REPASSIVATION AND CRACK-PROPAGATION OF ALLOY-600 IN 288-DEGREES-C WATER

Polarization and repassivation behaviors of alloy 600 (UNS N06600) wer e evaluated at 288 degrees C in 0.1 M boric acid (H3BO3) function of d issolved hydrogen (0 cm(3)/kg to 48 cm(3)/kg lo ppm to 2.7 ppm] H-2) a nd Zn (0 wppb and 60 wppb). Potentiodynamic scans measured polarizatio n behavior, while a combination of drop-weight straining and cathodic reduction/potential pulse techniques measured repassivation behavior. Potentiodynamic scans revealed larger current densities, especially ov er 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 repass ivation kinetics at 0 cm(3)/kg and 48 cm(3)/kg H-2. Repassivation kine tics 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 hig her levels of H-2 agreed with the Pourbaix diagram showing Ni metal be coming 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 w ith recent observations of a decrease in the crack growth rate (CGR) o f 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 f or crack advance.