Abstract
Selective dissolution of grain boundaries was observed on Alloy 600 after exposure to a 25 molal (50 wt pet) NaOH solution at 140 °C under applied anodic potentials. Three heat-treatment conditions were investigated in addition to the original mill-annealed material (carbon content 0.04 pct), namely: solution-annealed, sensitized (700 °C/1 hour), and thermally-treated (700 °C/16 hours). The material in both the sensitized and thermally-treated conditions was found to be more susceptible to grain boundary attack than that in the solution- and mill-annealed conditions. The degree of attack was extremely potential dependent, being more severe within the primary passive range at potentials close to the region where transpassive dissolution of chromium occurs. Anodic polarization curves were measured on chromium carbide (Cr23C6) and various Ni-Cr-Fe alloys, which are thought to simulate the grain boundary composition of Alloy 600 after heat-treatment in the sensitization range. In addition, galvanic current measurements were performed on couples formed between chromium carbide and the various Ni-Cr-Fe alloys. All these data are used to explore various mechanisms for the grain boundary attack of Alloy 600 in hot caustic solutions.
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