Grain-boundary chemistry and intergranular corrosion in alloy 825 |
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Authors: | Y -M Pan D S Dunn G A Cragnolino N Sridhar |
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Affiliation: | (1) the Corrosion Science and Process Engineering Element, Center for Nuclear Waste Regulatory Analyses, Southwest Research Institute, 78238 San Antonio, TX |
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Abstract: | Alloy 825, a former candidate material for radioactive high-level waste containers, was investigated to assess its thermal
stability and the time-temperature conditions for sensitization. Alloy specimens with a carbon content of 0.01 wt pct in the
mill-annealed (MA) and solution-annealed (SA) conditions were studied after thermal exposure to temperatures ranging from
600 °C to 800 °C for periods of up to 1000 hours. Sensitization was evaluated by using corrosion tests that were correlated
to grainboundary 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 °C for 100 hours
resulted in the highest sensitization. While heat treatment at 640 °C showed a progressive development of sensitization with
time, healing was found to occur after aging at 800 °C for 100 hours. The degree of sensitization, quantified by an equivalent
chromium-depleted-zone size, correlates well with the corrosion rate in the nitric acid test. Thermodynamic models were used
to calculate the interfacial chromium concentration, chromium depletion profile, and the depleted-zone width. Comparisons
between experimental measurements and model calculations indicate that reliable prediction depends on the selection of key
model parameters. |
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