The performance improvement of Zr conversion coating through Mn incorporation: With and without organic coating

In this paper, a hexafluorozirconic acid based conversion coating was applied through immersion method on steel substrate. The main aim related to how the concentration of MnSO4 additive (0.01, 0.1 and 1 g/l) affects corrosion resistance, morphology and composition of the coating. The properties of the coated surface was examined by a range of characterization techniques including DC polarization and EIS tests for comparing the corrosion resistance of differently coated samples, and SEM and AFM to evaluate surface morphology; in addition, EDS and XPS were used to analyze chemical composition. According to AFM and SEM results, more uniformity was observed for the zirconium conversion coating containing 0.01 g/l Mn ion compared to that with no Mn. The lower corrosion current density and higher charge transfer resistance of conversion coating were observed in the presence of Mn ions in the solution as reported by the DC and EIS results, respectively. The result of XPS for the sample coated in optimized Mn2 + containing solution revealed that the coating was composed of zirconium oxide states and no evidence of Mn peak was seen. The organic coating applied on the Zr conversion coating without Mn ion showed the highest adhesion strength of 4.97 MPa and the highest corrosion resistance with |Z| 0.01 Hz = 1.34 × 1010 after 6 weeks of immersion in corrosive media. Consequently, although a certain additive such as MnSO4 may modify conversion coating characteristics, it cannot necessarily improve corrosion performance and adhesion in an organic coated system.