CORROSION SCIENCE SECTION 932 CORROSION—OCTOBER 2007 Submitted for publication January 2007; in revised form, July 2007. ‡ Corresponding author. E-mail: stefanie.asher@gatech.edu. * School of Materials Science and Engineering, Georgia Institute of Technology, 771 Ferst Drive, Atlanta, GA 30332-0245. ** Batelle, 505 King Ave., Columbus, OH 43201. Investigating a Mechanism for Transgranular Stress Corrosion Cracking on Buried Pipelines in Near-Neutral pH Environments S.L. Asher, ‡, * B. Leis,** J. Colwell,** and P.M. Singh* ABSTRACT Buried carbon steel fuel transmission pipelines, protected by external coatings and cathodic protection, are known to experience transgranular stress corrosion cracking (TGSCC). Failure analysis of failed pipelines and laboratory tests have indicated that hydrogen plays an important role in the overall failure mechanism. However, the role of various groundwater constituents in TGSCC and the source of hydrogen are not well understood. The objective of this research was to further understand the mechanism of TGSCC by examining the en- vironments in which TGSCC can occur and near-neutral pH can be maintained. Specifically, this research focuses on the production of hydrogen through the interaction of iron with carbonic acid (H 2 CO 3 ) and bicarbonate (HCO 3 - ) as based on the Colwell-Leis mechanism. The Colwell and Leis mechanism attributes the hydrogen generation, in groundwater solutions, to the decomposition of bicarbonate and reaction with metal ions to form metal carbonate. Results from coupon exposure tests and slow strain rate tests indicate that TGSCC on pipe- line steels can be produced in simple bicarbonate solutions. Slow strain rate tests in simple bicarbonate solutions with 5% carbon dioxide (CO 2 ) produced cracks that have similar crack morphologies to what is found in the field. These results sug- gest that the environmental conditions for TGSCC and inter- granular stress corrosion cracking (IGSCC) on pipeline steel surface are related and may change from one to another by changes in the applied cathodic potential. KEY WORDS: carbon steel, iron carbonate, near-neutral pH stress corrosion cracking, pipelines, transgranular stress corrosion cracking INTRODUCTION Until the mid 1980s the dominant reported form of pipeline failure from the outside surface was in- tergranular stress corrosion cracking (IGSCC). The IGSCC mechanism was determined to be anodic dis- solution at the crack tip with preferential dissolution at the grain boundaries resulting in sharp intergranu- lar cracks. This type of cracking is associated with a concentrated environment of bicarbonate-carbonate solution with a pH greater than 9.3. 1 However, in the mid-1980s, a rupture of a natural gas pipeline in Northern Ontario revealed the presence of trans- granular stress corrosion cracking (TGSCC). Since then, TGSCC of pipelines has been reported in differ- ent parts of the world, including Australia, Iran, Iraq, Italy, Pakistan, Saudi Arabia, Russia, and the United States. 2 Subsequent reevaluation of prior failures has traced TGSCC of pipelines back to the late 1970s. Collection and analysis of solutions near failed pipe- lines suggest that the TGSCC failures are associated with dilute bicarbonate solutions with a pH around 7. It is this characteristic that leads to the alternative name for this failure of near-neutral pH stress corro- sion cracking (NNSCC). Significant amounts of white powder, predominantly iron carbonate (FeCO 3 ) with corrosion products, are typically found near TGSCC failures at the pipeline surface. 3 It has been reported that TGSCC was associated with pipeline sections 0010-9312/07/000169/$5.00+$0.50/0 © 2007, NACE International