Annealing effect on corrosion resistance of Bi$_x$Ti$_y$O$_z$ coatings

Abstract

Bismuth titanate (Bi$_x$Ti$_y$O$_z$) has received widespread attention due to the fact that during recent times it has found important applications in strategic research fields such as optics and optoelectronic, and more recently studies have shown how their physicochemical properties may be harnessed in order to be able to use Bi$_x$Ti$_y$O$_z$, as an anticorrosive coating. In this work bismuth titanate (Bi$_x$Ti$_y$O$_z$) coatings were grown on titanium alloy (Ti6Al4V) substrates, using RF magnetron sputtering at room temperature. The main objectives of the work were quantify the evolution of crystallographic phase formation, as a function of the annealing temperature, and establish the chemical composition in order to characterize the behavior of the bismuth titanate coating as a protective coating of the corrosion. The morphology of the coating was observed via Scanning Electronic Microscopy (SEM); the crystalline structure was characterized by X-ray Diffraction (XRD) and the chemical composition was analyzed by Rutherford Backscattering Spectrometry (RBS). The corrosion resistance of the coatings was studied by Potentiodynamic polarization (PP) test (Tafel extrapolation). SEM results showed that the surface roughness of the coatings changed when the temperature of annealing increased. Similar change occurred after PP tests. The XRD analysis revealed a change in the coatings microstructure as a function of the annealing temperature, since they evolved from a completely amorphous phase to a polycrystalline phase. RBS results indicate that coatings growing at high temperature have a complex chemical composition. Finally, the electrochemical analysis showed that the corrosion resistance of the coating is much better in the amorphous phases of bismuth titanate than in the polycrystalline phases.