Electrochemical Corrosion Behavior of Heat-Treated HVOF Coatings on ASTM SA213-T22 Steel

Abstract

Herein we report the electrochemical corrosion behavior of pre and post heat-treated composite coatings of NiCrMoFeCoAl-30%SiO2 and NiCrMoFeCoAl-30%Cr2O3 on ASTM SA213-T22 boiler tube steel by high velocity oxygen fuel (HVOF) spraying technique. The samples were subjected to hot molten salt (Na2SO4–60%V2O5) corrosion environment in a tubular furnace at 7000C under thermocyclic conditions. The microscopic, structural and electrochemical investigations of post-heat treated specimens reveal NiCrMoFeCoAl-30%Cr2O3 composite HVOF coating exhibits a superior corrosion resistance compared to NiCrMoFeCoAl-30%SiO2 composite coating and bare ASTM SA213-T22 steel boiler tube steel in neutral electrolyte. The room temperature potentiodynamic and impedance investigations of heat-treated samples suggest high interfacial charge transfer resistance for HVOF coatings over a wide anodic potential window. This could be ascribed to the protective nature of the chromium oxide containing coatings on high temperature treatment. AC impedance analysis reveals NiCrMoFeCoAl-30%Cr2O3 coating exhibits very high resistive behaviour with very high charge transfer resistance, in the order of 106 Ohm higher than the NiCrMoFeCoAl-30%SiO2 coating and uncoated ASTM SA213-T22 steel boiler tube steel. Furthermore, the high temperature induced formation of metal chromates/chromites along with the presence of Cr2O3 provides good resistance towards corrosion.