Effect of Radiation on the Oxide Film Behavior of Nickel-Based Alloys, 18-9421

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Principal Investigators
James F. Dante
Narasi Sridhar
Darrell S. Dunn

Inclusive Dates:  09/15/03 - Current

Background - The effect of illumination (particularly UV radiation) on the semiconducting nature and properties of metal oxides has been the subject of much interest over the years.

Approach - The main focus of this work is to gain an understanding of the effect of gamma radiation on the electrochemical and semiconducting properties of nickel-based alloys during passivating conditions and to separate the indirect effects of radiolysis from the more direct effects of electronic excitation. This is especially important for long-term storage of nuclear waste and has applicability in the development and selection of proposed materials for use in nuclear reactors. The relationship between the semiconductor properties and the electrochemical properties of various Ni alloys is also being studied as a function of solution chemistry. The relationship between solution chemistry, electrochemical parameters, and corrosion has been relatively well defined. By using these established relationships, the role of oxide semiconductor properties in various corrosion processes can be confirmed and then used to gain a basic understanding of radiation effects on corrosion processes.

Accomplishments - Figure 1 shows a simplified schematic diagram of a semiconductor band model for charge transfer across a solution/oxide interface. The relationship between the flatband potential (Efb), the Fermi energy, and the redox potential in solution governs charge transfer and hence corrosion phenomenon. Solution composition strongly affects the Efb as a result of adsorption. For the n-type oxide semiconductor shown, fixing the solution chemistry (and hence Efb) and applying a positive voltage (reverse bias) will move the Fermi energy to lower values, increasing the band bending. At a critical value of the band bending, electrical breakdown of the oxide film will occur allowing charge transfer and perhaps corrosion of the substrate. Electromagnetic radiation impinging on the solution/oxide interface will result in both a shift in the flatband potential and the creation of minority carriers that will in turn affect the passive current density and the corrosion potential in solution. Efb can be measured by performing Mott-Schottky (M-S) scans. Additionally, M-S measurements will allow the determination of the oxide charge carrier density (Ni). A simple schematic of an M-S measurement is shown in Figure 2.

In Figure 3, M-S scans for alloy 600, alloy 690, and alloy 22 are shown. Alloys 600 and 690 are much more susceptible to localized corrosion compared with alloy 22. Open circuit potentials for all three alloys are approximately -0.5 V (SCE). The data suggest that the semiconductor properties of these nickel alloy oxide films is a complex phenomenon and is dependant on potential. It is evident that alloy 22 has a much higher charge carrier density than either alloy 600 or alloy 690 duringr anodic polarization conditions. Thus, it would take a much larger positive voltage to induce enough band bending to produce electrical breakdown of the oxide film and hence localized corrosion. It is also clear (particularly in the cathodic potential regime) that there is a negative shift in the flatband potential for alloy 22.

Figure 1. A simplified schematic of a semiconductor band model for an n-type semiconductor oxide that defines the energy levels at the solution/oxide interface and describes charge transfer across the interface.
Figure 2. A simplified schematic of a Mott-Schottky measurement. The x-intercept is used to define the flatband potential and the slope of the curve is used to define the oxide charge carrier density.
Figure 3. Mott-Schottky plots for alloy 600, alloy 690, and alloy 22 showing a significant reduction in charge carrier density in the anodic region and a negative shift in flatband potential for alloy 22 compared with the other alloys.

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