The Effect Of Mn Ion On Zr Conversion Coating Properties: Corrosion Resistance, Morphology and Composition of Coating

In this paper, St-12 steel surfaces were coated using hexafluorozirconic acid conversion coatings. Influence of different pH (2, 4, 6) on corrosion resistance of coated samples was initially studied. Electrochemical and anti corrosion properties in solutions with different pH were studied by use of Polarization and EIS measurements. The results showed that pH=4 hinders corrosion of steel much better compared to pH= due to less acidity. On the other hand, pH=6 also couldn’t offer good corrosion protection for steel, since it is too alkaline to result in proper oxidation of substrate. Next, questions of interest associated with 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; AFM to study morphology and surface roughness; SEM to evaluate surface morphology; in addition to EDS and XPS to analyze chemical composition. Results showed that the sample coated in solution with pH=4 and Mn2+ concentration of 0.01 g/l possess improved corrosion resistance (current density decreased about fifty percent in comparison to steel coated just with Zr) and more uniform morphology. The result of XPS for the sample coated in optimized Mn2+ containing solution showed that the coating is composed of various zirconium oxide states and manganese compounds.