The Effects of Chromium on the High Temperature Corrosion of Ni–Cr Alloys Exposed to Calcium Sulfate

Calcium- and sulfur-rich deposits have been linked to failure of turbine components as a consequence of high temperature exposures (> 1000 °C). There are only limited studies on the effects of these deposits on the degradation behavior of turbine alloys. To gain further understanding of this phenomenon, a systematic study was undertaken with model binary nickel–chromium alloys. Three alloys with different chromium contents—low, medium and high—represented by Ni-5Cr, Ni-10Cr and Ni-18Cr, were exposed to CaSO₄-deposit-induced corrosion in the 900–1100 °C temperature range. At 1000 and 1100 °C, the decomposition of CaSO₄ (either by decomposition to CaO and SO₃ or by reacting with Cr₂O₃) led to the formation of calcium chromates and chromium sulfides. At the lower temperature, 900 °C, the limited decomposition of CaSO₄ allowed the formation of a continuous Cr₂O₃ scale.