Assessment on external corrosion rates for API pipeline steels exposed to acidic sand-clay soil

Purpose: The purpose of this paper is to study the susceptibility to corrosion processes of X60, X65 and X70 steels immersed in sand-clay soil with pH 3.0, using electrochemical techniques, scanning electron microscopy (SEM), energy dispersive spectroscopy and X-ray diffraction (XRD). Design/methodology/approach: Natural acidic soil sample was collected as close as possible to buried pipes (1.2 m in depth) in a Right of Way from south of Mexico. Both steels and soil were characterized through SEM and XRD. Then, open circuit potential was recorded for all steels exposed to soil at different exposure times. Thus, the electrochemical impedance spectroscopy (EIS) was traced, and anodic polarization curves were obtained. Findings: The steel corrosion processes started when the active sites were exposed to natural acidic soil. However, corrosion rates decreased for three steels as immersion time increased, obtaining the highest corrosion rate for X60 steel (0.46 mm/year for 5 h). This behavior could be attributed to corrosion products obtained at different exposure times. While, 5 h after removing corrosion products, X65 steel was more susceptible to corrosion (1.29 mm/year), which was corroborated with EIS analysis. Thus, corrosion products for the three steels exposed to natural acidic soil depended on different microstructures, percentage of pearlite and ferrite phases, in which different corrosion processes could occur. Therefore, the active sites for carbon steel surfaces could be passivated with corrosion products. Practical implications: The paper identifies the any implication for the research. Originality/value: Some anodic peaks could be caused by metallic dissolution and was recorded using high positive polarization (high field of perturbation). In addition, the inductive effects and diffusion process were interpreted at low frequency ranges using EIS. According to X-ray diffraction (XRD), acidic soil had Muscovite containing aluminum and iron phases that were able to generate hydrogen proton at the presence of water; it might be promoted at the beginning of deterioration on low carbon steels. Steel surface cleaning after removing corrosion products was considered to study the possible diffusion phenomena on damaged steel surfaces using EIS.