某N80油管接箍腐蚀断裂失效分析

Failure Analysis of Corrosion and Fracture of N80 Tubing Coupling

针对某N80 EU油管接箍腐蚀断裂的现象,通过宏观形貌分析、化学成分分析、金相及夹杂物分析、力学性能分析、扫描电镜和能谱分析、接箍螺纹结构分析,结合其服役工况,综合分析了其接箍腐蚀断裂的失效原因。结果表明:该失效接箍的化学成分、力学性能均符合相关标准要求,金相组织未见异常且无金属夹杂;其腐蚀是由内向外腐蚀,且以局部腐蚀为主,腐蚀产物是以铁的氧化物以及硫化物为主。该接箍失效原因是由于含有Cl^(-)、CO_(2)、SRB等腐蚀介质的井液在管柱内压作用下通过接箍中间缝隙进入齿顶和齿底的螺旋形泄露通道,在以缝隙腐蚀为主的腐蚀环境+轴向载荷共同作用下导致接箍断裂。建议优化螺纹结构,防止或减轻缝隙腐蚀对圆螺纹油管接头的影响,解决接箍腐蚀及断裂失效问题。

In view of the corrosion fracture of a N80 tubing coupling,the failure reasons of the corrosion fracture of the coupling are analyzed comprehensively through the analysis of its macroscopic morphology,chemical composition,metallography and inclusion,mechanical properties,SEM and energy spectrum,and the structure of the thread of the coupling.The results show that the chemical composition and mechanical properties of the failure coupling meet the requirements of relevant standards,and the metallographic structure is not abnormal and there is no metal inclusion;the corrosion is mainly from the inside to the outside,and the corrosion products are mainly iron oxides and sulfide.Under the action of internal pressure of pipe string,the well fluid containing Cl^(-),CO_(2) and SRB enters the screw leakage channel of the tooth top and the bottom through the gap between the coupling,and finally leads to the fracture of the coupling under the joint action of corrosion environment and axial load.It is suggested that the screw structure should be optimized,the influence of crevice corrosion on the round thread pipe joint should be prevented or reduced,and the corrosion and fracture failure of the coupling should be solved.