Corrosion behaviour of thermal spray-coated carbon steel in a saline environment

The electrochemical behaviour of two cermet coatings (WC/12%Co and WC/17% Co) applied by High Velocity Oxygen Fuel (HVOF) technique on carbon steel specimens has been studied in 3.5% sodium chloride solution, by both chemical and electrochemical techniques. The coating thickness ranged between 0.05 and 0.2 mm and efforts were devoted to keep the porosity at a constant and low level. The lowest corrosion current densities were measured on 0.2 mm WC/17% Co coated specimens. The coating microstructure was investigated by SEM and electron probe microanalysis (EPMA). The matrix was found to be affected by compositional inhomogeneities: carbide was found to dissolve partially in the matrix in both coatings, but rich cobalt regions were still present, particularly in the coating with the highest cobalt content. Further investigations were carried out on the coating constituents (cobalt and WC, sintered in the absence and in the presence of 5% Co) and the substrate. The results show the galvanic stimulation of steel dissolution in steel-WC coupling, while galvanic stimulation of cobalt dissolution is found in Co-WC coupling. Both phenomena can affect the studied materials when the coatings exhibit interconnected porosity, while in the presence of impervious coatings, only the latter galvanic coupling can be operative as under these conditions the sole possible anodic process is constituted by cobalt dissolution.