Corrosion behavior in artificial seawater of thermal-sprayed WC-CoCr coatings on mild steel by electrochemical impedance spectroscopy

The corrosion behavior in artificial seawater of different as-sprayed ceramic-metallic (cermet) coatings applied on low-alloy steel was studied. Five conditions, associated to modifications of the composition of the powder or deposition parameters were evaluated. The degradation mechanisms were studied during extended immersion tests using conventional electrochemical measurement and electrochemical impedance spectroscopy. The extended immersion tests reveal that these as-thermal-sprayed coatings present a cathodic behavior compared with steel. During the first hours of immersion, the electrolyte infiltrates the defects of the coatings, which then result to the local degradation of the substrate accelerated by the galvanic coupling with the cermet coating. Optical observations and Raman analyses reveal the formation of calcium carbonates like aragonite on the cermet surface, very close to the appearance of local anodic sites. The cross-sectioned views reveal the infiltration of the corrosive solution, and the depth penetration of the degradation of steel substrate probably due to the acidification of the anodic sites.