采用宏观形貌观察、化学成分分析、电化学性能测试、腐蚀产物分析等,对某天然气井口"Z"型管管异径接头环焊缝的泄漏原因进行分析,结合3D建模和腐蚀模拟试验对法兰面均匀腐蚀和焊缝局部腐蚀进行了综合分析。结果表明:法兰、异径接头、管体材质化学成分与组织正常,焊条选择与母材相匹配,未发生未焊透和未熔合缺陷;法兰装配后在两法兰端面间产生装配间隙,这是天然气中的腐蚀介质进入该间隙导致法兰面发生均匀腐蚀的原因;法兰与异径接头对焊错边量较大,异径接头与管体内焊缝余高明显,进而导致管件内错边和内焊缝余高处产生湍流和流体回流现象,在流体冲刷和腐蚀的作用下加剧了焊缝金属的损失,从而在焊缝内形成凹槽,导致焊缝壁厚快速减薄,最终在焊缝上发生穿孔泄漏。

The leakage reason of the circumferential weld of the “Z” type reducing joint at a natural gas wellhead was analyzed by means of macro morphology observation, chemical composition analysis, electrochemical performance test, corrosion product analysis, etc. The uniform corrosion of the flange surface and localized corrosion of the weld were also comprehensively analyzed by combining 3D modeling and corrosion simulation test. The results showed that the chemical composition and structure of flange, reducer joint and pipe body materials were normal, the selection of welding rod matched the base metal, and there were no incomplete penetration and incomplete fusion defects. After the flange was assembled, there was an assembly gap between the two flange end faces, which was the reason why the corrosion medium in natural gas entered the gap and led to uniform corrosion on the flange surface. The butt weld misalignment of flange and reducer joint was large, and the weld reinforcement of reducing joint and pipe body was obvious, which led to turbulence and fluid backflow in the pipe fittings and the weld reinforcement, which aggravated the loss of weld metal under the effect of fluid scouring and corrosion, thus forming grooves in the weld joint, resulting in rapid thinning of weld wall thickness, and finally perforation and leakage on the weld.