Effect of a localized thermal treatment process on the electrochemical behaviour of thermally sprayed nickel-based alloy

In present work, thermally sprayed Inconel 718 coatings were prepared by high velocity oxy-fuel (HVOF) spraying. To promote their corrosion resistance the effect of different associated processes (AP) to the torch have been investigated. The first one is a pre-heating maintained during the coating elaboration. The second one consists in a post-cooling process applied just after particle impingement on the substrate. The third one corresponds to an association of a pre-heating and post-cooling process to the thermal spraying torch to elaborate coatings. In each case, the associated processes were maintained during coating elaboration. Coatings were characterized by their metallographic microstructures, phase compositions, microhardness and fractal dimensions of their cross-section profiles. The electrochemical properties of the coatings were evaluated in sulfuric acid solution using free potential measurements and polarization curves. It was found that the cross-section profiles and the lamellar morphologies of the coatings prepared using the different combinations of pre-heating or post-cooling processes were similar to each other. However, the additional pre-heating process used with torch has notably improved the corrosion resistance of the coating opposite to the post-cooling process. These coatings were all preferentially corroded around the lamellar boundaries at the surfaces. The corrosion resistance can be related to the oxide contents in the thermally sprayed coatings and the different chromium contents around their lamellae boundaries due to the chromium depletion during the particle oxidation step. The heat–cooling process presented in this paper seems to have the same effect that the pre-heating process.