| As a part of the United States of America's Department of Energy (DOE) thrust to assess the long-term disposal of nuclear waste at the proposed waste repository in Yucca Mountain, Nevada, this study focuses on crevice corrosion of Ni-Cr-Mo alloys in particular, Alloy-22. Electrochemical data were complimented by surface analytical data to create a crevice corrosion damage function under exposure conditions similar to those anticipated in the repository. Due to the alloy's strong tendency to repassivate, even in extremely corrosive conditions (5 M NaCl, 120°C), crevice corrosion did not initiate and propagate at an appreciable rate. Consequently, a constant current was applied to initiate and sustain crevice propagation.;Imaging of crevice corroded surfaces by Confocal Laser Scanning Microscopy (CLSM) revealed that the damage morphology varies with applied current, with the applied current being inversely proportional to the maximum penetration depth but proportional to the corrosion-damaged area. Crevice corrosion products were shown to influence the propagation and repassivation processes and, consequently, the damage morphology. Raman imaging was used to characterize the corrosion products formed in areas of preferential attack. The peaks in the Raman signals were attributed to the formation of polymeric molybdates. These polymeric species appear to limit propagation, thereby allowing local repassivation and influencing the distribution of damage. The formation rate of insoluble polymeric molybdates, which is proportional to the applied current, appears to dictate the overall damage morphology.;Electron Back-Scatter Diffraction (EBSD) was used to assess whether grain orientation within the polycrystalline alloy was important in determining why crevice corrosion initiated at intergranular sites. The alloy was shown to posses a high fraction of sum3 grain boundaries, which are known as 'special' boundaries due to their extraordinary properties compared to 'non-special' random boundaries, and crevice corrosion was shown to initiate preferentially at these latter locations.;The anodic oxide film was characterized by Electrochemical Impedance Spectroscopy (EIS) and X-ray Photoelectron Spectroscopy (XPS). A potential threshold for oxide film breakdown (E ≥ 200 mVAg/AgCl) was established and linked to an increase in the dissolution rate of higher valence cations leading to the segregation of alloying components within the film. |
