Correct Coating for Outdoor Use


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Q. I am making sheet metal (cold-rolled steel) cabinets and boxes. The parts are prepared for coating in a seven-tank process, and then powder coating is applied. The powder coating is done within 24 hours of the phosphating process. The powder coating thickness is 85 microns, and the powder type is epoxy. These cabinets are generally kept outdoors (at the customer’s place), and at times the humidity is quite high.

I would like to have your input on following :
  1. Is powder coating on cold-rolled steel OK for outdoor usage in high-humidity areas like Dubai?
  2. What could be the reason for corrosion/rusting on the box in the field?
  3. At present we are using an epoxy powder. Is this OK, or do we need to select some other type of powder material?

A. Powder coating will work outside in high-humidity conditions, however the surface preparation and powder coating must be done correctly. To avoid rusting in extreme conditions you need to use a zinc phosphate treatment or apply a very thick layer over a very clean surface. You also need more powder thickness and good edge coverage for outdoor use. I would use a zinc phosphate, an epoxy primer at around 75 micron and then a polyester TGIC topcoat at around 50 to 75 micron. You definitely do not want to use an epoxy resin, because it will fade and wear out in sunlight. 

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