Stress Corrosion Cracking of Welded API X70 Pipeline Steel in Simulated Underground Water

This paper investigates the stress corrosion cracking (SCC) behavior of welded API X70 pipeline steel in simulated underground water using the slow strain rate test, fractographic characterization by scanning electron microscopy, and potentiodynamic polarization techniques. SCC susceptibility of the heat-affected zone (HAZ) is demonstrated to be dependent on two factors: the effect of the microstructure in the HAZ on electrochemical reactions and the effect of the mechanical property on SCC occurrence. Electrochemical experiments indicate that the microstructures in the HAZ, especially the softened microstructure, can significantly facilitate the processes of hydrogen evolution when cathodic potential is positive to −1050 mV_SCE. However, when the cathodic potential is below −1050 mV_SCE, the cathode current densities of different microstructures are close to one another and greatly increase because of the decrease of the applied potentials. The SCC behavior is consistent with the electrochemical results. Under −650 and −850 mV_SCE, SCC is most likely to occur in the softened region, and under −1200 mV_SCE, SCC occurs in both the softened and hardened regions.