轴向应变作用下含腐蚀缺陷X80管道氢渗透的有限元分析

目的 研究在役天然气管道掺氢输送条件下氢原子在管道腐蚀缺陷处的扩散和分布情况及应变对腐蚀缺陷处氢扩散行为的影响。方法 利用COMSOL软件，将固体力学模型和扩散模型相结合，建立了基于有限元的氢原子扩散渗透模型，研究了在纵向拉伸应变作用下X80钢制管道不同尺寸腐蚀缺陷处氢原子的分布情况。结果 在没有拉伸应变的情况下，氢原子一旦进入管道内，在浓度梯度的驱动下，沿径向梯度扩散到管道内。当在管道上施加应变时，氢原子的扩散受到静水应力的驱动。氢原子在腐蚀缺陷处的最大浓度超过了进入管道的氢原子初始浓度。结论 氢原子在腐蚀缺陷处聚集。此外，施加的拉伸应变也影响氢原子聚集的位置，随着缺陷长度的减小和深度的增大，在内壁腐蚀缺陷处，更多的氢原子会集中在缺陷中心和尖端。

Finite Element Analysis of Hydrogen Permeation in X80 Pipeline with Corrosion Defects under Axial Strain

One potential issue in using existing natural gas pipelines to transport hydrogen in the form of hydrogen blending natural gas is hydrogen damage of high-strength pipeline steel. The hydrogen damage of pipelines is closely related to the diffusion and trapping of hydrogen atoms in pipeline steel. The diffusion and distribution of hydrogen atoms at corrosion defects in in-service natural gas pipelines under the condition of hydrogen blending transportation, and the effect of strain on hydrogen diffusion behavior at corrosion defects are unclear. In this paper, a hydrogen atoms diffusion model was established based on the finite element method by COMSOL software to combine the solid mechanics model and the diffusion model. The distribution of hydrogen atoms at different sizes of corrosion defects in X80 pipelines under longitudinal tensile strain was studied. The mechanical curve of X80 steel was obtained through experiments before modeling. This article adopted the Ramberg-Osgood (R-O) relationship for X80 steel in order to obtain better computational efficiency and accuracy, and to better describe the nonlinear mechanical properties of the material. For further analyzing the effect of strain on the hydrogen atom diffusion at the corrosion defect, a geometric model containing corrosion defects on the inner wall of the pipeline was established, in which the thickness of the pipeline wall was 12.7 mm and the length of the pipeline section was 3 000 mm. For boundary conditions, assuming that no hydrogen atoms initially entered the steel, the initial diffusion surface was the interface between hydrogen and the inner surface of the pipeline (i.e. the inner wall of the pipeline), and the initial diffusion hydrogen atom concentration was 10 mol/m3. The loads were 0%, 0.1%, 0.2%, 0.3%, 0.5%, 0.8%, and 1% longitudinal tensile strains to simulate the effect of ground motion. The local strain at the corrosion defect was variable. Afterwards, the grid was divided and solved using the direct solver MUMPS. Analyzing the results could lead to the following conclusions:under the free state, once hydrogen atoms entered the steel, they would diffuse radially driven by concentration gradients. When the pipeline suffered from the strain, the diffusion of hydrogen atoms was driven by hydrostatic stress. The maximum concentration of hydrogen atoms at the corrosion defect exceeded the initial concentration of hydrogen atoms, indicating that the hydrogen atoms accumulated at the corrosion defect. For the corrosion defects on the inner wall of the pipeline, the application of tensile strain would greatly change the distribution of hydrogen atoms in the pipeline. The maximum concentration of hydrogen atoms always occurred near the center of the corrosion defect and near the outer wall of the pipeline, while the concentration of hydrogen atoms was lower in other areas of the pipeline. As the length of the inner wall corrosion defect decreased and the depth increased, more hydrogen atoms concentrated at the center and tip of the corrosion defect. When the depth of the defect was constant, the accumulation of hydrogen atoms at the corrosion defects on the inner wall of the pipeline became more and more obvious as the length of the defect decreased. In other words, narrow corrosion defects tended to aggregate more hydrogen atoms. Similarly, when the defect length was constant, the concentration of hydrogen atoms increased as the corrosion defect deepened.