Corrosion behavior of heat-treated NiCrMoAl alloy coatings produced via arc spraying

Arc-sprayed NiCrMoAl coatings were heat-treated at 500°C–700 °C for 10 and 30 days in an air atmosphere. The chemical compositions and microstructures of all coatings were characterized before and after heat treatment and after corrosion tests via scanning electron microscopy with energy-dispersive X-ray spectroscopy. The corrosion behavior of the coatings was studied through potentiodynamic polarization tests in a 20 vol.% H2SO4 solution at room temperature. Heat treatment at excessively high temperatures and for a long duration can create large amounts of oxides, leading to internal stress, which induced cracks between splat boundaries and intensified corrosion. Cracks allowed the penetration of the electrolyte into the substrate, inducing galvanic corrosion between the substrate and coating resulting into low corrosion resistance. Notably, heat treatment at 500 °C with high solute-atom (Cr, Mo, and Si) concentrations in splats and dense oxides around splats could enhance corrosion protection of the NiCrMoAl coatings.