Repassivation and Crack Propagation of Alloy 600 in 288°C Water

Polarization and repassivation behaviors of alloy 600 (UNS N06600) were evaluated at 288°C in 0.1 M boric acid (H₃BO₃) titrated with sodium hydroxide (NaOH, pH_25°C = 7.9) as a function of dissolved hydrogen (0 cm³/kg to 48 cm³/kg [0 ppm to 2.7 ppm] H₂) 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 H₂. At 0 cm³/kg H₂, dissolved Zn at 60 wppb reduced the current density at ~ –650 mV_SHE. However, 60 wppb Zn did not affect repassivation kinetics at 0 cm³/kg and 48 cm³/kg H₂. Repassivation kinetics experiments conducted slightly above the open-circuit potential (E_OC) revealed a monotonic decrease in the oxidation current transient with increasing H₂ at short times. Reduced current transients at higher levels of H₂ agreed with the Pourbaix diagram showing Ni metal becoming more stable at potentials associated with H₂ addition. Residual oxidation transients exhibited with 18 cm³/kg (1 ppm) and 48 cm³/kg H₂ likely were associated with H₂ 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°C as H₂ content increased from 0 cm³/kg to 18 cm³/kg, indicating slip oxidation was the mechanism for crack advance.