Abstract
The effects of various heat treatments on grain boundary structure and chemistry have been investigated for two series of nickel based alloys of types 600 and 690, using Auger electron spectroscopy, analytical transmission electron microscopy, and scanning electron microscopy. The alloys were also tested with respect to their resistance to intergranular corrosion in 30%NaOH + 10%Na2SO4 solution at 350°C. Results indicate that there are several microstructural conditions that are susceptible to intergranular corrosion in this environment. Grain boundaries that contain a high concentration of segregated impurities, such as phosphorus without carbide precipitates, resulted in the highest susceptibility to corrosion. Thermal treatment at 704°C for 16 h following solution annealing at high temperatures caused the precipitation of chromium rich carbides at the grain boundaries with no significant chromium depletion. Such a treatment dramatically improved the corrosion resistance of both alloys. A comparison of the corrosion behaviour of the two alloys shows alloy 690 to have performed considerably better than alloy 600 in the test environment used.
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