Abstract
The effect of strain distribution on stress corrosion cracking (SCC) susceptibility of Inconel® Alloy 600 c-ring was investigated both in caustic (50% NaOH+ 1% Na2CO3 at 320°C) and acid (boiling 25% HNO3) exposures. By using the computer-aided x-ray rocking curve analysis method sampling a large grain population, the determination of strain distribution of laboratory c-ring deformed to closure was carried out as a function of the angular position along the periphery of the c-ring surface layer. The correlation between SCC susceptibility and applied strain was evaluated by scanning electron microscopy, x-ray photography and the x-ray rocking curve analyzer. It was found that the SCC mode was strongly related to the exposure environment, and the susceptibility depended significantly on the heat treatment and the applied strain level. Micropitting occurred in the mill annealed and the thermally treated Inconel Alloy 600 when they were exposed to the caustic solution. The largest micropitting density was found at or near the apex region, and the micropitting density decreased as a function of the position away from the apex. Thermally treated Inconel Alloy 600 seemed to be more resistant to caustic pitting corrosion. Intergranular attack of Inconel Alloy 600, however, occurred in acid exposure. The results of x-ray topographs and x-ray rocking curve halfwidth implied that sub-domains and residual elastic strains in the deformed grains were created in the caustic attack, whereas planar dislocation density increased during intergranular attack in acid exposure. These tendencies were more significent as the applied strain increased.
