Production experience has aver time demonstrated that a uniform phosphate coating, iron or zinc, has a significant impact on the quality and performance of the treated surface, when processing mixed metals there are certain parameters that need to he considered in each step of the process to ensure the proper coating. Although automatic controls are available, the whole process can he controlled through simple titrations and pH measurements. These values determine the amount of chemical necessary to maintain the bath's chemical equilibrium and prevent any of the possible problems indicated earlier. Within the phosphate system the most important step is cleaning, and this becomes even more critical in mixed metal lines although the function remains the same, removal of all types of soils on all incoming surfaces. If the surface is properly cleaned and rinsed of any and all residual chemicals, a uniform phosphate coating will result. The phosphate-coating bath requires some adjustment when processing high amounts of either galvanized or aluminum and proper controls can achieve the proper coating. After coating, the surface is generally posttreated with a product containing either an organic polymer, chromium as Cr~(+3), or a combination of Cr~(+3) and Cr~(+6) in order to achieve increased performance. Extensive testing has shown, if the phosphating process is properly applied on the surface to be painted, a quality base for the subsequent electrocoat paint operations is the result.
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