Repassivation and crack propagation of alloy 600 in 288 C water
- GE Corporate Research and Development Center, Schenectady, NY (United States)
The polarization and repassivation behavior of Alloy 600 was evaluated at 288 C in 0.1 M boric acid titrated with NaOH (pH{sub 25 C} 7.9) as a function of dissolved hydrogen (0 to 48 scc/Kg) and zinc (O 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 {minus}800 to {minus}550 mV{sub SHE}, with the addition of H{sub 2}. At a H{sub 2} content of 0 scc/Kg, dissolved zinc at 60 wppb reduced the current density at {approximately}{minus}650mV{sub SHE}. However, 60 wppb zinc did not effect the repassivation kinetics at 0 and 48 scc/Kg of H{sub 2}. Repassivation kinetics experiments conducted slightly above the open circuit potential, E{sub OC}, revealed a monotonic decrease in the oxidation current transient with increasing H{sub 2} at short times. Reduced current transients at higher levels of H{sub 2} agree with the Pourbaix diagram that shows Ni metal becoming more stable at potentials associated with the addition of H{sub 2}. The residual oxidation transients exhibited with 18 and 48 scc/Kg of H{sub 2} are most likely associated with H{sub 2} 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 H{sub 2} 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.
- OSTI ID:
- 367698
- Report Number(s):
- CONF-960389--
- Country of Publication:
- United States
- Language:
- English
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