In this case study, we have conducted the failure analysis of corroded steel tubes of natural gas risers subjected to a period of service in urban distribution network. Visual inspections and optical microscopy indicated that a severe corrosion was detected on the contact area covered with voids, cracks and corrosion products under a protective rubber cap inserted between the steel tubes and an outer freestanding supportive steel tube. The corrosion products were composed of rust (Fe₂O₃ and Fe₃O₄) as well as elemental chlorine, sulphur and phosphor. The water adsorption examination conducted on tape-coated tubes acquired from operating sites revealed remarkably more water adsorption compared to samples prepared in laboratory according to the standard. Based on potentiometric and potentiodynamic polarization studies, we find that water ingress and its adsorption on the tubes cause the failure. The contaminated water then accumulates under the rubber cap and corrosion under insulation (CUI) initializes the corrosion, which will then be associated with crevice corrosion owing to the presence of atmospheric corrosive species. Prevention of corrosion primarily include changing the design of risers by the elimination of rubber cap to avoid water penetration and accumulation, the implementation of standard tape wrapping in combination with additional anti-corrosion coatings, and application of cathodic protection.

在本案例研究中，我们对在城市配电网中服务了一段时间的天然气立管腐蚀的钢管进行了失效分析。目视检查和光学显微镜检查表明，在插在钢管和外部独立式支撑钢管之间的防护橡胶帽下，在覆盖有空隙，裂纹和腐蚀产物的接触区域上检测到严重腐蚀。腐蚀产物由铁（Fe ₂ O ₃和Fe ₃ O ₄）以及元素氯，硫和磷。对从操作现场获取的胶带涂层管进行的吸水检查显示，与实验室根据标准制备的样品相比，吸水量明显增加。基于电位和电位动力学极化研究，我们发现进水及其在管上的吸附会导致失效。然后，被污染的水积聚在橡胶盖下，绝缘层腐蚀（CUI）引发腐蚀，由于大气腐蚀性物质的存在，这将与缝隙腐蚀相关。防止腐蚀主要包括通过消除橡胶盖来改变立管的设计，以避免水渗透和积聚，