Blisters And Creep On Aluminum Castings

Q. We have repeatedly failed a 336-hour salt spray test on an aluminum casting. Our pretreat system and coating have met the same requirement on steel and a shorter salt spray test on aluminum castings. We use an immersion bath iron phosphate conversion coating (soap, rinse, phosphate, rinse) and urethane powder coating. The results have consistently been blisters and significant creepback from scribe. How can we improve our adhesion and weather resistance on these die castings? What is causing the blistering? We could instead use a TGIC Polyester? Would these powders perform better? M.S.

A. My first impression is that you may have more of a problem with your casting and pretreatment than you probably do with your powder coating. The fact that you are not having the same problems on your steel parts (presumably not cast) would also indicate that it is an issue with the casting. When parts are cast, there will always be some degree of porosity that may trap impurities or gas. These impurities will inhibit the performance of the powder. There could be two ways to approach this problem.

One way would be to focus on the casting itself, since the casting method will have an impact on the problem. For instance, an investment casting will rely on the casting shell being developed around a wax replica of the part. When that cast shell is complete, the wax should be melted out of the interior, however, based on the time, temperature and drainage points, there could be a small or even large amount of wax remaining in the cast shell. When the metal is poured, depending on the temperature and time, some of that wax may break down into a hydrocarbon gas while some could remain behind coating individual voids and porosities within the cast metal structure.

While each of the casting processes has inherent drawbacks regarding contamination and porosity, two methods that would minimize both of these factors would be permanent mold and diecasting. Both would be preferred methods to use to minimize both natural porosity of the metal and the potential for contamination by residual wax, foam or sand from making the casting shell.

While these may be preferred methods, their starting costs are higher (tooling) and are not completely devoid of their own problems since they may have tenacious surface residues from mold release agents. If the casting is the source of the problem and changing type is not practical from a time or financial standpoint, you may want to try pre-baking the part prior to painting. This could be incorporated into your pretreatment process or done offline as a batch. The idea behind prebaking of parts would be to outgas as much residue from the casting as possible prior to application of the paint. I would suggest you get the metal temperature to about 100°F above your maximum metal temperature the casting reaches at cure. Try holding this for various lengths of time (possibly 5, 10, 15 minutes) and evaluate the effect on the performance of your powder coating process. Ideally you should outgas the contaminants from the interior porosity of the part so that it either volatilizes or stays at the surface so your pretreatment can remove it.

You could try changing powder, but that would be a longer-term prospect that may require some amount of trial and error with your supplier. Is your current supplier aware that their current product is being used over castings? If so, they may have already taken that into account when they introduced the product you are using. The potential benefit that could be derived from changing the powder or formulation would be to slow the gel time such that any contaminants can outgas prior to the coating fully crosslinking and curing.