The influence of initial microstructure on interaction behaviour of Alloy 693 in borosilicate glass melt

Abstract

Alloy 693 is emerging as a better choice of structural material over Alloy 690 for the vitrification of nuclear waste in molten glass. Alloy 693 derives this advantage because of its superior high temperature corrosion resistance and mechanical strength due to the addition of Al and Ti. Alloy 693 forms coherent precipitates of strengthening γ′-phase particles, and also improves corrosion properties due to enhanced growth kinetics of the Cr₂O₃. Alloy 693 also tends to form precipitates of the Cr-rich α-phase, in addition to γ′-phase particles, both of which dissolve at temperatures above 1000°C. A limited published work reports better glass corrosion resistance of Alloy 693 over Alloy 690 in a borosilicate and Fe-phosphate glass melts, however, only at temperatures higher than 1000°C, which stabilizes two alloys in a single γ-phase. Present study reports the effect of γ′- and α-phase precipitates on the corrosion behaviour of Alloy 693 in a borosilicate glass melt at 900°C and 1000°C. Alloy 690 exposed under identical glass exposure condition has been used as a benchmark for comparison. The study demonstrates that, during initial period of glass exposure, Alloy 693 exhibits a corrosion resistance much lower than Alloy 690. The identification of corrosion mechanism has shown that the presence of unbounded Al in the alloy promotes higher metal loss, at least during initial periods of glass exposure, while the metal loss significantly reduces after the precipitation of γ′-phase particles in Alloy 693. Effect of γ'-phase and α-phase particles on the corrosion behaviour of Alloy 693 is discussed in the light of availability of Al and Cr solutes on the development of protective Cr₂O₃ layer.