EFFECT OF HYDROGEN ON THE INTERGRANULAR STRESS-CORROSION CRACKING OF ALLOY-600 IN HIGH-TEMPERATURE AQUEOUS ENVIRONMENTS

Slow strain rate tests (SSRTs) on differently shaped tensile specimens of Alloy 600 tubing were conducted at 350 C in deaerated aqueous solutions containing small amounts of lithium hydroxide and boric acid at different partial pressures of hydrogen. In addition, tube specimens were subjected to reverse U-bend testing in pure water and steam at 363 and 400 C, respectively. Intergranular stress corrosion cracking (IGSCC) was found to occur predominantly at sites where a complex state of stress and strain existed. During the SSRTs, the strained portions of specimens absorbed more hydrogen than the unstrained portions. Still greater contents of hydrogen were found at sites close to the fracture zone. The IGSCC susceptibility and crack growth rate increased with increasing partial pressure of hydrogen; however, absorption of hydrogen also occurred in the absence of hydrogen gas in the system. Results suggest that hydrogen plays an essential role in the IGSCC mechanism.