Paint Adhesion on Aluminum

Q. We have an issue with paint adhesion on aluminum. Here is what is going on: Three stage washer: Cleaner Phosphate, rinse, rinse. Phosphate stage specs: pH running at 4.1 – 4.2
Fluoride level at 285 ppm; Temp = 140°F; Time = Uncertain but I estimate 30 seconds. Rinses are overflowing with second rinse cascading into the first.

The main part in question is an aluminum diecast office chair base. The part has five legs coming out from a hollow circular mount. The legs are curved on the top and flat on the sides. Another part in question is an arm rest mount for the same chair. These are also aluminum diecast. They are essentially solid round pieces of aluminum rod that are bent into a wide U shape. Other die cast parts from the same customer have no adhesion issues. As a matter of fact there are no adhesion issues on any other aluminum part from any customer other than these two parts.

The paint is powder coat. The paint manufacturer recommends a 20 minute cure at 350°F but the customer is running his oven at 385°F and the line speed is such that the cure time is 10 minutes.

Their customer requires a minimum 2 mil thickness for the paint. Flat areas typically have 3 to 6 mils while curved areas have anywhere from 8 to 14 mils.

The customer performs a standard crosshatch test for adhesion using a 2mm crosshatch scribe tool. Adhesion testing on the flat areas is generally excellent but on the curved areas the paint seems to peel right off the aluminum as the customer makes his scribe.

1. Is the fluoride concentration high enough and if we should go higher, how high could I reasonably go before no added benefit is seen?
    
2. Should the customer lower his cure temperature and slow his line speed down or does the higher cure temperature make up for the shorter than recommended cure time?
3. Should the customer back off on the amount of powder being applied to the parts? My unproven theory is the heavy thickness of paint on the curved areas along with the higher cure temperature and shorter cure time is not providing for well cured paint on the curved areas. Could this be the issue?

 

A. I appreciate the thorough amount of background information. It certainly helps when trying to get to the root cause(s) of a manufacturing problem. I will answer your specific questions and give you a couple more things to think about and try.

1) The fluoride concentration seems to generally be in the correct range, however, you should also ask the pretreatment supplier the same question, or at least check their technical data sheet recommendations. Since it’s their formulation, they should have the best idea exactly where the operating range should be. You could also run a test to verify the phosphate coating weight to insure that you are actually applying a measurable iron phosphate coating. If the coating weight is below spec (or less than about 20-30 mg/ft2) you could try increasing the fluoride content until the coating weight is higher (closer to 50 mg/ft2). Since the flat parts are acceptable, this is likely not the problem.

2) There are two points to investigate here. You should check with the powder supplier since they should be able to provide you a cure ladder, instead of simply just one point. It is possible to cure for a shorter time at a higher temperature, but they would have to advise you. They should be able to supply a set of time-temperature pairs that will provide adequate cure (for example 40 minutes at 330°F, 20 minutes at 350°F, 15 minutes at 375°F, etc.). This will give you a better idea if they are close to the correct time-temperature combination. My gut feeling is that they are not high enough over the 350°F (385°F) to be able to cut the cure time in half.

Also related to this, you should obtain a cure profile of the actual parts. If the oven set point is 385°F and the part is only in there a total of 10 minutes, I can guarantee the heavy diecast parts are not coming up to the required cure time and temperature. There are commercially available temperature profiling systems and software that can be purchased for about $5,000. The powder supplier should be able to bring one in to profile your parts and oven for a day and answer this question. Longer term, the coater would probably be interested in obtaining this equipment themselves.

3) The high coating thicknesses could be a problem. If you are not getting adequate cure, the problems will be worse with the thicker coating. I would also think there may be some internal “stress” issues with the film build this thick. Not to mention, they are likely throwing money away by coating this thick when half this much would likely perform just as well.

The other items for consideration are outgassing and cleanliness (or both). Castings are porous and when heated (at curing), can outgas and cause adhesion problems with paints. To check this, try pre-baking a part through your cure oven, allowing it to cool, then running it through the pretreat and powder coating. It is also possible that a single stage clean/coat phosphate stage may not be getting your parts adequately clean in order to develop a phosphate coating. Possibly try an alternate cleaning method offline (alkaline cleaner, solvent wipe, etc.), then process the part through pretreat and powder. Sounds like you are asking the right questions and are on the right track to solve the problem.