Analyzing the effect of long-term operation of gas pipeline of X70 steel on stress-corrosion cracking susceptibility in near-neutral pH solution under cathodic polarization

Stress-corrosion cracking of the parent metal of linear section of main gas pipeline of X70 steel after operation for 40 years in near-neutral pH solution NS4 was investigated in comparison with the pipe that was not in operation and sheet steel. The tensile strength and yield strength all studied specimens meet the requirements of ISO 3183, but the relative elongation the specimens from pipes is slightly below the normalized value. Corrosion potentials are similar and equal to -0.697 V for sheet steel, -0.681 V for new pipe, -0.689 for exploited pipe, and the corrosion rates are 0.053, 0.064 and 0.056 mm/year, respectively, which corresponds to the resistance group "stable". Stress-corrosion cracking at potentials negatively than the maximum protective -1.050 V occurs by the hydrogen mechanism for a new pipe, and for the sheet and pipe after 40 years of operation - by mechanism of anodic dissolution and hydrogen embrittlement with a small contribution of anodic dissolution. At the potential of -1.050 V, hydrogen penetration is greatest for the sheet steel (0.045 mol/dm3), for the new pipe and pipe after 40 years of operation - values are similar, 0.0296 and 0.0241 mol/dm3, respectively.