Phosphating of Carbon Steels in Solutions Containing Zinc and Zinc-Manganese Phosphates

The formation of phosphating coatings on carbon steels by novel baths containing zinc and zinc-manganese phosphates has been investigated. By gravimetric, chemical, electrochemical and physical methods are determined the characteristics of preparations (density, pH, conductivity, total and free acidity) and those of the produced coatings (thickness, phase and chemical compositions, structure, protection ability). The concentration range has been varied from 5 to 20% vol., while the temperature effects have been studied in the range from 20 to 80 degrees C. The thicknesses of the coatings and the amount of the substrate dissolved have been determined. It is proved that Mn-phosphate decreases the coating thickness for all working solution concentrations and temperatures, at the same time increases the mass of dissolved metal of the substrate. The results obtained indicate that the coatings developed in zinc and manganese baths contain the following phases: hopeite, phosphophyllite, quasihopeite, strunzite and mixtures of them.