Effect of Hydrogen on the Intergranular Stress Corrosion Cracking of Alloy 600 in High Temperature Aqueous Environments Available to Purchase

Slow strain rate tests (SSRTs) on differently shaped tensile specimens machined from two heats 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, using a recirculating autoclave system. In addition, split half tube specimens cut from the same two heats of Alloy 600 were subjected to reverse U-bend testing in pure water and steam at 363 and 400 C, respectively, using two static autoclaves. In both types of tests, intergranular stress corrosion cracking (IGSCC) was found to occur predominantly at sites in the alloy where a complex state of stress and strain existed. During the SSRTs, the strained portions of specimens absorbed much more hydrogen than the unstrained portions. Still greater contents of hydrogen were found at sites close to the fracture zone. The IGSCC susceptibility and the crack growth rate both increased with increasing partial pressure of hydrogen; however, absorption of hydrogen by the alloy also occurred in absence of hydrogen gas in the system. Results of this study suggest that hydrogen plays an essential role in the IGSCC mechanism.