Effects of MgO and CaO on hot corrosion of Fe base superalloy Superfer 800H in Na₂SO₄-60%V₂0₅ environment

Abstract

Molten vanadate induced hot corrosion is a serious problem in power plants using residual fuel oil. In this investigation the effects of MgO and CaO on the hot corrosion behaviour of an iron based superalloy, Superfer 800H (equivalent to Incoloy 800) has been investigated using an aggressive environment of Na₂SO₄-60 wt-%V₂O at 900°C for up to 50 cycles. Weight change has been plotted with respect to time to establish the corrosion kinetics. Corrosion products were analysed by SEM, X-ray diffraction, and electron probe microanalysis (EPMA). X-ray diffraction identified the presence of the phases FeV₂O₄, Fe₂O₃, FeS, NiO, and Cr₂O₃, together with Mg₃V₂O₈ in the case of MgO coated specimens and Ca₃V₂O₈ in the case of CaO coated specimens. These latter phases remain solid at 900°C, explaining the inhibiting behaviour of CaO and MgO. The presence of various oxides and spinels was identified by EPMA. It is concluded that MgO and CaO injected onto hot section surfaces can be used to counteract hot corrosion of Superfer 800H owing to trace impurities of Na, V, and S in residual fuel oil.