Effect of Surface Activation Agents on Manganese Phosphate Coating of Carbon Steel

The surface activation agents have been used to form a high quality coating layer on manganese phosphate process. The role of surface activation agents increases the nucleation sites, which leads to obtain a finer phosphate coating. In this study, the effects of surface activation agents on manganese phosphate coating were investigated by changing the chemical composition ratio between sodium pyrophosphate and manganese carbonate. The morphology, chemical composition and corrosion resistance of the coatings were analyzed by SEM, XRD, EDS, XPS and electrochemical polarization method, respectively. Also, the tribology property of manganese phosphate coating was tested by ball-on disk. In the results of EDS analysis, coating layer consists of elements such as Mn, P, Fe, O, and C. XRD showed that (Mn,Fe)5H₂(PO₄)₄ · 4H₂O in manganese phosphate coating layer was formed by the chemical reaction between manganese phosphate and elements in carbon steel. With regard to the effects of surface activation agents on the manganese phosphate coatings, it can be seen that there is an increase of the crystal size on phosphate coating as the content of sodium pyrophosphate increased. The increase of sodium pyrophosphate had effect on the tribology property under the condition of spindle oil retention. Corrosion resistance was improved for manganese carbonate (3 g/L) and sodium pyrophosphate (3 g/L) coating with the ratio of 1:1. Also, better tribology property was observed for manganese carbonate (3 g/L) and sodium pyrophosphate (15 g/L) with the ratio of 1:5.