Q. I have three powder coating plants in Pakistan (Karachi) where I am powder coating various types of steel products. I currently use a hot zinc phosphate for pretreatment, but I would like to use a lower-temperature product for this process. Can you give me any advice on lower temperature zinc phosphate products?—M.S.
A. You do not supply the temperature that you are using or the temperature you consider “low,” so your question is a little harder to answer accurately. For good reaction and deposition of the phosphate coating, zinc phosphate should be applied at 130°F (55°C) or higher. Many efforts have been made to apply it at cooler temperatures, but the amount of zinc in the deposited coating drops dramatically when the temperature is lowered. At temperatures lower than 115°F (46°C), the coating is almost all iron phosphate, and the solution will rapidly build sludge. Some chemical companies do work in these low temperatures, but they must use a special formulation of zinc phosphate to partially account for the loss of thermal energy in the reaction. The phosphate coating that is deposited includes some sludge and oxide, so the resulting coating is not as good as a higher-temperature product, even if the measured coating weight looks good. Some immersion products work reasonably well between 115 and125°F, but for the best possible corrosion resistance, the higher temperature is still preferred. Try researching pretreatment chemical suppliers from the pfonline.com site and see what they have to offer, then get samples and run trials.