Electrochemical Corrosion of HVOF-Sprayed NiCoCrAlY Coatings in CO2-Saturated Brine

The effect of pre-oxidation treatment and surface preparation of optimized NiCoCrAlY coatings deposited by high-velocity oxygen fuel (HVOF) spraying and exposed to a low-temperature corrosive environment is reported herein. Coatings with two surface finish conditions (as-sprayed and ground) were heat treated under two different oxygen partial pressures (air and argon). The electrochemical corrosion behavior was evaluated in CO2-saturated brine via potentiodynamic polarization, polarization resistance, and electrochemical impedance measurements. The results show that the grinding process and pre-oxidation treatment in argon enhanced growth and formation of alpha-Al2O3 scale. The potentiodynamic polarization results show that both pre-oxidation and surface treatment had a positive influence on the corrosion resistance of the coating. The reduction of the porosity and the formation of a dense, uniform, and adherent oxide scale through pre-oxidation treatment led to an increase of the corrosion resistance due to a decrease in active sites and blocking of diffusion of reactive species into the coating. However, according to the results, complete transformation from metastable alumina phases to alpha-Al2O3 in addition to formation and growth of dense alpha-Al2O3 is required to ensure full protection of the coating and base material over long periods.