Understanding the Nature of the Manganese Hot Dip Phosphatizing Process of Steel

In this work, the phosphatizing process of steel is investigated using open circuit potential and Tafel curves as well as scanning electron microscopy and energy dispersive X-ray spectroscopy. The results reveal that a pH of 2.57 in the phosphatizing solution promotes the dissociation of phosphoric acid which assists the formation of the manganese tertiary salt (Mn₃(PO₄)₂), which is deposited on the substrate. It was also observed that an increase in the temperature from 25 to 90°C and the presence of HNO₃ as catalyst enhances the manganese phosphatizing kinetics. On the other hand, the generation of iron phosphates and oxides is predominant at a pH of 1 and 90°C. These observations are supported by species distribution and Pourbaix thermodynamic diagrams.