Experimental study coupled with thermodynamic assessment of the NiSO₄–K₂SO₄ quasi binary system

Thermodynamic properties of the NiSO₄–K₂SO₄ system play significant roles in various applications, e. g. in gas turbine components. In these components, Ni-base superalloys and coatings often suffer from the so-called hot corrosion induced by alkali sulphate deposits. In the present work, the phase equilibria in the system NiSO₄–K₂SO₄ were experimentally determined for the first time using differential thermal analysis (DTA), dilatometry (DiL) and X-ray diffraction method (XRD). Two eutectic points and two stoichiometric compounds, K₂Ni₂(SO₄)₃ and K₂Ni(SO₄)₂, were found in this system. The temperature and composition of the lower eutectic were determined to be 642 °C and 40 mol.% NiSO₄, respectively. Another eutectic between NiSO₄ and K₂Ni₂(SO₄)₃ was found at 819 °C, and the corresponding composition is around 75 mol.% NiSO₄ and 25 mol. % K₂SO₄. Based on the obtained experimental data, the NiSO₄–K₂SO₄ system was thermodynamically assessed using the CALPHAD methodology. The modified associate species model was used for the description of the liquid phase. Using the optimized dataset, the calculated phase diagram showed a good agreement with the experimental results. This dataset extends the previously developed general sulphate database including alkali- and alkaline-earth sulphates (CaSO₄–MgSO₄–Na₂SO₄–K₂SO₄) with the addition of NiSO₄.