Investigation of Hydrogen Behavior in Relation to the PWSCC Mechanism in Alloy TT690

The mechanism of primary water stress corrosion cracking (PWSCC) in Alloy TT690 was studied from the viewpoint of the influence of hydrogen behavior and cavity formation. Four parameters were examined: cavity formation at crack tips; deformation in the presence of hydrogen gas; dynamic strain effect on mechanical properties; and nickel plating effect on PWSCC propagation. The correlation that was observed between cavity formation at crack tips and crack growth rates indicates the cavities have an important influence on cracking. Results for creep tests done in the presence of hydrogen gas reveal hydrogen lowers the elasticity and makes the alloy brittle at the stress concentrated region. Results for slow strain rate tensile tests indicate that the dynamic strain which accompanies the hydrogen source increases the apparent yield strength. Additionally, nickel plating, which might change corrosion behavior on the alloy surface, suppresses the crack propagation. These observations provide direct support for hydrogen and cavity related mechanism that account for PWSCC of Alloy TT690.