High-temperature reactions of CaSO₄and Na₂SO₄in aircraft engines

CaSO₄ and Na₂SO₄ are often associated with CMAS deposits and hot corrosion in gas turbine engines. They are formed from ingested airborne contaminants, CaSO₄ mainly through the direct dehydration of ingested CaSO₄·2H₂O (gypsum), and Na₂SO₄ through reaction of sulfur- and sodium-bearing compounds. A strong candidate for generating Na₂SO₄ is reaction between gypsum and halite (NaCl). In this study, the effects of temperature, pressure and bulk composition on the formation of Na₂SO₄ have been investigated using gypsum and halite mixed with CaO–MgO–Al₂O₃–SiO₂ (CMAS) glasses. Experiments were conducted at 900–1200 °C under open and encapsulated conditions. In open experiments containing only gypsum and halite, gypsum dehydrated to form anhydrite and halite decomposed. In contrast, when gypsum and halite were mixed with CMAS glasses, Ca from anhydrite reacted to form silicates, and SO₃ from anhydrite reacted with halite to form Na₂SO₄. In open experiments without halite, anhydrite remained stable to a much higher temperature than in experiments with halite. Under encapsulated conditions, the high Pₛₒ₂ suppressed the reaction of anhydrite, resulting in no Na₂SO₄ being formed. Overall, the results show that in the presence of NaCl and provided a source of SiO₂ is available, CaSO₄ loses 50–100% of its SO₃ at 900–1200 °C, providing the sulfur needed for the formation of Na₂SO₄. At these temperatures, Na₂SO₄ is molten and therefore capable of causing hot corrosion. The incorporation of Mg from reactant minerals into some of the sulfate melts stabilises them to lower temperatures than pure Na₂SO₄ melts, with greater potential for causing hot corrosion.