In-situ Observation and Atomistic Modelling of Early Stage Oxidation / Corrosion Behavior of Nickel Base Alloys in Nuclear Power Plants

Abstract

Atomistic modelling and experimental observation has been conducted in order to investigate the early stage oxidation/corrosion behavior on nickel-base alloy in nuclear power plants. In order to fundamental study for the oxidation phenomena on the nickel, high resolution X-ray reflectivity is adopted. To examine the atomistic structure at the Ni(110)/NiO-liquid interface using X-ray reflectivity, the crystallinity of the Ni(110) surface is a very important parameter. To improve the surface crystallinity of Ni(110), the surface treatment by electro-polishing and sequential Ar sputtering/annealing in an ultra-high vacuum chamber is developed. After the successful surface pre-treatment, X-ray measurement at room temperature is conducted using synchrotron X-ray at advanced phonon source in US. Firstly, Crystal Truncation Rod(CTR) test for Ni(110)/NiO surface at room temperature condition in helium gas environment is conducted to obtain reference condition, and pure water is injected into the test cell to measure the water effects on the sample surface. CTR data was measurable with low errors even at lower intensity region. This CTR data also confirms that the surface pre-treatment procedure developed for Ni(110) is suitable for the surface X-ray study. According to results from helium gas environment, there are relaxed Ni layers on the Ni(110) surface with 5Å thickness and then there are stressed Ni + NiO layers on the relaxed Ni layers with 5 Å, and13Å NiO amorphous or polycrystalline layer formed on the top layer. While, the peak position and value is changed when the water is applied. In other word, the position and electron structure of Ni layer at the middle are changed due to the water contact. It can be considered that the water makes change the lattice structure of the Ni surface and NiO To investigate the chromium effect on the oxidation behavior of Ni, atomistic modelling using first principle method is conducted. Using first principles approach, It has been simulated that the atomistic diffusion of oxygen in Ni-Cr binary alloy to understand the role of chromium during the oxidation of nickel-based alloys which are versatile in a wide range of application. The activation energy of oxygen diffusion is calculated by varying the number and position of the nearest-neighbor (NN) chromium atoms relative to oxygen along the diffusion pathway. The activation energy of oxygen diffusion is found to decrease with the increase in the number of NN chromium in front of oxygen, while that increases with increase in the number of NN chromium at the back of oxygen. Therefore, in this study, the first principles calculation confirms the role of chromium as a barrier for the atomistic diffusion of oxygen in Ni-Cr binary alloy. Finally, in-situ Raman spectroscopy has been applied in order to characterize the surface oxide film of nickel-base alloy/low alloy steel dissimilar metal weld interface in simulated primary water conditions of pressurized water reactors (PWRs). In order to directly examine the oxide film in high temperature aqueous conditions, an in-situ Raman spectroscopy system has been developed by constructing a hydrothermal optical cell with direct contact immersion optics.. For the verification of the constructed Raman system, high purity NiO, NiFe2O4, Cr2O3, and NiCr2O4 powders are examined to obtain reference spectra in room temperature air environment). The specimens were exposed to typical PWR water with 1,200ppm H3BO4 and 2 ppm of LiOH at a pressure of 15MPa and 300°C. In-situ Raman spectra were collected for interfaces of as-welded/thermally aged DMW in PWR water condition at 300°C during 50hrs. Cr2O3, Fe3O4 and FeCr2O3 were measured on as-welded DMW, while FexNi1-xCr2O4 and NiFe2O4 were measured on thermally aged DMW. From the ex-situ EDS measurements, the main compositions of the oxide layer after oxidation experiment are Ni and Cr on the as-welded Alloy152 and Fe for A533Gr.B, respectively, while Ni, Cr and Fe on the both of thermally aged Alloy152 and A533Gr.B. The difference of oxidation behavior by thermal aging was found and it was caused by diffusion-assisted chemistry redistribution by thermal aging. And the thermally aged DMW