THE COMBINED EFFECT OF BICARBONATE AND CHLORIDE IONS ON THE STRESS CORROSION CRACKING SUSCEPTIBILITY OF ALLOY 22

摘要

The potential engineering barriers for the permanent disposal of high-level waste at the Yucca Mountain site include an Alloy 22 outer container. One of the anticipated degradation modes for the container is stress corrosion cracking of Alloy 22. Slow strain rate tests were performed to evaluate the combined effect of bicarbonate and chloride ions on the stress corrosion cracking susceptibility of mill-annealed Alloy 22. Tests were conducted in which the solution chemistry, electrochemical potentials, and solution temperatures were varied. Stress corrosion cracking was observed in simulated concentrated water and in bicarbonate solutions containing various chloride concentrations at 75 and 95°C [167 and 203°F] under applied potentials of 200 to 400 mV{sub}(SCE). Bicarbonate and chloride ions act synergistically to promote transgranular stress corrosion cracking of mill-annealed Alloy 22. Stress corrosion cracking susceptibility was found to be strongly dependent upon electrochemical potential. For simulated concentrated water, the transition from no cracking to stress corrosion cracking occurs between 200 and 400 mV{sub}(SCE). For a bicarbonate solution containing concentrated 7.2 m (mol/kg H{sub}2O) chloride, the transition occurs between 100 and 200 mV{sub}(SCE). At a constant applied potential of 400 mV{sub}(SCE), the susceptibility of Alloy 22 to stress corrosion cracking decreases with decreasing temperature.