Grain-boundary chemistry and intergranular corrosion in alloy 825

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 ЊC to 800 ЊC 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 M 23 C 6 -type carbides and a chromium-depleted region in the vicinity of the grain boundaries. Thermal aging at 700 ЊC for 100 hours resulted in the highest sensitization. While heat treatment at 640 ЊC showed a progressive development of sensitization with time, healing was found to occur after aging at 800 ЊC 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.

Prediction of Corrosion of Advanced Materials and Fabricated Components

“…For this alloy, a fair amount of scattering is observed in the experimental data and some asymmetry in the profiles is noticeable (Pan et al, 2000). The model is in a reasonably good agreement with the data.…”

“…Figure 45 shows the difference between the local repassivation potential in alloy 825 and the E rp value of a homogeneous alloy. These calculations have been performed for alloy 825 sensitized for 15h and 100h at temperatures ranging from 873 K to 1073 K. These sensitization conditions correspond to the chromium depletion profiles that have been experimentally obtained by Pan et al (2000) and reproduced by the sensitization model described in the previous quarterly report. It should be noted that the reduction in the local E rp value qualitatively follows the chromium depletion profiles.…”

Electrochemical methods to detect susceptibility of Ni-Cr-Mo-W alloy 22 to intergranular corrosion

“…Other researchers also observed similar results for alloy 825. [26] The addition of activators such as thioacetamide and thiourea did not show any reactivation peaks for the specimens aged at 650 °C and 700 °C. Faint reactivation peaks could be observed for specimens aged at 750 °C.…”

Topologically close-packed phase precipitation and thermal stability in alloy 22

“…[15,16,17] The principal feature responsible for sensitization is the existence of a narrow Cr-depleted zone along grain boundaries as a result of chromium carbide precipitation. Therefore, it is generally expected that incorporation of Mo in the TCP phases is likely to induce the depletion of Mo in the matrix immediately adjacent to the precipitate.…”