Q. We powder coat thicker gauges of steel (10-gauge to ¼-inch plate), using a blast operation for some of the heavier parts. We use a multi-stage wash system with iron phosphate on everything. Our pretreatment chemistry is fairly aggressive because of weld areas and other imperfections that do not go through the blast process. We want to test the parts to confirm proper pretreatment coating performance before shipment, but we have some problems with impact and conical mandrel bend testing. The aggressive reaction of the washer chemistry can cause flash rust or other problems on the lightweight panels needed for the bend tests. Is there some way to create a bend test for our heavier materials? How can we confirm that our pretreatment and powder are working correctly? J.J.
A. A bend test or impact test on heavier-gauge material is not practical. The point of these tests is to deform the metal and see if the powder adheres. If you hit a 10-gauge plate hard enough to impact or bend it, the powder will be damaged and the results will not be clear. Dry adhesion is a good test for these surfaces. Also, you can do corrosion testing to find out how the pretreatment and powder are working together to provide good performance. Salt spray is one test, but it takes a long time to get the results, and it is only good for comparison and cannot predict field life. Cyclic testing is better for actual performance, but it also takes time. You may want to try a Machu test. Machu combines glacial acetic acid, salts, hydrogen peroxide and water to brew a highly corrosive solution. A scribed panel in the solution may show signs of failure in 24 to 48 hours of immersion if the pretreat/powder combination is weak. You should be able to find the directions on the web, but if you cannot, contact me and I will send it to you. It is a strong solution that must be used in well-ventilated areas with proper personal protection equipment and disposed of properly, but it will work quickly. You should also do a cure test to make sure the powder is fully cured. If you use the chemical rub test as described by PCI #8, you should be sure to compare the results to a known cured sample. The results of the chemical rub test reflect the chemical resistance of the powder, so comparison is essential to understand the results.