Yi-Ming Pan

Localized Corrosion Susceptibility of Alloy 22 in Chloride Solutions: Part 1—Mill-Annealed Condition

Abstract Alloy 22 (UNS N06022) is a Ni-Cr-Mo-W alloy developed for corrosion resistance in a variety of aggressive environments. Because of its excellent corrosion resistance, Alloy 22 was selected as a candidate container material for the disposal of high-level nuclear waste at the proposed repository at Yucca Mountain, Nevada. The localized corrosion susceptibility of mill-annealed Alloy 22 was assessed in chloride-containing solutions at temperatures ranging from 60°C to 150°C by comparing the crevice corrosion repassivation potentials with corrosion potentials measured in separate tests. Crevice corrosion repassivation potentials were found to be strongly dependent on temperature, chloride concentration, and nitrate-to-chloride molar concentration ratio. Corrosion potentials were dependent on solution pH but independent of chloride concentration.

Localized Corrosion Susceptibility of Alloy 22 in Chloride Solutions: Part 2—Effect of Fabrication Processes

Abstract The effect of fabrication processes on the localized corrosion susceptibility of Alloy 22 ([UNS N06022] 56Ni-22Cr-13.5Mo-3W-4Fe), a candidate container material for the disposal of high-level nuclear waste in the proposed repository at Yucca Mountain, Nevada, was assessed in chloride-containing solutions at temperatures ranging from 60°C to 95°C. The results of tests covering a range of metallurgical conditions, including as-welded, welded plus solution-annealed, and thermally aged materials, were compared with those for the as-received mill-annealed alloy. The localized corrosion susceptibility of Alloy 22 was determined by comparing the crevice corrosion repassivation potentials with corrosion potentials measured in separate tests. The crevice corrosion susceptibility was found to be dependent on temperature, chloride concentration, and the nitrate-to-chloride molar concentration ratio. Welding and short-term thermal aging decreased the localized corrosion resistance of Alloy 22 compared with t...

Fatigue crack growth mechanisms in HSLA-80 steels

Fatigue mechanisms of large and small cracks in Cu-bearing HSLA-80 steels were studied at ambient temperature. Fatigue striations were measured for both large and small fatigue cracks by SEM fractography, while dislocation structures adjacent to the fatigue surfaces were characterized by TEM. The results of the fatigue striation and dislocation structure characterization are compared with crack growth data to assess the fatigue mechanisms in HSLA-80 steels and the cause for the lack of a threshold in the small cracks. Comparisons revealed that both large and small cracks propagated via an intermittent growth mechanism. The growth kinetics of small cracks were consistent with the extrapolation of the power-law regime of the large crack data to stress ranges below the large-crack threshold. The absence of a growth threshold in small cracks was discussed in conjunction with the large-crack threshold.

Computation of Ni-Cr phase diagramvia a combined first-principles quantum mechanical and CALPHAD approach

A first-principles quantum-mechanical computational code has been used to compute the energy of formation for selected ordered and topologically close-packed (TCP) phases in a Ni-base alloy. The thermodynamic data are incorporated into an existing database for Ni alloys and used in conjunction with the CALPHAD approach to compute the binary Ni-Cr phase diagram. In addition, a thermodynamic model is used to treat long-range ordering and the formation of the Ni2Cr. The phase field for ordering is compared against that predicted by the Thermo-Calc to elucidate possible implications on the long-term phase stability of Ni-base alloys in a nuclear waste repository environment.

Localized Corrosion Susceptibility of Alloy 22 as a Waste Package Container Material

Alloy 22 is the material preferred by the U.S. Department of Energy for the waste package outer container for geological disposal of high-level radioactive waste at the proposed repository site in Yucca Mountain, Nevada. The susceptibility of Alloy 22 to localized corrosion is an important consideration in the evaluation of the waste package behavior and the assessment of the overall performance of the proposed repository. The effects of the environment chemical composition and temperature on localized corrosion susceptibility were examined by measuring the repassivation potential for crevice corrosion in chloride-containing solutions at temperatures ranging from 80 to 150°C. The effect of potentially inhibiting anionic species, such as nitrate, was also determined. In addition to the mill annealed material, tests were conducted on both welded and thermally aged material to evaluate microstructural effects related to container fabrication processes. The resistance of Alloy 22 to localized corrosion decreased with increasing temperature and chloride concentration. Welding and thermal aging also decreased the localized corrosion resistance of the alloy.

