Corrosion behavior of nickel and cobalt coatings obtained by high-velocity oxy-fuel (HVOF) thermal spraying on API 5CT P110 steel

The API 5CT P110 steel is employed in oil and gas industry due to its excellent mechanical properties. However, its poor corrosion resistance makes necessary the employment of a protection. Although nickel-based and cobalt-based coatings obtained by HVOF have been widely studied regarding corrosion resistance, they are normally associated with other elements. Pure nickel and/or pure cobalt HVOF-obtained coatings have not yet been studied. Therefore, in this work, the corrosion resistance of nickel and cobalt coatings obtained by high-velocity oxy-fuel thermal spraying on API 5CT P110 steel was evaluated. The coatings were characterized regarding morphology and structure by SEM, EDS, XRD, roughness, and Vickers microhardness. The corrosion resistance was evaluated by OCP monitoring and potentiodynamic polarization in a 3.5wt% sodium chloride solution. The results showed that both coatings acted as a barrier and avoided the contact between the steel substrate and the electrolyte due to the low melting point of the metals employed, which resulted in effective fusion of the particles. Besides, the nickel coating, promoted a better corrosion resistance compared to cobalt coating.