Abstract
The polarization and repassivation behavior of Alloy 600 was evaluated at 288°C in 0.1 M boric acid titrated with NaOH (pH25°C 7.9) as a function of dissolved hydrogen (0 to 48 scc/Kg) and zinc (0 and 60 wppb). Potentiodynamic scans measured the polarization behavior, while a combination of drop-weight straining and cathodic reduction/potential pulse techniques measured the repassivation behavior. Potentiodynamic scans revealed larger current densities, especially over the range of -800 to -550mVSHE, with the addition of H2. At a H2 content of 0 scc/Kg, dissolved zinc at 60 wppb reduced the current density at ~-650mVSHE. However, 60 wppb zinc did not effect the repassivation kinetics at 0 and 48 scc/Kg of H2. Repassivation kinetics experiments conducted slightly above the open circuit potential, EOC, revealed a monotonic decrease in the oxidation current transient with increasing H2 at short times. Reduced current transients at higher levels of H2 agree with the Pourbaix diagram that shows Ni metal becoming more stable at potentials associated with the addition of H2. The residual oxidation transients exhibited with 18 and 48 scc/Kg of H2 are most likely associated with H2 oxidation on the oxide surface, which is unaffected by time. The repassivation results are consistent with recent observations of a decrease in the crack growth rate of Alloy 600 in water at 288°C as the H2 content increased from 0 to 18 scc/Kg, indicating that slip oxidation is the mechanism for crack advance under the conditions investigated in this study.
