Metal Finishing Guide Book

2012-2013

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ZINC PHOSPHATE COATINGS Coatings composed of zinc phosphate have been successfully used in all areas of the cold forming of nonalloy and low-alloy steels. The main reasons for the special suitability of zinc phosphate as a separating medium and as a wear-reducing material are as follows: zinc phosphate coatings are associated with strong binding forces with the iron surface; the specific structure of the zinc and zinc-iron crystal structure enables the plastic deformation of crystals under the action of the compressive and shear stresses arising during forming operations; zinc phosphate coatings are able to react with alkali metal soaps to give high-efficiency lubricant systems; the actual phosphating process is performed by immersion or by an in-line method. The workpieces are treated for a period of 5 to 10 minutes (dip process) or 20 to 30 seconds (in-line process) with zinc phosphating processes working on the ���iron side��� or accelerated with nitrite. Coating weights can vary as much as 300 to 2,500 mg/ft2. The properties of the zinc phosphate coatings produced are mainly determined by the following factors: type of phosphate forming the coating (zinc phosphate, zinciron phosphate, and zinc-calcium phosphate); type of accelerators used (nitrite/nitrate, chlorate, and nitrate); concentrations of components of the process (total acid, free acid, metal components in the bath, and accelerators); process parameters such as process temperature and process time; type of application (dip operation versus in-line process); application of activation prerinses; type and mode of previous pickling processes; and type and mode of previous annealing. Lubrication and its different processes will not be discussed in detail here. Plastic forming of a zinc phosphate coating after being treated with metallic soap is shown in Fig. 4. The SEM pictures show the internal surface of a steel pipe before and after drawing and also after cleaning. Some application differences are listed in Table VI. MANGANESE PHOSPHATE COATINGS TO IMPROVE SLIDING PROPERTIES Manganese phosphate coatings are extensively employed to improve the sliding properties of engine, gear, and power transmission systems. Manganese 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 carburized 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 solution, which, besides the necessary free acid for setting the phosphating equilibrium, contain primary manganese phosphate as the determining compo134

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