Development of permanganate assisted manganese phosphate coating on mild steel

Manganese phosphate coatings are widely used as pretreatment for corrosion and wear resistance of numerous automotive components. Manganese phosphating formulations are established with various additives to improve the coating weight besides achieving the required amount of iron dissolution in a shorter time. The aim of our investigation is to use permanganate as an additive to increase iron dissolution and to achieve subsequent improvement in the quality of manganese phosphate coating. Manganese carbonate, phosphoric acid, nitric acid are used to create a basic formulation in which permanganate concentration is varied. The composition of the final formulation is optimised by giving due consideration to higher iron dissolution and improved coating weight. The resultant coating is characterised with polarisation, electrochemical impedance spectroscopy (EIS), FT-IR, XRD, SEM and EDX. The kinetics of the coating formation is also followed using potential–time measurement for all the experimental baths. Kinetic studies showed earlier attainment of point of incipient precipitation of manganese phosphate coating in the optimised formulation when compared to other formulation under investigation. The study revealed that the addition of KMnO₄ in the manganese phosphating bath enabled an increase in the rate of metal dissolution, and enrichment of ferrous ion concentration at the metal/solution interface thus, favoured precipitation of corrosion and wear resistant Hureaulite ((Mn,Fe)₅H₂(PO₄)₄.4H₂O). For a given coating weight, addition of KMnO₄ substantially reduced the processing time.