Effect of inclusions on initiation of stress corrosion cracks in X70 pipeline steel in an acidic soil environment

The effect of inclusions on the initiation of stress corrosion cracking (SCC) X70 pipeline steel was investigated in an acidic soil solution using slow strain rate test, scanning electron microscopy and energy-dispersive X-ray techniques. The results demonstrated that stress corrosion cracks are not initiated in X70 steel when it is under anodic polarization. At cathodic polarization, hydrogen evolution is enhanced, and hydrogen is actively involved in SCC processes. Two types of inclusions exist in the steel and play different role in crack initiation. The inclusions enriching in Al are brittle and incoherent to the metal matrix. Microcracks and interstices are quite easily to be resulted in at the boundary between inclusions and metal. There is no crack initiating at inclusions containing mainly Si.