Ni-base Inconel 625 alloys with a composition of 58.9Ni-22Cr-9Mo-5Fe 3.5Nb-0.4Si-0.4Mn-0.4Al-0.4Ti (wt.%) corroded at 600, 700 and 800 °C for up to 30 h in 1 atm of N2/H2O/H2S-mixed gases. They corroded fast owing to the formation of sulfides and the presence of hydrogen and water vapor in the gas. Triple layered scales formed. Nickel corroded to NiS to form the outer NiS layer, where Fe was dissolved. Chromium corroded to Cr-sulfides to form the middle layer. Other alloying elements such as Mo, Nb, Si, Mn, Al, and Ti corroded to form the inner layer that consisted primarily of intermixed sulfides and oxides. Since sulfides were present throughout the scale and hydrogen made the scale porous and fragile, Inconel 625 was nonprotective during high-temperature corrosion in N2/H2O/H2S gases.
Effect of Alloying Elements on the High Temperature Corrosion of Heat Resisting Alloys
