Preparing Steel for Powder Coating


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Q. We powder paint steel machinery structures and have several questions. 1) Which would be the better preparation process: alkaline/IO rinse/iron phosphate/ IO rinse or alkaline/IO rinse/NT1/IO rinse? 2) Which powder paint supplier would you recommend: Valspar or Akzo Nobel? 3) Why is my paint chipping off (peeling)? P.D.

A. Your questions are tough to answer. Iron phosphate vs. NT1 compares a number of companies and their technologies with one specific product and one company. I will compare iron phosphate vs. zirconium oxide and talk about a few pros and cons.

1. Iron phosphate is a little easier to control and works well on ferrous metal. It can be used in a three-stage washer, and it does not require stainless steel or exceptionally clean water. Zirconium requires somewhat more careful control, better cleaning and better rinsing following the alkaline cleaner. You should have two rinses between an alkaline cleaner and a zirconium bath. Zirconium can be better for multiple metals like aluminum and iron. It requires less maintenance for the plumbing (no sludge), and it does not need heat. It also has little or no phosphate. Zirconium works best in a stainless steel washer that is designed to provide the specific cleaning, rinsing and treatment that work best with transitional metal products. Zirconium oxide may be challenging to convert to if the washer does not have the specific features mentioned. It can also be used with other metals like titanium. Testing is the best way to determine what is best for a given situation. 

2. Valspar and Akzo are both good companies, and I would not recommend one over the other. Look at the different products they offer and the applied cost of the material. Evaluate that applied cost and the services they have to offer.

3. Paint chipping off can be caused by a number of things. First check the pretreatment. The parts need to be clean, and you need a roughened surface (sanded, blasted, etc.) or a good conversion coating (phosphate, zirconium, chrome, etc.). You also need to confirm that you have the right cure schedule (time and temperature). Did you pick the right material for the job? Is the paint chipping at sharp edges or on flat areas? If it is at sharp edges, the coating thickness may be inadequate or poor handling may have caused excessive impact. What environment was the product designed to work in, outdoor or indoor? Is it in an environment that is too tough for the pretreatment you used or too tough for the coating you used? Poor pretreatment, inadequate film thickness, voids in the coating or undercure are the most likely causes of chipping.

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