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 2N load and 20 rpm or 10 N and 100 rpm) stimulate the cathodic process and 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. Optical microscope and SEM observations of the wear track evidence that on both coatings abrasion of the surface asperities occurs, with a consequent surface polishing. On Al2O3/13%TiO2 the wear debris detached at 10 N and 100 rpm is found to partially fill up the surface porosity by plastic deformation and embedment in the coating, so reducing the amount of surface defects and limiting the corrosion attack to a certain extent.