Dependence of Stress Corrosion Cracking for Cold-Worked Stainless Steel on Temperature and Potential, and Role of Diffusion of Vacancies at Crack Tips

The growth rate of stress corrosion cracking (SCC) was measured for nonsensitized and sensitized, cold-worked Type 316 (UNS S31600, CW316) and Type 304 (UNS S30400, CW304) in hydrogenated and oxygenated pressurized water reactor (PWR) primary water, each containing standard boron and lithium additions. First, the dependences of crack growth rates on temperature were measured in low- and high-potential water in the range from 250°C to 360°C. Intergranular morphology was observed for nonsensitized CW316 in both low- and high-potential water. However, no significant growth of SCC was observed for sensitized CW316 in low-potential water. Similar 1/T temperature dependencies were observed both in hydrogenated PWR primary and oxygenated water for nonsensitized CW316 in the temperature range between 250°C and 330°C. However, the peak rate occurred about 330°C to 350°C. The peak temperature seems to shift to a lower temperature with decreasing cold work in hydrogenated PWR primary water. Second, to assess what appeared to be a common temperature dependency in both hydrogenated and oxygenated water, grain boundary creep (GB creep) was studied in air using CW316; intergranular creep cracking (IG creep cracking) was observed after low-temperature creep tests in air. The crack occurred perpendicular to the direction of tensile stress. Cavities were observed ahead of creep crack tip and near the creep crack. Finally, to determine the cause of formation of the crack embryos, Auger electron spectroscopy (AES) analyses were performed at the tips of the creep and SCC. Significant nickel enrichment and iron and chromium depletion were observed at the tips of the creep crack, near the cavities, and at the tip of the stress corrosion crack when specimens were tested in hydrogenated and oxygenated, high-temperature water. These results suggest that diffusion of vacancies to the tip could be considered as one of the important growth processes for IGSCC in hydrogenated and oxygenated high-temperature water on nonsensitized CW316/304. The extent to which grain boundary diffusion interacts with other electrochemical processes is probably important but is not defined.