Effect of MgSO₄ addition on alkali sulphates induced hot corrosion of a β-NiAl coating

The effect of MgSO₄ addition on alkali sulphates induced hot corrosion of a β-NiAl coating was studied by performing a series of tests with deposits of Na₂SO₄, Na₂SO₄-20 %K₂SO₄ and Na₂SO₄-12 %K₂SO₄-35 %MgSO₄ (mol.% for all) at 700 °C in air with addition of SO₂. For elucidating the observed corrosion phenomena, various salt-oxide/NiAl powder mixtures were exposed to the same hot corrosion test conditions. Two pseudo-binary phase diagrams of the Na, K, Mg-sulphate systems were calculated using an in-house developed thermodynamic database. Addition of MgSO₄ to Na₂SO₄-20 %K₂SO₄ deposit salt inhibited the hot corrosion attack during 24 h exposure, which is related to consumption of aggressive K₂SO₄ by the formation of solid K₂Mg₂(SO₄)₃. However, extending the exposure with Na₂SO₄-12 %K₂SO₄-35 %MgSO₄ deposit to 100 h resulted in severe corrosion attack. The latter observation is explained by a reaction between K₂Mg₂(SO₄)₃ and NiAl causing formation of a Mg, Ni and Al containing spinel accompanied by release of the aggressive K₂SO₄ into the liquid phase.