The relationship between relative oxide ion content of Na₂SO₄, the presence of liquid metal oxides and sulfidation attack

Abstract

According to the “oxide ion” theory, sulfidation attack does not occur until oxide ions present in the fused Na₂SO₄ melt react with the normally protective oxide scale. It has already been shown that chromia reacts with and decreases the oxide ion content of sodium sulfate and inhibits sulfidation attack. Based upon the results reported herein, the reduction of the oxide ion content of sodium sulfate is a necessary but not sufficient condition for sulfidation inhibition. It is shown that the oxides of molybdenum as well as vanadium react with and decrease the oxide ion content of Na₂SO₄. It is shown that the addition of either Mo or V to nickel imparts sulfidation resistance. However, it is also shown that whereas the addition of Cr₂O₃ to Na₂SO₄-coated nickel-base superalloys prevents or inhibits sulfidation attack, no beneficial effects are noted when either MoO₃ or V₂O₅ are codeposited with Na₂SO₄ onto nickel-base superalloy substrates. The reactions between V₂O₅ with metal oxides were also studied. V₂O₅ readily fluxes Al₂O₃ and slowly reacts with Na₂SO₄. The relationship between accelerated oxidation, oxide ion content of a fused melt and the fluxing of the normally protective oxide scale by liquid metal oxides is discussed.