Effects of Surface Chromium Depletion on Localized Corrosion of Alloy 825 as a High-Level Nuclear Waste Container Material

Effects of the chromium-depleted, mill-finished surface on the localized corrosion resistance of alloy 825 (UNS N08825) were investigated. Tests were conducted in solutions based on the ground water at Yucca Mountain, Nevada, but with a higher concentration of chloride (Cl⁻). Results indicated that breakdown (E_p) and repassivation (E_rp) potentials for mill-finished surfaces were more active than those for polished surfaces. Potentiodynamic polarization tests indicated pits could be initiated on the chromium-depleted surface at potentials of 220 mV_SCE in a solution containing 1,000 ppm Cl⁻ at 95°C. Potentiostatic tests identified a similar pit initiation potential for the mill-finished surface. However, under long-term potentiostatic tests, a higher potential of 300 mV_SCE was needed to sustain stable pit growth beyond the chromium-depleted layer. An increase in surface roughness also was observed to decrease localized corrosion resistance of the material.