锆合金初始氧化行为的原位近常压XPS研究

锆基合金由于具有低的热中子吸收截面、良好的耐腐蚀性能和力学性能等优点，通常被用于水冷核反应堆中的核燃料包壳和其他结构材料。通过在合金中添加适量的Nb元素可以有效地降低锆合金的氧化速率和吸氢分数，从而改善锆合金的耐腐蚀性能。尽管对锆合金的耐腐蚀性能得到了广泛的认识，但关于其在接近真实氧化腐蚀条件下的原位研究一直是具有挑战性的课题。本工作中利用近常压X射线光电子能谱(NAP-XPS)原位研究了1.3 × 10^-8 - 1.3 × 10^-1 mbar (1 mbar = 100 Pa)连续分压下室温到623 K温度时两种锆基合金表面在水，氧中的初始氧化腐蚀行为。结果表明，未添加Nb和添加1%Nb的锆合金表面在初始氧化过程中锆元素都会由金属态向多种氧化态过渡。水蒸气环境下两种合金的氧化速率都要低于氧气环境。室温下无论水蒸气还是氧气环境两种合金的氧化速率都要比623 K高温情况下的慢。在623 K的氧气气氛下，未添加Nb的锆合金相较于添加1%Nb的锆合金更容易被氧化，Nb的添加一定程度上会降低氧物种的吸附能力。在室温下和623 K低水蒸气压力下，1%Nb锆合金氧化速率更快，Nb促进OH^-在表面生成。而在623 K高水蒸气压力下，未添加Nb的锆合金有更易于被氧化的倾向，Nb在高温下向表面扩散并抑制OH^-键的断裂，但两种样品表面短时间内都无法被完全氧化。

In Situ Study on the Initial Oxidation Behavior of Zirconium Alloys with Near-Ambient Pressure XPS

Zirconium alloys are often used to fabricate nuclear fuel cladding and other structural materials because of their low thermal neutron absorption cross section, satisfactory corrosion resistance, and decent mechanical properties. The oxidation rate and hydrogen-absorption fraction of zirconium alloys can be reduced by adding moderate amount of Nb to them, and the corrosion resistance of zirconium alloys can be improved as well. Although the corrosion resistance of zirconium alloys has been widely recognized, the in situ study of zirconium alloys in conditions that resemble real oxidative-corrosion environments has still been a challenging subject. The initial oxidation behavior of zirconium alloys might affect the subsequent generation of oxides in the form of the element valence and type of surface oxides changes, resulting in the long-term corrosion-behavior changes. In addition, the reaction mechanism of Nb in zirconium alloys is still controversial. To investigate the influence of the alloy composition and environmental conditions on the initial oxidation behavior of zirconium alloys, in situ initial oxidation experiments were performed on two different Zr alloys in a near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) chamber. The samples were cut to the size of 12 mm × 3 mm, and the primary oxide film was removed via pickling, argon etching and annealing. Oxygen or water vapors with the pressure in the range of 1.3 × 10^-8-1.3 × 10^-1 mbar (1 mbar = 100 Pa) were gradually introduced into the NAP-XPS chamber after sample-surface cleaning. The experiment was repeated at room temperature (300 K) and 623 K. The results showed that both Nb-containing and Nb-free zirconium-alloy surfaces transitioned from a metallic state to various oxidation states during the initial oxidation process. The oxidation rates of both the alloys were lower in water vapors than those in oxygen. In the presence of water vapors or oxygen, both the alloys oxidized more slowly at room temperature than at 623 K. Compared with 1%Nb zirconium alloys, the Nb-free zirconium alloys were more easily oxidized and had a denser oxide layer, in the oxygen atmosphere at 623 K. To some extent, the presence of Nb would reduce the adsorption capacity of oxygen atoms. The oxidation rate of 1%Nb zirconium alloy was quick at room temperature and also at low water vapor pressures at 623 K; Nb promoted the formation of OH^- at the surface. Under the high pressure vapor atmosphere at 623 K, the Nb-free zirconium alloys were more prone to be oxidized; Nb diffused to the surface at high temperatures and inhibited the breaking of the OH^- bond; however, the surfaces of both the samples could not be completely oxidized in a short time.