Metal Finishing Guide Book


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nents. Nitrates and nickel compounds and possibly other additives are also included for acceleration and modification of the coating formation. The phosphate layers precipitated mainly consist of (Mn,Fe)5H2(PO4)4.4H2O and manganese or iron hureaulith mixed crystal and varying contents of manganese and iron. When adapted to the particular application and the permissible dimensional tolerances, manganese phosphate coatings are produced with a coating weight between 500 and 2,200 mg/ft2. The coating weight and crystal size of the manganese phosphate coatings are influenced to an even greater extent than in zinc phosphating by the mechanical, thermal, and chemical pretreatment of the workpiece surface prior to phosphating. For example, cleaning in alkaline aqueous cleaning agents or pickling in acids produces coatings with a much coarser texture. Even after such treatments, however, fine crystalline phosphate coatings are still obtainable if the workpieces are prerinsed in an activation rinse prior to the phosphating process. A typical flow chart for the process would be as shown in Fig. 5. 135

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