Effect of sulfate-reducing bacteria and cathodic potential on stress corrosion cracking of X70 steel in sea-mud simulated solution

The submarine oil and gas pipelines steel buried in sea mud are always influenced by microorganisms and cathodic protection potential. In this paper, the single and combined effect of sulfate-reducing bacteria (SRB) and cathodic potential on stress corrosion cracking (SCC) behavior of X70 pipeline steel in sea-mud simulated solution was investigated by slow strain rate tests and fractographic observation. Whether in sterile or SRB-inoculated solution, the SCC susceptibility at − 850 mVSCE showed a decline compared with that at open circuit potential. The reason is that the lower number of corrosion pits and the slighter hydrogen evolution at − 850 mVSCE inhibited both of the cracks nucleation and propagation. The SCC susceptibility of X70 steel increased drastically as the cathodic potential shifted from − 850 mVSCE to − 1200 mVSCE in sterile or SRB-inoculated solution. SRB assisted pitting corrosion and the promotion effect on hydrogen permeation make SRB plays a positive role in promoting SCC susceptibility. The SCC susceptibility can be elevated in the co-existence of SRB and cathodic potential but the combined action between them became limited as the potential shifted negatively, which was attributed to the fact that alkalization of solution caused by cathodic potential restricts the growth of SRB.