Corrosion behavior of Cr3C2-NiCr vacuum plasma sprayed coatings

The objective of the present work is to determine the influence of the heat treatment on the corrosion resistance of a Cr₃C₂-NiCr coating of 450 μm thickness, deposited by a vacuum plasma spray process (VPS) on a steel substrate. The post-heat treatment of the as-deposited coating was carried out in Ar at 400 °C and 800 °C, respectively. The coatings were characterized by means of an electron probe micro analyzer (EPMA) with wavelength dispersive X-ray spectrometers (WDS). It was found that no significant changes were produced as a consequence of the heat treatment carried out at 400 °C. Therefore, the corrosion experiments were conducted for the substrate, the as-deposited coating and the post-heat treated coating at 800 °C. Potentiodynamic polarization showed that the annealed coating at 800 °C has a better corrosion resistance than the as-deposited coating. The corrosion current density (I_corr) of this coating was approximately 3 and 4 times smaller than that corresponding to the as-deposited coating and steel substrate, respectively. This significant improvement of the corrosion behavior of the post-heat treated coating is mainly due to both the microstructural changes that take place in the coating and the diffusion of Ni into Fe at the coating-substrate interface, which ensures the presence of a metallurgical bond.