Phosphate Surface Treatments on Steel.

A review of U.S. Patent literature concerned with phosphate coating of metal summarizes 179 patents in the field, with special attention given to the time factor, temperature and recommended pH. A critical review of the classification of iron phosphate coating materials and zinc phosphate coating materials as pre-paint treatments, in respect to their chemical composition is given. Here, it is understood that iron phosphates might contain primarily alkali phosphates together with small amounts of metal ions derived from metals which can be defined as to their standing in certain specific ranges of the electromotive series. Zinc phosphates, also, can be produced in many different ways whereby, in reaction with steel surfaces, iron ions will participate in the reaction of the bath and in the formation of the coating on the steel. It was found to be typical for metal phosphates high in zinc content to form a medium coating weight during the first 60 sec. of spray application. Between 60 sec. and 120 sec. spray-time, the coating weight increases rapidly to and above 150 mgper sq. ft. Metal phosphates which are low in zinc content and are classified as iron phosphates, produce a medium coating weight during the first 120 sec. of spray-time. The coating weight does not increase considerably with longer spray-time.