The plasma sprayed HA ndash nanostructure
The corrosion behavior of the substrate and coatings were examined with the help of dc potentiodynamic polarization and AC electrochemical impedance measurements (EIS). Obtained results reveal that there was insignificant variation with respect to change in the weight ratio of TiO2 in HA. Hence, here the electrochemical corrosion behaviors of HA–nanostructure TiO2 composite coating was compared with pure HA coating and blank SS-316L substrate for better understanding of this NVP-BGT226 composite coating. Fig. 11 and Fig. 12 show the Tafel polarization curves and Nyquist plots for the tested samples in SBF solution respectively. The corrosion current densities were obtained from the polarization curves by the extrapolation of the cathodic branch of the polarization curves to the corrosion potential. A lower passive current density and a shift in corrosion potential to more positive potential is observed from Tafel plots, implying that the HA coating improves the corrosion resistance of SS-316L substrate. The passivation behavior with a significantly low current density as given in Table 3 indicates binary fission highly protective and uniform surface films have been formed on the surface of the HA coated specimen exposed to the SBF solution. Lower porosity was observed for HA–nanostructure TiO2 composite coating (HA–10 wt% TiO2) than for the pure HA coated substrate and hence the higher protective efficiency as observed from Table 3. The noble behavior of the coated SS-316L samples can be attributed to the crystalline and denser nature of the coating surface, by the reaction between the calcium and phosphate ions from the SBF, apatite precipitates are formed on the coating surface . The limited passive regions of the SS-316L exhibit a lower ability to endure localized corrosion. The electrolyte infiltrates into the inner portion of the coating through the structural imperfections such as pores and cracks or pinholes existing in the coating and comes into contact with the deeper portion of the coating , causing pitting corrosion.