The Role of Calcareous Soils in SCC of X52 Pipeline Steel

  • PDF / 1,016,482 Bytes
  • 12 Pages / 432 x 648 pts Page_size
  • 31 Downloads / 183 Views

DOWNLOAD

REPORT


The Role of Calcareous Soils in SCC of X52 Pipeline Steel A. Contreras1*, L. M. Quej-Aké1, C. R. Lizárraga2, T. Pérez2 1

Instituto Mexicano del Petróleo, Eje Central Lázaro Cárdenas Norte 152, Col. San Bartolo Atepehuacan, C.P. 07730, México D.F. * Contacting author email: [email protected] 2

Universidad Autónoma de Campeche, Campeche, México, ave. Agustín Melgar s/n, Col Buenavista, P.O. Box 24039, Campeche, México.

ABSTRACT Susceptibility to external stress corrosion cracking (ESCC) of API X52 pipeline steel in calcareous soil containing different moisture content has been investigated using slow strain rate tests (SSRT). This type of soil is common of the state of Campeche Mexico and has a pH around 8. The results indicate that X52 pipeline steel was susceptible to external SCC only in the saturated calcareous soil, showing some micro cracks in the gage section of the SSR specimen. It was observed that some micro cracks were found at the bottom of micro-pits. Which indicate that first develop a pit and this evolved with time and micro-strain like a crack. Few micro cracks were observed as initiation of SCC close to surface failure. The mechanism of SCC may be influenced by formation and rupture film of carbonates.

INTRODUCTION The investigation and studies on SCC originated by the soils in the current days have become of interest in the oil industry in Mexico due to several cases of SCC have been detected through smart pigs before to occur a failure in the pipeline [1]. SCC is dependant of the time and can develop in pipelines under normal operating condition when a coating disbonded and ground water comes into contact with the outside surface of the pipe. SCC is a type of environmentally assisted cracking that occurs when tensile stresses (mechanical factor) are applied to a susceptible material (metallurgical factor) which is exposed to corrosive environment (electrochemical factor). SCC can develop in pipelines under normal operating condition when a coating disbonded and ground water comes into contact with the outside surface of the pipe. SCC processes involve complex interactions between metallurgy, operational and residual stresses, external soil environment, and the electrolyte chemistry beneath disbonded coatings. During the years has been observed that some pipelines suffer cracks when they are subject to stresses (residual or operational) in a corrosive soil. SCC in pipelines has been found to occur in a range of soils, in a range of diameters, thicknesses, grades, manufacturing processes and coatings [2]. SCC produced by the soils must taking account many factors like the moisture content, pH, temperature, concentration of dissolved ions, carbonates content, redox potential, resistivity, level of cathodic protection, operational and residuals stresses and time of electrolyte in contact with the steel.

95

All pipe steels used for oil and gas transmission pipelines are susceptible to SCC [3]. The lower SCC susceptibility observed on some of the most recent pipes grades (X80 and X100) might be due t