Low Alloy X100 Pipeline Steel Corrosion and Passivation Behavior in Bicarbonate-Based Solutions of pH 6.7 to 8.9 with Gr
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INTRODUCTION
WITH increased demand for energy in the form of hydrocarbon fuels, greater amounts need to be transported in pipelines existing between basins and demand centers. The development of high strength low alloy (HSLA) steels is motivated by the need for newer pipeline steels with higher strength but also adequate toughness and good corrosion resistance.[1] HSLA steels provide increased strength-to-weight ratios over conventional low-carbon steels for only a modest price premium.[2] Their superior mechanical properties are achieved through thermo-mechanical rolling and accelerated cooling, whereas low amounts of alloying can improve corrosion resistance amongst other properties. This improved performance creates opportunities for thinner-walled pipelines with greater design throughput, since the material can withstand higher stresses and resist chemical deterioration and mechanical degradation better. This has encouraged pipeline companies to rely on HSLA steels for their upcoming large projects.[3] API-X100 is a new grade in a series of HSLA steels, IBRAHIM M. GADALA, PhD Candidate, is with the Department of Materials Engineering, The University of British Columbia, 3096350 Stores Road, Vancouver, BC V6T 1Z4, Canada. AKRAM ALFANTAZI, Professor, is with the Department of Materials Engineering, The University of British Columbia, and also with the School of Materials Science and Engineering, China University of Mining and Technology, Xuzhou 221116, P.R. China. Contact e-mail: [email protected] Manuscript submitted December 23, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
including X80 and X70, made for pipeline applications. It is characterized by a minimum yield strength of 100 ksi (690 MPa). Increased grain refinement, higher strength, and different alloy compositions of X100 affect its mechanical and corrosion performances. Hence, characterization of its corrosion resistance and electrochemical behavior in field-relevant environmental conditions is needed, and is valuable to industry. When the external coating protecting buried pipelines deteriorates or disbonds, corrosive groundwater electrolyte comes into contact with exposed steel sections. Research has shown that among the anions which are present in the surrounding soil, bicarbonate (HCO3) has a strong influence on the external corrosion of buried pipelines. Linter and Burstein reported that HCO3 is a major agent in the dissolution of pipeline steels, and contributes in the formation of iron carbonate (siderite, FeCO3).[4] For common pipeline steel grades such as X70 or X80, passivation occurs in concentrated HCO3 solutions simulating the electrolyte in contact with exposed areas of buried pipelines[5] and responsible for high pH (9 to 11) stress corrosion cracking (SCC). In such situations, field investigations have found that the steel is typically covered in white FeCO3[6] and most often shows signs of localized pitting. In spite of accelerated corrosion at local sites, the protectiveness of the FeCO3 limits the corrosion rate (CR) in these cond
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