Cathodic Protection Effect on the Assessment of SCC Susceptibility of X52 Pipeline Steel
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Cathodic Protection Effect on the Assessment of SCC Susceptibility of X52 Pipeline Steel A. Contreras, E. Sosa and M. A. Espinosa-Medina Instituto Mexicano del Petróleo, Programa de Integridad de Ductos, Eje Central Lázaro Cárdenas Norte No. 152, Col. San Bartolo Atepehuacan, C.P. 07730, México. Tel: +52 (55) 9175-8194, Fax: +52 (55) 9175-6429 E-mail: [email protected]
ABSTRACT Assessment of anodic and cathodic potentials on stress corrosion cracking (SCC) of API X52 pipeline steel through slow strain rate tests (SSRT) was studied. The SSRT were carried out in a NS4 solution to simulated dilute ground water that has been found to be associated with SCC of pipelines. SSRT were performed and evaluated in air and in the NS4 solution at room temperature at an extension rate of 1x10-6 in/sec. Tests were performed at controlled electrochemical polarization potentials, both anodic and cathodic (100, 200, 400 mV) versus the open circuit corrosion potential. The results of reduction in area ratio (RAR), time to failure ratio (TFR) and plastic elongation ratio (PER) of the specimens tested in the soil solution indicate that X52 pipeline steel was susceptible to SCC at cathodic potentials. These specimens showed a brittle type of fracture with transgranular appearance. The SCC proceess and mechanism of X52 steel in the NS4 solution is mixed-controlled by both anodic dissolution and the hydrogen involvement. At positive potentials the SCC is based mainly on the anodic dissolution mechanism. When the applied potentials shifted negatively, the SCC on the steel follows mainly hydrogen embrittlement mechanism. This mechanism was confirmed through the internal cracks observed in the specimens. Keywords: Steel, Coating, Corrosion, Embrittlement, Stress/Strain relationship. INTRODUCTION Stress corrosion cracking (SCC) in transmission pipelines is a complex failure mode for both gas and liquid pipelines. It occurs under two broad conditions: alkaline pH and near-neutral pH [1-2]. As many existing gas and liquid pipelines approach the end of their design life, pipeline operating companies are looking for ways of extending lives of these pipelines through detection and prediction of the main factors affecting SCC. Stress-corrosion cracking and external corrosion have been implicated in several field failures. Consequently, the researchers have invested considerable effort to both understand and mitigate future related failures. To establish the effectiveness of cathodic protection in mitigating stress corrosion cracking, the X52 pipeline steel was studied. To assess the effects of overprotection and underprotection several SSRT were performed at different cathodic and anodic potentials. SSR tests were conducted in simulated soil solution called NS4, and service experience should be obtained so that a correlation between SSR test results and anticipated service performance can be developed. In many cases the SSR test has been found to be a conservative test for SCC. Therefore, it may produce failures in the laboratory under conditions whic
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