Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 46 of 331

phosphating is capable of application to carbon steels with a maximum extraneous element content (i.e., Ni + Cr + Mo + V, etc.) of approximately 5% to 6%, and also to cast iron. The additive quantities of the individual alloying components here amount to a maximum of approximately 1.5% manganese, 0.4% silicon, 0.2% chromium, 4% nickel, and 0.6% molybdenum. In the case-hardening steels the carbon content amounts to approximately 0.2%, whereas in the tempering steels it is between 0.2 and 0.5%. Of the elements mentioned, chromium and molybdenum, particularly in the upper concentration range, may interfere with the phosphating process. The existing surfaces in many cases are in the carbur- ized or nitrided state, without significantly affecting the phosphating capacity. In contrast, a working layer produced by mechanical means seriously affects the formation of the phosphate coatings. The Process The manganese phosphate process is performed by immersion using water so- lution, which, besides the necessary free acid for setting the phosphating equi- librium, contain primary manganese phosphate as the determining compo- nents. Nitrates and nickel compounds and possibly other additives are also in- cluded for acceleration and modification of the coating formation. The phosphate layers precipitated mainly consist of (Mn,Fe)5 H2 (PO4 )4 .4H2 O and manganese or iron hureaulith mixed crystal and varying contents of manganese and iron. When adapted to the particular application and the permissible dimen- sional tolerances, manganese phosphate coatings are produced with a coat- ing 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 pretreat- ment 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 ac- tivation rinse prior to the phosphating process. A typical flow chart for the process would be as shown in Fig. 5. 45

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