High-temperature passivation mechanism for recycling of refractory metals from Inconel 718 via sulfurization methods

Nickel-based superalloys contain substantial amounts of refractory metals such as Cr, Nb, and Mo. Recovering these refractory elements is a critical approach to promoting resource utilization and ensuring the secure supply of strategic metals. Among the various methods, the sulfurization corrosion technique offers distinct advantages for the recovery of nickel-based superalloys. However, research on the high-temperature sulfurization corrosion behavior of nickel-based superalloys remains limited. To realize the recovery of Inconel 718 alloys at lower temperatures, this study systematically investigated the hot sulfurization corrosion behavior of Inconel 718 alloys in low-melting-point metal sulfides. The findings revealed that the optimal recovery temperature of Inconel 718 via high-grade nickel matte is around 900 °C, accompanied by significant morphological changes due to the aggressive penetration and erosion by sulfides. Upon increasing the temperature to 1000 °C, the superalloy demonstrated a distinct passivation effect, which became progressively more pronounced with rising temperatures, resulting in a marked decrease in the corrosion rate. Further analysis indicated that the formation of Ni₃Fe during the hot sulfurization corrosion process is the primary factor responsible for the observed decline in corrosion rate. As the temperature increased, the growth rate of Ni₃Fe grains accelerated due to Ostwald ripening, leading to the development of a continuous Ni₃Fe layer, which inhibited the inward penetration and erosion of metal sulfides. In the sulfurization corrosion process, the conversion of refractory metals into low-melting-point sulfide phases enhances the extractability of refractory metals in the Inconel 718 alloys during the subsequent hydrometallurgical recovery process.