Cleaning Aluminum Castings

Q: Our customer finishes our aluminum lamp surface with powder. We apply dipped degreasing solution with alkali solution and acid treating of the surface and rinsing. We have had a lot of nonconformity reports about adhesion or other covering problems. I’d like to know what cleaning problems result in bad powder coating, or what tests we can do to ensure good cleaning. We apply a heat treatment (250°C, 14 hr) before cleaning to remove trapped gas from aluminum pores. Thanks for your help. M.S.

 

A: There are some powder coatings that perform better on aluminum than others. Some of these powders are specially formulated to provide superior adhesion on aluminum, which can sometimes be difficult to adhere to. If this is not a new problem, I suggest asking your customer if he has alternative products with an adhesion promoter that would perform better on your aluminum cast parts. If the powder has performed well in the past, then you do need to take a look at your pretreatment process. It could be that the alkaline cleaning alone is marginal in preparing the part for powder coating and a slight change to that process was enough to make it perform poorly. I would suggest starting by examining tank maintenance. What is different about your process? Are the tanks on the same dump frequency? Is production at the same level it has been in the past, or has it increased? Answering some of these questions may get you to the root cause.

If not, or if you cannot consistently produce a part with adequate adhesion, then you may want to consider changing the cleaning process to something more aggressive. An alkaline or acidic (preferred) etch would better prepare the surface to accept paint or powder. The extra etch can also increase surface area of the base material that will in turn improve adhesion.

This could be especially beneficial given the long, elevated temperature you are soaking the aluminum castings at prior to cleaning (to eliminate gas in the casting).

Given potential differences in atmosphere, time and temperature during this heat treatment, that could introduce a significant source of variability into your process. That will tend to grow a thicker oxide layer that is likely not being removed in the current cleaning process. The more aggressive etch will produce a more consistent surface, regardless of incoming material oxide thickness.