Pinholes in Bend Area

Q. I work at a powder coating shop in Michigan, and we have had a consistent problem with a particular part we are coating. The part is approximately 1/8-inch thick hot-rolled, pickled and oiled steel. We are getting some “pinholes” through the coating in the bend radius after the part is cured. It seems to be consistent in the (outside) radius only. We do not get this problem when we run lighter-gauge steel, only when we run 1/8-inch or greater. We are using a five-stage washer with an iron phosphate conversion coating. I think this problem has to do with the material being stretched and opening some pores on the surface, but I am not sure. I do know that we do not see this problem when there is a larger radius in the bend. It only occurs when we bend the radius fairly tight. Have you seen this before? Is there something I can do to prevent this?—B.C. 

A. You have most likely hit on the root cause of your problem based on what you write. I have seen this before in different places. The steel fractures slightly in the outside of the bend area and traps air and contamination. The air escapes due to exposure to high temperature and blows small ruptures in the coating.

There are several options that may help you to avoid the craters. You could see if the radius of the bend area could be less aggressive so the micro-fractures do not occur. If that is not practical, try a lower-temperature cure for a longer cycle time. This will give the trapped air more time to release before the coating starts to harden. You could also try a low-temperature powder. Some epoxy formulas can be cured below 300°F, and some polyester materials can be cured at 325°F. Your powder supplier may also be able to work on a formula that is more tolerant of the outgassing of air by adjusting the flow cycle and allowing more time to relive the air from the fractured area.