Oil and gas pipelines manufactured from API‐5L Grade X65 steel are generally subjected to cyclic loading and their internal surfaces are frequently exposed to corrosive sour fluids. Exposure of pipelines to these environments often leads to localized corrosion (pitting) and decreased fatigue life. Corrosion pits are geometrical discontinuities that may promote fatigue cracking by acting as stress raisers. In order to optimize asset inspection and repair scheduling, it is important to understand the fatigue behavior of X65 steel and in particular, the ability to predict the crack initiation from corrosion pit. To achieve this level of understanding, conducting fatigue tests in an environmental condition replicating the field environment is important. This paper presents the test protocol and results of environmental fatigue testing using bespoke laboratory apparatus to undertake in situ corrosion fatigue tests in a sour corrosive environment under uniaxial loading. The environment selected represent processes that are likely to occur at internal surfaces of oil and gas pipelines exposed to production fluids. The tests were carried out on smooth samples to obtain S‐N curve in this specific environment as well as on pre‐pitted samples. An electrochemical method is used to create corrosion pits on the samples. Also, a model is proposed to predict the crack initiation life from corrosion pit, using a local stress‐based technique, which has been validated by experimental test results. Post‐test fractography was carried out by scanning electron microscopy (SEM). The performance of our approach is demonstrated. The innovation is anticipated to encourage other workers to employ similar small‐scale tests requiring toxic and challenging test environments.

中文翻译：

---

X65钢在酸性腐蚀环境下的疲劳—预测腐蚀坑疲劳裂纹萌生寿命的新实验和分析方法

由API-5L X65级钢制造的油气管道通常会承受周期性载荷，其内表面经常暴露于腐蚀性酸性流体中。管道暴露在这些环境中通常会导致局部腐蚀（点蚀）并降低疲劳寿命。腐蚀坑是几何上的不连续点，可通过充当应力提升剂来促进疲劳裂纹。为了优化资产检查和维修计划，重要的是要了解X65钢的疲劳性能，尤其是能够预测腐蚀坑产生裂纹的能力。为了达到这种理解水平，在复制现场环境的环境条件下进行疲劳测试非常重要。本文介绍了使用定制实验室设备在酸性腐蚀环境下单轴载荷下进行原位腐蚀疲劳测试的环境疲劳测试的测试规程和结果。选择的环境表示可能在暴露于生产流体的油气管道内表面上发生的过程。测试是在光滑的样品上进行的，以在此特定环境中以及在预先挖坑的样品上获得S-N曲线。使用电化学方法在样品上产生腐蚀点。此外，还提出了一种模型，该模型使用基于局部应力的技术来预测腐蚀坑的裂纹萌生寿命，该模型已通过实验测试结果进行了验证。测试后的断层扫描是通过扫描电子显微镜（SEM）进行的。我们的方法的性能得到了证明。预计这项创新将鼓励其他工人采用类似的小规模测试，这些测试需要有毒且具有挑战性的测试环境。