Comparison of Na2SO4, K2SO4 and Na2SO4-K2SO4 deposit induced hot corrosion of a β-NiAl coating

Hot corrosion behaviour of a β-NiAl coating induced by Na2SO4, K2SO4, and Na2SO4-K2SO4 deposits was investigated at 700 °C in air + 300 ppm SO2. To provide an insight into the corrosion mechanisms, phase equilibria of the Na2SO4-K2SO4-NiO-SO3 system were calculated using an in-house developed thermodynamic database. The attack rates and morphologies could be correlated with the deposit compositions and their reactions with oxides. K2SO4 and Na2SO4-K2SO4 deposits are more corrosive than Na2SO4. This is attributed to a reduction of the melting temperature of the deposit and the formation of Ni-containing mixed sulphate phases in the presence of K2SO4. •The rate and morphology of type II hot corrosion attack strongly affected by pSO3 and deposit composition.•K2SO4 and Na2SO4-K2SO4 deposits are more corrosive than Na2SO4.•Formation of Ni-containing mixed sulphates and liquid phases in the presence of K2SO4 accelerate the corrosion attack.•Stability diagrams of the Na2SO4-K2SO4-NiO-SO3 system were calculated using an in-house developed thermodynamic database.•The calculated stability diagrams agree well with the hot corrosion results.