Q. We have been coating a lot of different substrates for some time now including hot-dipped galvanized (HDG) steel with no major problems. We just took on a new job for some parts that are HDG and we are having big problems with adhesion failure. We clean the parts in our five-stage washer and apply the coating. The parts look good but often fail the adhesion test.
What is wrong with these parts? G.N.
A. There are several potential problems (poor cleaning for example) on any part that has trouble with adhesion. However, the most likely problem in your case is that the parts have been quenched after they were galvanized. Some galvanized steel is quenched in water or chrome solution to quickly cool the surface. This is well known to cause problems with powder adhesion. Check to see if these parts were quenched and see if they can change that process.
Also, it is often helpful to roughen the surface with a light media blast to remove excess zinc deposits and polish the surface before coating.
You should try a few parts with a brush blast and see if that improves the adhesion.