Questions and Answers for Pretreatment Options

Q. Are you aware of any uncomplicated, user-friendly pretreatments that are compatible with powder coating, will inhibit rust on surfaces with deep inset corrosion and will accelerate beneath the coating after the process is complete? 

A. Iron phosphate works well with powder but has modest corrosion resistance if the coating is broken. Zirconium oxide products can be modestly better, and some contain other metals such as copper or titanium and are somewhat more resistant to oxidation. Zinc phosphate has very good corrosion resistance, but it also requires waste treatment and would not be considered simple or user-friendly. One of the best options is a primer. Thoroughly clean the surface by chemical or mechanical cleaning, then you can have the parts electrocoated or you can apply a powder primer. The primer provides exceptional resistance to moisture penetration and a thicker film with better edge coverage. The primer does a great job of stopping the undercoat creep you refer to.

Q. Are there any new methods other than or in addition to phosphating which act as bonding media between powder and a metal surface? 

A. Powder will bond effectively on a mechanically clean surface that has been abraded or roughened with a blast media or other surface preparation method. It is important to be sure that the media used to roughen the surface is clean and that it cuts the surface. Spherical shot may peen the surface and leave some oils imbedded in the metal. A grit blast media like aluminum oxide is usually better because it cuts into the surface more effectively. Many automated blast systems use a blend of shot and grit. Grinding or sanding can be effective, but they are not suitable for a higher-volume operation. There are also chemical conversion coatings that will enhance adhesion. In addition to iron phosphate, zinc phosphate, manganese phosphate and chrome, new pretreatments such as zirconium oxide will enhance adhesion to a metal. These newer treatments do not require heat and they do not generate the sludge that is a normal byproduct associated with phosphate solutions. Phosphates have been connected to eutrophication in fresh water lakes, while the zirconium-based products have not been a problem.