Painting Magnesium Castings

Q. For the past several months, we have been trying to supply a new customer with a die-cast magnesium product that is coated with a baked epoxy enamel. We have had no success and would like to know what we are doing wrong. The parts are cast, trimmed, pickled and then sent to the paint shop for coating. The results are a very high scrap rate because of blisters in the painted surface. We tried a number of other treatments such as pickling and sanding after pickling without success. Microscopic examination of the die-cast surface shows that it has a very porous surface. We tried without success to modify the casting process by increasing die temperature and changing shot speeds and dwell time using pure magnesium. I find this to be a very difficult problem. I would welcome any changes you can suggest. H.S.

 

A. After reading your question, I agree with you that the problem with blisters in the baked epoxy paint over die-cast magnesium is very difficult to solve. It is also very puzzling, partly because I know very little about die casting and am unable to evaluate the effects of the process changes you made. On the other hand, I am familiar with the coating defect you described, which is common when painting magnesium as well as other castings. One cause of this condition is expansion of gasses in the pores during curing. Another cause is the expansion of entrapped pretreatment solutions or their residues during the thermal curing process. Preheating the castings before coating will eliminate the first cause. Extra care in rinsing and drying after pretreatment, then preheating, should solve the second.