Chemical synthesis and characterization of nanosized titanium aluminide

Abstract Titanium aluminides are among the more promising intermetallics for use in aerospace and automotive applications; however, their acceptance has been hampered by their lack of ductility. Significant improvement in ductility may be obtained from nanostructured intermetallics. Reaction of Ti[N(SiMe 3 ) 2 ] 3 with excess alane produces a precursor (Compound 1) to titanium aluminide. We propose compound 1 to be a loose cluster (or family of clusters) in which titanium and aluminum atoms are both bonded directly and bridged by imidosilanes. This chemically synthesized precursor, when heated to 1000 °C, produces nanosized particles of TiAl 3 . Nanosized TiAl 3 has been characterized by chemical analysis, solid-state NMR, X-ray diffraction, energy dispersive spectroscopy, and high-resolution electron microscopy.

Independent Evaluation of Waste Package Corrosion Performance Under Potential Repository Conditions

Abstract Long-term corrosion performance of the waste package is among the key engineered barrier system attributes of a potential high-level waste repository at Yucca Mountain, Nevada. Waste package degradation processes are evaluated on the basis of independent investigations conducted at the Center for Nuclear Waste Regulatory Analyses. This paper summarizes the results of laboratory measurements and model analyses focused on uniform, localized, and microbially influenced corrosion, and stress corrosion cracking of Alloy 22 (UNS N06022).

EVOLUTION OF CHEMISTRY AND ITS EFFECTS ON THE CORROSION OF ENGINEERED BARRIER MATERIALS

The evolution of environmental conditions within the emplacement drifts of a potential high-level waste repository at Yucca Mountain, Nevada, may be influenced by several factors, including the temperature and relative humidity within the emplacement drifts and the composition of seepage water. The performance of the waste package and the drip shield may be affected by the evolution of the environmental conditions within the emplacement drifts. In this study, tests evaluated the evolution of environmental conditions on the waste package surfaces and in the surrounding host rock. The tests were designed to (i) simulate the conditions expected within the emplacement drifts; (ii) measure the changes in near-field chemistry; and (iii) determine environmental influence on the performance of the engineered barrier materials. Results of tests conducted in this study indicate the composition of salt deposits was consistent with the initial dilute water chemistry. Salts and possibly concentrated calcium chloride brines may be more aggressive than either neutral or alkaline brines.

Corrosion Behavior of Waste Package and Drip Shield Materials

Abstract The susceptibility to various forms of corrosion that could be experienced by the alloys considered by the U.S. Department of Energy for the waste package and drip shield, the principal components of the engineered barrier system for the proposed repository at Yucca Mountain, is evaluated on the basis of experimental studies conducted at the Center for Nuclear Waste Regulatory Analyses. Environmental, metallurgical, and mechanical conditions for the occurrence of uniform corrosion, localized corrosion, and environmentally assisted cracking of Alloy 22 (58Ni–22Cr–13Mo–3W–4Fe), the preferred material for the outer container, and Titanium-Grade 7 (Ti–0.15 Pd), the alloy proposed for the drip shield, are reported.

Electrochemical Evaluation and Surface Characterization of Josephinite as a Natural Analog for Container Materials

A sample of josephinite, a rock containing predominantly a Ni-Fe metallic phase, was evaluated as a natural metal analog to increase confidence in the assessment of waste package performance for the potential high-level radioactive waste repository at Yucca Mountain, Nevada. The josephinite sample was characterized electrochemically in simulated groundwater environments using cyclic potentiodynamic polarization and potentiostatic tests. The passive surface layers formed potentiostatically were examined by X-ray photoelectron spectroscopy. These results were compared to those obtained with a synthetic Ni 3 Fe alloy with a chemical composition similar to that of josephinite. Electrochemical studies showed that josephinite exhibited passivity at a slightly higher pH than did the cast Ni 3 Fe alloy and was found to be slightly more susceptible to pitting corrosion. The passive films formed on the josephinite and the cast Ni 3 Fe alloy have a duplex structure consisting of an Fe-rich hydroxide outer layer and a Ni-rich oxide inner layer. Results obtained from this study provide an appropriate characterization of the environmental conditions leading to the passivity and localized corrosion of josephinite. Extended persistence of a stable passive film, however, is essential for the long-term stability of the josephinite sample. The relationship between the passive behavior and the formation of alteration layers needs to be established for assessing the survivability of josephinite.