Hydrostatic pressure effects on stress corrosion cracking of X70 pipeline steel in a simulated deep-sea environment

The stress corrosion cracking (SCC) properties of X70 pipeline steel in simulated deep-sea environments was investigated in a high-pressure vessel with a constant loading device. The results reveal that the threshold stress, σSCC, for SCC is 0.89σb, 0.78σb and 0.68σb in a 3.5% NaCl solution (pH = 7) of 0.1 MPa, 5 MPa and 10 MPa, respectively. When the 3.5% NaCl solution pH value decreased to 3, σSCC decreased further to 0.83σb, 0.68σb and 0.53σb, corresponding to 0.1 MPa, 5 MPa and 10 MPa, respectively. These results indicate that the hydrostatic pressure (HP) promotes SCC, particularly in an acidified sodium chloride solution. Scanning electron microscopy (SEM) and confocal scanning laser microscopy showed that on the sample surface, the HP promoted pitting propagation and linkage, which then initiated microcracks in SCC. The hydrogen concentration was 0.10 ppm, 0.17 ppm and 0.23 ppm after a 200-h immersion in a 3.5% NaCl solution (pH = 7) of 0.1 MPa, 5 MPa and 10 MPa, respectively, revealing that the hydrogen concentration increased approximately linearly with the HP. This result indicated that much more hydrogen was involved in the SCC process in a solution under a higher hydrostatic pressure. The SCC mechanism is controlled by both anodic dissolution and hydrogen-induced cracking in deep-sea environment.