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 3days 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 2N load and 20rpm or 10N and 100rpm) 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 10N and 100rpm, while on Al2O3/13% TiO2-coated specimens, a stronger corrosion attack is achieved at 2N and 20rpm. 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. ►Cr2O3 and Al2O3/13% TiO2 coatings only partially protect steel substrate from corrosion. ►On Cr2O3 coatings tribocorrosion is most severe at high load and rotation speed. ►On Al2O3/13%TiO2 coatings at high loads a tribofilm obstructs surface pores and limits corrosion. ►Wear rates of the coatings decrease at increasing rotation speed and at increasing load.