Powder Coating Q&A: Pretreatment for Different Metals

Q. We are currently planning to install a powder system for our HRS structural parts. We also run aluminum extrusions. Please tell me what my options are for the different metals. Also, our parts are used outdoors, and we want to know how to test them for quality.

A. These two metals are quite different, and reliable treatment for outdoor use is not the same for these different substrate characteristics. HRS is rough and has inorganic soils such as rust and scale metal imperfections that can interfere with adhesion and corrosion resistance. You will need a strong acid or an abrasion process to get these soils off. Abrasion is usually preferred because it is effective and no strong chemical is needed.

If you chose to abrade the part, you have a couple of options with steel. You can blast the surface to remove the inorganic soils, blow off the dust and then apply the powder. If you want exceptional corrosion protection on that surface, I would recommend a primer coat and a top coat. The primer will enhance resistance to moisture and build more powder on the sharp corners. Always be sure that the part is clean after the blast. If the blast media is kept clean and relatively free of oil, a blow-off with clean, dry compressed air is probably all you need after blasting.

Then, chemically clean and treat the steel before coating. If you can use a zirconium oxide or similar product and apply a thick coating, you may be able to use the top coat only without the primer. However, in harsh environments (think sea coast) or if the quality expectations are high, you may still want a primer coat.

The oxide layer that is normally present on the surface is the challenge when preparing aluminum for powder. It will need to be cleaned, and you should use a suitable aluminum conversion coating to prevent the oxide layer from interfering with adhesion. Chrome or chromate have been popular conversion coatings for aluminum through the years. The chrome does a great job of altering the surface of the aluminum so the powder will adhere and the surface will have great corrosion protection for outdoor use.

More recently, non-chrome options have become much more popular than chrome, because they perform very well without the challenges associated with using a toxic product. Zirconium oxide conversion coatings also work well on aluminum. You can run a multi-stage washer to clean and treat the part.

In summary, remove all inorganic soils from the steel, and then clean it. Clean the aluminum, and add a conversion coating. The conversion coating is optional on the steel, but it may enhance corrosion resistance.

For the steel, a dry adhesion test is good. Run a panel with your parts and conduct the test as soon as it is cured and cooled. There are a lot of ASTM tests that you may want to adopt, but adhesion is critical.

To confirm proper adhesion on aluminum, you should us a boiling water adhesion test as described in the American Architectural Manufacturers Association (AAMA) specifications.

Prepare the surface of a panel as you would for a normal dry adhesion test. Then place the test panel into boiling (210–212°F) DI or distilled water for 20 minutes. Remove the test panel, dry it off and then do a tape pull. This is a much more severe test than the dry test, and it will more accurately predict field failure. Continue to conduct short- and long-term corrosion tests to verify that your process is working. Cyclic testing may be the best option for your product.

Originally published in the March 2016 issue.