Environmentally assisted cracking in linepipe steels

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The design of linepipe steel microstructures that resist environmentally assisted cracking is important for the construction of reliable gas transmission pipelines that carry sour gas. The phenomena of hydrogen induced cracking (HIC) and stress corrosion cracking (SCC) involve interactions between hydrogen originating in the sour gas and impurities in the steel such as non-metallic inclusions and precipitates. Special 'sour service' steels are produced that are alloyed to reduce the absorption o hydrogen and have high internal cleanness. The current general understanding of the precise mechanisms involved in HIC and SCC initiation limit steel producers' ability to develop more effective products at higher strengths or lower cost. The proposed project plan is designed to provide a detailed understanding of the chemical/metallurgical mechanisms involved in the initial stages of HIC and SCC. Micro-galvanic activity in the vicinity of native and synthetic inclusions will be measured as a function of inclusion and base steel composition. At the same time bulk steel behaviour in standard HIC/SCC tests will be assessed and related to the micro-galvanic understanding to predict test results from inclusion statistics. This relationship between micro-scale and bulk behaviour will represent an advancement in the field that will provide guidance to steel producers for targeted inclusion engineering in ladle treatment operations that will improve HIC and SCC performance. As a result of improved steel qualities in respect of environmentally assisted HIC and SCC Canadian production and application of value-added steel materials will be enhanced so as to benefit domestic manufacturing activities as well as to lower long term corrosion related costs in the operation of Canadian energy distribution infrastructure.