Corrosion and tribocorrosion behaviour of thermally sprayed ceramic coatings on steel

This research aims at investigating the corrosion and tribocorrosion behaviour of thermally sprayed ceramic coatings deposited on steel specimens and exposed to a 3.5% NaCl solution. The coatings have been prepared by plasma spraying Cr2O3 and Al2O3 /13%TiO2 powders on a Ni/20%Cr bond coating. Combined wear–corrosion conditions have been achieved by sliding an alumina antagonist on the lateral surface of coated steel cylinders, during their exposure to the aggressive solution. Polarization resistance values monitored during 3 days exposures and polarization curves recorded at the end of the immersion period show that both coatings only partially protect steel substrate from corrosion. Sliding conditions (under 2 N load and 20 rpm or 10 N and 100 rpm) induce a limited increase of the substrate corrosion rates, likely as a consequence of an increase in the defect population of the ceramic coatings. On Cr2O3-coated specimens, tribocorrosion is more severe at 10 N and 100 rpm, while on Al2O3/13%TiO2-coated specimens, a stronger corrosion attack is achieved at 2 N and 20 rpm. Profilometer analysis and wear track observations by optical and scanning electron microscopes evidence that on both coatings abrasion of the surface asperities produce both a surface polishing effect and, at high loads, the formation of a tribofilm, more continuous on Al2O3/13%TiO2. On this coating the tribofilm reduces the amount of surface defects and limits the corrosion attack to a certain extent.