The Oxygen and Hydrogen Diffusion Behavior in Cr-Coating by Density Functional Theory

In a pressurized water reactor, the corrosion chemical reaction between zirconium alloy cladding and water will adversely affect the mechanical properties of the cladding, thus limiting the service life of the fuel elements. In order to slow down the oxidation rate of the cladding and prevent the risk of hydrogen explosion, a conceptual design of accident tolerant fuel was proposed. Chromium metal has excellent corrosion and oxidation resistance, and has been widely used as cladding coating in the field of nuclear power. At present, the micro-mechanism of corrosion and oxidation resistance of chromium coating at high temperature is not clear, so it is urgent to carry out relevant research. In this paper, the diffusion mechanism of oxygen and hydrogen in chromium crystals has been investigated on the electronic scale by using the first principles method. Studies have shown that oxygen occupies the most stable position in the octahedral interstitial site, and hydrogen tends to occupy the tetrahedral interstitial site. The solubility of hydrogen is much lower than that of oxygen. The negative dissolution energy of oxygen in the interstitial site indicates that there is a strong mutual attraction between oxygen and the first nearest neighbor chromium. Further, the reaction-diffusion paths and migration energy barriers of oxygen and hydrogen are calculated by elastic band method. The oxygen diffuses from the tetrahedral interstitial site to the first nearest tetrahedral interstitial site along the reaction path, exhibiting a migration migration energy barrier of 0.79eV. Simultaneously, it also migrates from the tetrahedral interstitial site to the first near octahedral interstitial site with a migration energy barrier of .65eV. These findings suggest a preferential diffusion pathway from tetrahedral to octahedral interstitial site. Notably, hydrogen demonstrates comparable migration energy barriers (0.17eV) when moving along both the tetrahedral and octahedral interstitial sites in close proximity to each other. By employing Arrhenius diffusion equation, we establish a fitted relationship between temperature and diffusion coefficient, providing theoretical support for investigation coating corrosion properties at elevated temperatures.

在压水反应堆（pressurized water reactor）中，锆合金包壳与水发生的腐蚀化学反应会劣化包壳的力学性能，进而限制燃料元件的服役寿命。为减缓包壳的氧化速率、规避氢爆炸风险，学界提出了事故容错燃料（accident tolerant fuel）的概念设计方案。金属铬具备优异的耐腐蚀与抗氧化性能，已在核电领域被广泛用作包壳涂层。当前，高温环境下铬涂层的耐腐蚀与抗氧化微观机制尚未明晰，因此开展相关研究迫在眉睫。
本文采用第一性原理方法（first principles method），从电子尺度出发研究了铬晶体中氧与氢的扩散机制。研究表明，氧在八面体间隙位点（octahedral interstitial site）中占据最稳定的位置，而氢则更倾向于占据四面体间隙位点（tetrahedral interstitial site）。氢的溶解度远低于氧。氧在间隙位点中的溶解能为负值，表明氧与第一近邻铬原子间存在较强的相互吸引作用。
进一步地，本文采用弹性带法（elastic band method）计算了氧与氢的反应扩散路径及迁移能垒。氧沿反应路径从四面体间隙位点扩散至第一近邻四面体间隙位点，迁移能垒为0.79电子伏特（eV）。同时，氧也可从四面体间隙位点迁移至第一近邻八面体间隙位点，其迁移能垒为0.65电子伏特。上述结果表明，氧存在从四面体间隙位点向八面体间隙位点扩散的优先路径。值得注意的是，当氢在彼此邻近的四面体间隙位点与八面体间隙位点间迁移时，其迁移能垒相当（0.17 eV）。
本文采用阿伦尼乌斯扩散方程（Arrhenius diffusion equation）建立了温度与扩散系数间的拟合关系，为高温环境下涂层腐蚀性能的研究提供了理论支撑。