Pretreatment Process for Electrocoat Paint Application
In the pretreatment process, metal surfaces are cleaned and prepared for electrocoat paint application. Oils and other contaminants are removed from the part. Existing rust spots may be removed and a phosphate or conversion coating is applied. The pretreatment process is necessary to improve electrocoat performance, ensure a high quality electrocoat finish and to keep the paint bath clean of unwanted contaminants. The number and type of pretreatment stages depends greatly on the type and condition of metal being introduced to the system. It also depends on the part configuration and the overall performance requirements of the phosphate or conversion coating.
Zinc and iron phosphates are the most widely used conversion coatings in the electrocoat industry. Zinc phosphate is the preferred pretreatment choice in the electrocoat industry because it provides the best corrosion resistance and paint adhesion. Iron phosphating has had a long history of being the process of choice for applications where overall cost considerations override performance requirements. Although iron phosphates provide reduced corrosion resistance in comparison to a zinc phosphate process, it is less expensive to install and operate.
Non-phosphated, zirconium-based conversion coatings are becoming more common in the finishing industry. For several years, they have been replacing iron phosphates because of their low operating costs, multi-metal applications and environmental friendliness. When used in electrocoat applications, their performance usually falls short of a zinc-phosphate process. However, because of their installation and operating cost savings, zirconium processes are now being considered and evaluated where only zinc phosphate is currently approved.