Stress Corrosion Cracking of X70 Pipeline Steel immersed in Synthetic Soil solution

摘要

A study of stress corrosion cracking (SCC) susceptibility of API X70 pipeline steel exposed to a synthetic soil solution at room temperature and atmospheric pressure through slow strain rate tests (SSRT) was carried out. Electrochemical noise (EN) measurements were recorded during SSRT in order to study the SCC process. Additionally, polarization curves (PC) to evaluate the corrosion were carried out. Characteristic current transients were identified and assigned to different processes. EN measurements were analyzed in the time and frequency domain. The currents transients and localization index (LI) indicate that at the beginning of the test, in the elastic and yielding strength zone, the corrosion is dominant by not much transients with low amplitude and frequency. At the maximum strength (UTS) and before fracture, the presence of transients increased. The highest corrosion rate, obtained by R_n and Z_n, were obtained in the plastic zone before of the facture. A superficial analysis obtained by scanning electron microscopy (SEM) was carried out on surface of the samples and it revealed the presence of some micro-cracking in the gage section of SSRT specimens. According to SCC index and SEM observations the X70 steel has low SCC susceptibility at the conditions studied.