Environmentally-assisted cracking of 316 L stainless steel exposed to gallium suboxide

Failure of stainless steel furnace parts has been observed following treatment of gallium-containing plutonium compounds at 800 degreesC to 1200 degreesC. Our previous work examined the corrosive effects of gallium suboxide (Ga2O) gas on 316 L SS in an effort to elucidate a failure mechanism. Elemental segregation, oxidation, and Ga uptake were found to occur following exposure. Qualitative mechanical tests using four-point bend specimens suggested that stainless steel was severely embrittled following exposure to Ga2O at 1150 degreesC. The objective of this work is to quantitatively detail the effects of Ga2O on crack initiation and propagation in 316 L SS using fracture mechanics specimens. Samples were exposed to Ga2O gas and tested in laboratory air. Results indicate that increasing Ga uptake is detrimental to the crack initiation and propagation resistance of 316 L SS. Increasing Ga absorption decreased the threshold stress intensity required for crack initiation. Increasing Ga uptake also promoted unstable crack propagation and intergranular fracture. A combination of micro X-ray fluorescence and SEM suggests that compound formation and intrinsic Ga embrittlement result in increased cracking susceptibility. Ga embrittlement was shown to parallel hydrogen embrittlement, with reduction of a monovalent oxide resulting in specimen corrosion, uptake, and intrinsic and extrinsic embrittlement, depending on concentration. The combination of results from specimens Ga loaded by exposure to Ga liquid and to Ga2O gas suggest that the corrosion reaction is detrimental to fracture resistance. That is, for the same Ga concentration, specimens exposed to liquid metal Ga were more failure resistant than those exposed to Ga2O gas.