Influence of coating microstructure on the corrosion behavior of Inconel 625 coatings fabricated by different thermal spraying processes

Dense microstructures of Inconel 625 coatings (porosity <2 %) were successfully fabricated on AISI 304 L stainless steel substrates using flame spraying (F), twin wire arc spraying (A), plasma spraying (P), and high-velocity oxygen fuel spraying (H). The quantity of oxide within the coating layers followed an ascending order of F > A > P > H, with Cr2O3 identified as the predominant oxide in the Inconel 625 coating layers. Corrosion resistance of the Inconel 625 coating specimens exhibited a decreasing trend in the order of H > A > P > F. The outstanding corrosion resistance of the Inconel 625 coating prepared by high-velocity oxygen fuel spraying was attributed to the compact coating layer with very low oxide content. It is suggested that localized galvanic corrosion around the oxide/matrix interface may induce the deteriorated corrosion resistance of the coating layer, representing the primary factor contributing to the low corrosion resistance of the F specimen. Despite the P specimen exhibiting a lower oxide content than the A specimen, it demonstrated inferior corrosion resistance. This phenomenon can be attributed to the significant formation of microcracks within the P specimen, creating penetration tunnels for the solution and consequently reducing its corrosion resistance.