Nickel oxide as a new inhibitor of vanadium-induced hot corrosion of superalloys—comparison to MgO-based inhibitor

Abstract

The combustion of vanadium- and sodium-contaminated fuels in gas turbines provokes an accumulation of ash on metallic surfaces of the hot gas path. This ash material causes fouling and corrosion as a result of the formation of low melting point reactive compounds. This paper presents comparative investigations into the effects of adding nickel oxide and magnesium oxide to inhibit this form of high temperature corrosion. The chemistry in corrosive Na₂SO₄–V₂O₅ ash materials is discussed in terms of acid–base reactions in the presence or absence of SO₃ gas using Lux–Flood acid–base theory. It has been found that the addition of NiO leads to the formation of Ni₃V₂O₈, a refractory compound that dramatically reduces the corrosiveness of the ash materials by trapping vanadium. In contrast, the effect of MgO on the ash materials is to stabilise the vanadium by reacting with orthovanadate anions (VO₄³⁻) to form NaMg₄(VO₄)₃, which dramatically lowers the corrosive effect. However, the sulfation of a fraction of the MgO results in compaction of the ash materials on the metallic parts, which means that frequent cleaning of the machine is required. It is concluded that nickel oxide is an efficient alternative to magnesium as an inhibitor of hot vanadium corrosion, even in presence of sodium.