Environmental aspects of near-neutral pH stress corrosion cracking of pipeline steel
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INTRODUCTION
PIPELINE steel for high-pressure gas transmission is susceptible to stress corrosion cracking (SCC) in two basic forms: high pH (or classical) SCC and low (or near-neutral) pH SCC. The former has been extensively studied since the mid-1960s and is related to intergranular fracture in concentrated carbonate-bicarbonate solutions with a pH in the range of 9.5 to 12.5.[1,2,3] The latter was first reported in 1985[4] and has been responsible for an increasing number of pipeline failures in recent years. In comparison to the high pH form of SCC, relatively little research has been performed to understand the mechanism of nearneutral pH cracking or the controlling factors in crack initiation and propagation. [5] Extensive field investigation[4,5,6] has shown that nearneutral pH SCC is characterized by wide transgranular cracks with evidence of quasicleavage fracture. Colonies of longitudinal cracks form in response to the hoop stress resulting from the internal pressure. The electrolyte found under disbonded coatings is a dilute carbonate-bicarbonate solution containing small quantities of chloride and sulfate W. CHEN, Assistant Professor, is with the Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB, Canada T2E 7K7. Contact e-mail: [email protected] F. KING, Senior Research Scientist, and T.R. JACK, Team Leader, are with NOVA Research & Technology Corporation, Calgary, AB, Canada T6G 2G8. M.J. WILMOTT, Senior Research Scientist, formerly with NOVA Research & Technology Corporation, is with Bredero Price, Houston, TX 77032-3130. Manuscript submitted January 17, 2000.
METALLURGICAL AND MATERIALS TRANSACTIONS A
with a pH in the range from 5.5 to 8.5. Moreover, a relatively thick, white deposit of iron carbonate is observed on removing the coating from those regions where cracking is present. The development of near-neutral pH SCC is a synergistic result of stress, material, and the environment. Among these factors, however, a prerequisite for crack initiation is that an appropriate environment has come into contact with the surface of the pipeline. For this to occur, the soil electrolyte must penetrate any protective coating that has been applied to the pipeline. Coatings that disbond allowing groundwater to access the underlying steel surface and that shield the pipe surface from the cathodic protection system are most likely to allow near-neutral pH SCC.[4] The role of the local soil environment in the initiation and early growth of near-neutral pH SCC is not clear. Site selection models have been developed to help predict the location of SCC on a pipeline, but it is not certain which specific parameters are responsible for making a soil environment conducive to SCC. To date, site selection models have been based on the correlation of site characteristics with the occurrence of near-neutral pH SCC based on a large amount of field information gathered at past SCC sites. Underlying processes that cause SCC are not explicitly included in this approach. The aim of the
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