Powder Coating Clinic: Avoiding Rust on Parts
Q. We apply lighter colors of powder to our ticket machines for parking lots. Some go into covered garages, but many of them are exposed to the outdoor elements. We have some problems with rust around the base of the machines and also on corners. We currently run the steel parts through a five-stage iron phosphate washer, then apply a coat of TGIC polyester powder to a thickness of around 3 or 4 mils. What else should we be doing to prevent rust?
A. Outdoor performance is a bigger challenge for any steel part that is coated with an organic finish. The exposure to sunlight and rain creates a corrosive environment that can break down a finish at its weakest points and cause corrosion cells to break through the coating. The thinnest coating is often at the sharper edges or corners of the part. The problem is compounded when the base is fastened onto a surface and the corners and holes are exposed to standing water or corrosive elements present on the ground around the product. If you use the product in a northern climate, chances are that one of those elements is salt or another corrosive chemical used to melt winter ice.
What can be done to protect steel in a harsh outdoor environment? The first thing to concentrate on is understanding that powder coating is an engineered process. The steps in the process need to be carefully planned to meet a specific outcome. A five-stage iron phosphate pretreatment followed by a single coat of powder is outstanding for indoor product but not adequate for harsh outdoor situations.
There are different ways to achieve the performance needed for outdoor installations. It starts with the pretreatment process. Steel can be blasted to remove inorganic soils, leaving a relatively clean, rough surface. This creates a great anchor pattern for the coating because as it cures, it hardens. Blasting does not add additional corrosion protection, but does remove elements that would otherwise interfere with adhesion or create a nucleus for a corrosion cell under the film. If you blast and then add a primer coat before the topcoat, you will drastically improve your corrosion resistance. If you want to get the best possible corrosion on a blasted surface, use a zinc-rich powder primer. The zinc in the prime layer provides barrier protection against moisture penetration to the steel surface.
If you cannot blast or choose not to blast, you can increase the performance by improving the pretreatment. A zirconium oxide treatment or zinc phosphate will increase the corrosion resistance when compared to iron phosphate. Again, the addition of a primer coat will improve corrosion resistance a lot, used with any of the aforementioned pretreatment options. One of the reasons that the double coat (primer plus topcoat) is so effective is that it increases the coverage on the sharp edges where you are most vulnerable to corrosion. If you do not have a system that lends itself to the addition of a primer coat, you must enhance the pretreatment and make sure to get a thick coating on the edges. If you can, add a radius to the corners for better coverage with a single coat.
Originally published in the November 2016 issue.
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