THERMODYNAMICS OF LOW-TEMPERATURE (700-850{degree}C) HOT CORROSION

Existing phase diagrams in the systems Na{sub 2}SO{sub 4} - MSO}{sub 4} (M=Ni, Co) and Na{sub 2}SO{sub 4} - M{sub 2}(SO{sub 4}){sub 3} (M=Al, Fe, Cr) have been used to calculate the thermodynamic properties of the molten sulfate systems. The calculated thermodynamic data show satisfactory agreement with most of the available experimental observations. The calculations have shown that the activity of Al{sub 2}(SO{sub 4}){sub 3} and Fe{sub 2}(SO{sub 4}){sub 3} in the melt can be lowered to such an extent that liquid sulfate solutions can be formed at P{sub SO{sub 3}} levels that are prevalent in marine gas turbine operations, and this has been explained on the basis of complex formation in the melt. Thermodynamic analysis of the interaction of the Na{sub 2}SO{sub 4} - MSO{sub 4} (M=Co, Ni) melt with protective oxides Al{sub 2}O{sub 3} and Cr{sub 2}O{sub 3} has demonstrated the vulnerability of Al-containing alloys to hot corrosion attack