Hot corrosion behavior of HVOF-sprayed carbide based composite coatings for boiler steel in Na2SO4–60 % V2O5 environment at 900 °C under cyclic conditions

Hot corrosion behavior of HVOF sprayed coatings, namely NiCrAlY-SiC and NiCrAlY-B4C deposited on T22 steel were studied for 50 cycles at 900 °C in an Na2SO4-60 wt%V2O5 molten-salt environment. Compound formations and microstructure were analyzed using XRD, SEM-EDS and EDS mapping techniques. Uncoated T22 steel suffered an accelerated corrosion attack due to the formation of unprotective FeO, Fe2O3, FeS and FeSO4 phases. NiCrAlY-SiC coatings showed formation of a protective layer containing SiO2, Al2O3, Cr2O3, NiO with spinals phases, whereas NiCrAlY-B4C coatings featured layers comprising B2O3, Al3BC, Al2O3, Cr2O3 and spinals phases. NiCrAlY-SiC coatings delivered greater hot corrosion resistance than NiCrAlY-B4C coatings.