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

Inconel 625 coatings with dense microstructures (porosity <2 %) were fabricated on AISI 304L stainless steel substrates by 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 the descending order of F > A > P > H, with Cr2O3 identified as the predominant oxide in the Inconel 625 coating layers. The corrosion resistance of the Inconel 625 coating specimens decreased 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 a very low oxide content. It is suggested that the localized galvanic corrosion around the oxide/matrix interface may deteriorate the corrosion resistance of the coating layer, which is the primary factor contributing to the low corrosion resistance of the F specimen. Although the P specimen exhibited 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 corrosive solution and consequently reducing its corrosion resistance.