Failure analysis and preventive recommendations against corrosion of steel tubes of gas risers in natural gas urban distribution lines

Abstract 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.