Effect of pH and Temperature on Stress Corrosion Cracking of API X60 Pipeline Steel

Abstract

Stress corrosion cracking (SCC) is the result of the combined influence of tensile stress and a corrosive environment on a susceptible material. This paper analyzes the understanding mechanisms of stress corrosion cracking (SCC) of API X60 pipeline steel. The susceptibility to SCC and mechanism was investigated using slow strain rate tests (SSRT). The SSRT were performed at strain rate of 25.4 × 10−6 mm/sec in a glass autoclave containing a soil solution called NS4 with pH of 3 and 10 at room temperature and 50°C. Cathodic polarization potentials of −200 mV referred to Ecorr was applied. SCC properties were evaluated by SSRT in simulated soil solution (NS4 solution) that is typical environment for SCC under cathodic charging. The results of ratio reduction area (RRA), time to failure ratio (TFR) and elongation plastic ratio (EPR) indicate that X60 pipeline steel was more susceptible to SCC at pH 3 independently of the temperature. At high pH the SCC susceptibility was higher at 50°C. Scanning electron microscopy (SEM) observations of these specimens showed a brittle type of fracture with transgranular appearance. The failure and SCC mechanism of X60 steel into NS4 solution was hydrogen based mechanism. Hydrogen absorption by X60 pipe steel may result in local corrosion (primarily, SCC) of underground pipeline steel. SEM observations revealed the presence of internal cracks in some of these specimens.