Crack Growth Behavior of Pipeline Steel in Near-Neutral pH Soil Environments
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I.
INTRODUCTION
ALTHOUGH it has been two decades since the cracking of pipeline steels in near-neutral pH environments was first discovered, mechanisms governing the cracking are still being debated. The cracking has been referred to as stress corrosion cracking (SCC),[1,2] probably because steel pipelines, particularly those for high-pressure gas transmission, are operated under a near-monotonic loading condition and because high-pH SCC in steel pipelines can be reproduced in the laboratory under static loading.[3,4,5] Crack growth models for pipeline steels exposed to near-neutral pH soil environments, such as the superposition model[6,7] and the crack tip strain rate model,[7,8,9] have also been developed on the assumption of SCC. The superposition model acknowledges the significance of cyclic loading by compounding a cyclic growth parameter with the contribution of crack growth under monotonic loading. The crack tip strain rate model has been successfully applied to several corrosion systems[10,11] under monotonic loading; this is a model in which the crack growth is a synergistic balance between the oxidation of metal-forming passivating films and the film fracture by plastic strain at the crack tip. For the cracking of pipeline steels in near-neutral pH soil environments, the crack tip strain rate has been correlated solely with the growth rate obtained under cyclic loading. The correlation is generally very poor, with scatter of up to several orders of magnitude.[7] For the current environment-metal system, no convincing experimental data are available to support the SCC mechanism. In fact, crack growth under monotonic loading WEIXING CHEN, Associate Professor, is with the Department of Chemical and Materials Engineering, University of Alberta, T6G 2G6, Edmonton, AB, Canada. ROBERT L. SUTHERBY, Senior Engineer, is with the TransCanada PipeLines Limited, T2P 5H1, Calgary, AB, Canada. Contact e-mail: [email protected] Manuscript submitted August 10, 2006. Article published online June 7, 2007. 1260—VOLUME 38A, JUNE 2007
has never been observed in pipeline steels when the steels are exposed to near-neutral pH soil environments. The investigation reported here involves the extensive experimental characterization of crack growth behavior of a pipeline steel in near-neutral pH environments and the development of a true corrosion fatigue crack growth model.
II.
EXPERIMENT
An X-65 pipeline steel was used in this study, the chemical composition (wt pct) of which is as follows: 0.12C, 1.5Mn, 0.017P, 0.0046S, 0.26Si, 0.014Cu,
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