SCC of X70 and Its Deteriorated Microstructure in Simulated Acid Soil Environment

In order to study the stress corrosion cracking (SCC) of X70 pipeline steel and its weld joint in acid soil environment of southeast of China, two simulating methods were used here. The one was to obtain the bad microstructures in heat affected zone by annealing at 1300 degrees C for 10 min and air cooling to room temperature, the other was to get a series of simulating solutions of the acid soil environment. SCC susceptibilities of X70 pipeline steels before and after being normalized in the simulated solutions were studied by slow strain rate test (SSRT) and microstructural observation of fracture areas. Potentiodynamic polarization curves were used to study the electrochemical behaviour of different microstructures. SCC does occur to both the as-received material and normalized microstructure after heat treatment as the polarization potential decreased. Hydrogen embrittlement (HE) is indicated occurring to all tested materials at -850 mV (nu s SCE) and -1200 mV(nu s SCE). The SCC mechanisms are different within varying potential range. Anodic dissolution is the key cause as polarization potential higher than null current potential, and HE will play a more important role as polarization potential lower than the null current potential.