Hydrogen Pickup During Oxidation in Aqueous Environments: The Role of Nano-Pores and Nano-Pipes in Zirconium Oxide Films
Hydrogen Pickup During Oxidation in Aqueous Environments: The Role of Nano-Pores and Nano-Pipes in Zirconium Oxide Films
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DOI：
10.1016/j.actamat.2019.09.005
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发表时间：
2019-04
期刊：
影响因子：
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通讯作者：
Jing Hu;Junliang Liu;S. Lozano-Perez;C. Grovenor;M. Christensen;W. Wolf;E. Wimmer;Erik V. Mader
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文献类型：
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作者：
Jing Hu;Junliang Liu;S. Lozano-Perez;C. Grovenor;M. Christensen;W. Wolf;E. Wimmer;Erik V. Mader
金属被水氧化会产生氢，氢可以进入固体，导致其机械性能严重退化，还可能影响腐蚀速度。本工作主要研究水环境中锆合金腐蚀过程中的吸氢行为。利用菲涅耳成像的透射电子显微镜对三种不同的氧化锆样进行了研究，研究了氧化层中纳米孔隙率的类型、分布、浓度和连通性随氧化层深度的变化。在氧化物的非保护性外层发现了大量相互连接的纳米管，而在靠近金属-氧化物界面的保护性阻挡层中，连续的纳米管变成了单独的纳米孔。从头算表明，分子氢是由水与氧化锆中的氧空位反应自发形成的。分子动力学模拟表明，这些H_2分子可以通过氧化层中小至0.5 nm的纳米孔和纳米管快速扩散。计算表明，分子氢在实验上发现的金属-氧化物界面上的亚氧化物表面自发解离。氧化锆中的氧空位使氢原子能够进入和扩散，其势垒约为65molkJ/ 。氢在氧饱和的α-Zr金属中的进一步扩散速度很快，导致热力学上稳定的锆氢化物的形成。因此，分子氢的形成和扩散通过大块氧化物中的纳米孔和通过亚氧化物进入H原子是任何被包含纳米孔隙率的氧化膜覆盖的金属或合金中氢吸收的可能机制。
Oxidation of metals by water generates hydrogen which can enter the solid causing serious degradation of its mechanical properties and may also influence the corrosion rate. The present work focuses on hydrogen pickup during the corrosion of zirconium alloys in an aqueous environment. Transmission electron microscopy using Fresnel imaging on three different samples of oxidized Zr has been used to study the type, distribution, concentration and connectivity of nano-porosity as a function of depth through the oxide layer. Extensive interconnected nano-pipes are found in the non-protective outer part of the oxide, while in the protective barrier layer closer to the metal-oxide interface, continuous nano-pipes turn into individual nano-pores.Ab initiocalculations show that molecular hydrogen is formed spontaneously by the reaction of water with oxygen vacancies in zirconium oxide. Molecular dynamics simulations reveal that these H2molecules can diffuse rapidly through nano-pores and nano-pipes as small as 0.5 nm in the oxide layer. Calculations demonstrate that molecular hydrogen dissociates spontaneously on surfaces of suboxides found experimentally at the metal-oxide interface. Oxygen vacancies in ZrO enable the ingress and diffusion of H atoms with an energy barrier of approximately 65 kJ/mol. Further diffusion of hydrogen through oxygen-saturatedα-Zr metal is fast, leading to the formation of thermodynamically stable zirconium hydrides. Thus, formation and diffusion of molecular hydrogen through nano-pores in the bulk oxide and ingress of H atoms via suboxides is a possible mechanism of hydrogen pickup in any metal or alloy covered by an oxide scale that contains nano-porosity.