mils thick in all areas. The iron phosphate coating adds marginal additional corrosion protection and is not needed if the surface is completely dry, free from soil, grease, and dust, and has a good, roughened surface to enhance adhesion.

Change the washer over to use a dry-in-place nonchrome seal rinse. If your current supplier does not have this type of product, you should shop for someone who does.

To confirm proper adhesion on aluminum, you should change the procedure. A boiling water adhesion test as described in the American Architectural Manufacturers Association (AAMA) specifications is more conclusive. Prepare the surface as you would for a normal dry adhesion test. Then place the test panel into boiling (210 to 212°F) DI or distilled water for 20 minutes. Remove the test panel, dry it off and then do your tape pull. This is a much more severe test than the dry test, and it will more accurately predict field failure.

Unwanted Moisture in the Lines

We have an air dryer in line to take moisture from our compressed air, and we use coalescing filters. We still seem to get some moisture through the lines. What are we missing?

There could be several reasons why the current system is not properly removing moisture from the lines. For example, the dryer or filters may be undersized or in need of replacement or repair. But the first thing that you should check is the distance between the air dryer and the outlet at the booth. If you have a long run of pipe from the dryer to the booth, you can get moisture in the line from condensation. Adding a high-quality coalescing filter at the booth can help if this is the problem. Have a qualified compressor supplier evaluate your system for flaws.