Repassivation and crack propagation of alloy 600 in 288 degrees C water

Polarization and repassivation behaviors of alloy 600 (UNS N06600) were evaluated at 288 degrees C in 0.1 M boric acid (H3BO3) function of dissolved hydrogen (0 cm(3)/kg to 48 cm(3)/kg lo ppm to 2.7 ppm] H-2) and Zn (0 wppb and 60 wppb). Potentiodynamic scans measured polarization behavior, while a combination of drop-weight straining and cathodic reduction/potential pulse techniques measured repassivation behavior. Potentiodynamic scans revealed larger current densities, especially over the range from -800 mV(SHE) to -550 mV(SHE), with addition of H-2. At 0 cm(3)/kg H-2, dissolved Zn at 60 wppb reduced the current density at similar to -650 mV(SHE). However, 60 wppb Zn did not affect repassivation kinetics at 0 cm(3)/kg and 48 cm(3)/kg H-2. Repassivation kinetics experiments conducted slightly above the open-circuit potential (E-oc) revealed a monotonic decrease in the oxidation current transient with increasing H-2 at short times. Reduced current transients at higher levels of H-2 agreed with the Pourbaix diagram showing Ni metal becoming more stable at potentials associated with H-2 addition Residual oxidation transients exhibited with 18 cm(3)/kg (1 ppm) and 48 cm(3)/kg H-2 likely were associated with H-2 oxidation on the oxide surface, which was unaffected by time. Repassivation results were consistent with recent observations of a decrease in the crack growth rate (CGR) of alloy 600 in water at 288 degrees C as H-2 content increased from 0 cm(3)/kg to 18 cm(3)/kg, indicating slip oxidation was the mechanism for crack advance.