Influence of anion and sulfate-reducing bacteria on the stress corrosion behavior and mechanism of X70 steel in a marine mud environment

Subsea pipeline steel was severely corroded due to the presence of huge quantities of Cl-, SO42-, and sulfate-reducing bacteria (SRB) in marine mud. In this paper, the synergistic effects of Cl-, SO42- and SRB on the stress corrosion cracking (SCC) behavior of X70 steel in a simulated solution of marine mud in the South China Sea were examined using slow strain rate tensile (SSRT) tests, surface morphology analysis, and combined electrochemical experiments. The results indicated that the sensitivity of the specimens to stress corrosion increased as the concentration of Cl- rose. Cl- not only encouraged pitting and intensified the corrosion autocatalytic process but also influenced the number of surviving SRB. At a concentration of 20 g/L, the greatest number of SRB survived in the present investigation. However, when the Cl- concentration was 30 g/L, the anodic dissolution rate was the quickest and the SCC sensitivity was the highest, due to the synergistic impact of the high Cl- concentration and SRB. SO42- functions as an electron acceptor for SRB in the electrochemical reaction process and influences the adsorption of Cl-. Its rising concentration sped up the anodic dissolving process and increased the sensitivity of SCC.