Q: In our plant, we powder coat over aluminum and steel poles, but we have run into trouble because the paint is peeling. We do pretreatment with an alkaline pH 11-15 solution, and then we use a phosphate solution. Any suggestions on what we are doing wrong?
A: I wonder what the condition of your poles is prior to pretreatment and if your process is cleaning and phosphating adequately. I also wonder if the problem is both the steel and aluminum or just one of them.
Does your phosphate (I’m assuming iron phosphate) have a fluoride additive to allow etching of the aluminum? If the system doesn’t contain fluoride, it won’t work well for the aluminum parts.
I suggest that you send a few test panels through your pretreat and powder system and see if you experience the same lack of adhesion. If so, you may have a pretreat system seriously out of control (I doubt it), or you could be experiencing under-cure of the coating. If the test panels look good, your pretreat system needs to be more aggressive in cleaning and phosphating.
Reader response: The problem is on aluminum poles (and sometimes on lamps). And the problem represents about 2% of our sales. But it is currently with the black and brown colors.
Perhaps the problems just appear only at some part of the pole. For background, we cast the base of the pole and the body is extruded, which we buy from a supplier. We weld both parts together.
Is the shape important (because we have smooth and fluted)?
To prevent more claims, we are using a steel wheel abrading to do more “surface,” but this is mechanical then too slow. Perhaps I could use this pre-treatment to paint galvanized and it’s possible powder paint over anodized?
Joe Powder response: If I understand, the base is cast in a foundry and then welded to an extruded pole. And the problem is exclusively with the aluminum parts. It probably has something to do with your iron phosphate pretreatment. Please determine if you have the fluoride additive in your system. Without it, the phosphate doesn’t work at all on the aluminum.
As for the cleanliness of the immersion system, please ensure that you clean the tanks regularly. Oils and contaminants can accumulate in your solutions and interfere with the deposition of the phosphate.
Regarding pretreating galvanized with iron phosphate, you need a five-stage system that includes a high-quality final rinse. Chrome final rinses work best, but non-chrome types are nearly as good. It’s best to preheat the galvanized parts prior to powder coating. This allows the zinc coating to expel any entrapped gases and moisture. And yes, you can powder coat clean anodized aluminum. Use common sense in cleaning and prepping the surface, as powder doesn’t like to adhere to dirty surfaces.