Powder on Zinc

Q. Can zinc castings be powder coated? We have been coating some small zinc parts, and they out-gas and do not hold powder very well. We have a multi-stage washer we use for steel parts that applies an iron phosphate coating. Our chemical supplier tells us it should work, but we keep getting adhesion failures and bubbles in the coating. Any suggestions on methods to treat the zinc? Is iron phosphate effective for zinc? M.C.

A. Yes, zinc castings can be powder coated. A cast part will have porosity that may cause blemishes in the coating at high temperature. Air that is trapped near the surface may expand and rupture the film during the cure process. There are several ways to mitigate the issue. You can preheat the part to drive off some of the trapped air that causes the problem. Heat the part to a temperature about 50°F greater than the cure temperature, cool it down, and apply the coating. Cure at the lowest temperature possible to limit the problem. You also can use a powder that is formulated for a flow cycle that will help release air without leaving a blemish.
The adhesion is another issue. If the powder does not stick, it is because you have not cleaned and prepared the surface correctly. You need to get rid of all organic soils (grease, oil, dirt), and you may have to polish or blast the surface to get rid of inorganic soils (die release or similar compounds). Look at the nature of the adhesion failure. Is it on all parts of the surface or more likely to occur in the same areas all the time? If it is everywhere, the part is not getting clean, and you need a more aggressive cleaner with more heat. If it is in isolated areas, it is probably a die-release product. Iron phosphate leaves a film on zinc, but it is not a true conversion coating for zinc. I suggest you try a polish step (tumble the parts in a vibratory device, ultrasonic cleaning or some similar method) to see if the problem is die-release agents. Talk to the chemical supplier again about a thorough audit of the part and options for preparation.