Initiation of Stress Corrosion Cracking of Sensitized Type 304 Stainless Steel in Dilute Thiosulfate Solution

The corrosion potential of stressed sensitized Type 304 stainless steel has been studied in solutions containing oxygen to determine the relation between potential fluctuations and the initiation of stress corrosion cracking. An in situ scanning vibrating electrode technique was shown to detect currents coming from growing stress corrosion cracks in dilute thiosulfate solutions. It was observed that cracks, which had grown for a limited period and had repassivated did not act as reinitiation sites for continued stress corrosion cracking. Rather cracking progressed from other previously inactive sites. The onset of cracking was clearly observable by marked decreases in the potential which continued to drop if cracking remained active and increased when cracks repassivated. The crack sizes derived from potential transients were consistent with logarithmic nominal statistics indicating a broad distribution of sites susceptible to crack initiation.