Grain-boundary chemistry and intergranular corrosion in Alloy 825

Alloy 825, a former candidate material for radioactive high-level waste con tainers, was investigated to assess its thermal stability and the time-temp erature conditions for sensitization. Alloy specimens with a carbon content of 0.01 wt pct in the mill-annealed (MA) and solution-annealed (SA) condit ions were studied after thermal exposure to temperatures ranging from 600 d egrees C to 800 degrees C for periods of up to 1000 hours. Sensitization wa s evaluated by using corrosion tests that were correlated to grain-boundary chemistry analyses. Sensitized microstructures were found to contain M23C6 -type carbides and a chromium-depleted region in the vicinity of the grain boundaries. Thermal aging at 700 degrees C for 100 hours resulted in the hi ghest sensitization. While heat treatment at 640 degrees C showed a progres sive development of sensitization with time, healing was found to occur aft er aging at 800 degrees C for 100 hours. The degree of sensitization, quant ified by an equivalent chromium-depleted-zone size, correlates well with th e corrosion rate in the nitric acid test. Thermodynamic models were used to calculate the interfacial chromium concentration, chromium depletion profi le, and the depleted-zone width. Comparisons between experimental measureme nts and model calculations indicate that reliable prediction depends on the selection of key model parameters.