The Crevice Corrosion Behaviors And Mechanism Of The Steel X70with Sulfate-reducing Bacteria

Sulfate-reducing bacteria (SRB) is common microbe in soilenvironment. As it’s metabolism products—sulfides have strongcorrosivity, toxic and reactivation, SRB and the metabolic activitiesresult in serious environment problems and economic losses. Theberried pipelines under the disbanded coating contacts the soil withSRB unavoidably, it is necessary to research the effect of SRB oncrevice corrosion behavior of the pipeline steel.The weight-loss, SEM, EDS analysis and electrochemicalimpedance spectroscopy(EIS) technology were used to study corrosionbehavior of X70steel in the corrosive soil simulation solution with andwithout sulfate-reducing bacteria(SRB). The results showed that thecorrosion depth and speed were increased with time in the solutionwithout SRB. The corrosion product film was loose and protectiveness.In the solution with SRB, however, the compact and homogeneousbiofilm was formed, which could prevent mass transfer to mitigatecorrosion. The product film loosen and fallen down because ofcorrosion product absorption and sulfide increasing with time, the basesteel corrosion was further promoted. The stability of corrosion productfilm was related to its deposited and distributed on the steel in thesolution without SRB. While the porous extracellular polymericsubstances(EPS) played an isolation role to the mass transfer during theactivated process.A rectangle crevice assembly was used to research the crevicecorrosion behavior of the steel of X70under the effects of cathodic polarization potential in0.1mol/LNaHCO3+Na2SO4+NaCl solution withand without sulfate-reducing bacteria(SRB). The change of potential ofsteel X70, pH value in the crevice with time were measured. It is foundthat the potential of steel X70decreased pH value of the solution andpotential difference increased when the applied cathodic potential wasmore negative. The potential of the X70steel in the crevice was morepositive without SRB than that in the solution with SRB, pH value ofthe solution decreased without cathodic potential. Potential difference inthe crevice was no much lower with SRB under-850mV appliedpotential. Potential of X70steel in the solution without SRBwas morenegative than that one, and pH value of the solution without SRBwashigher.Stress corrosion behaviors of X70steel are researched in thecorrosive soil simulation solution with and without SRB by SSRT andSEM methods. The results showed that X70steel had low SCCsensitivity and was protected better without SRB under-775mV. WhileX70steel had good cathodic protection under-850mV potential withSRB. Obviously, protective potential is more negative with SRB. TheSEM results showed that SCC sensitivity increased with the appliedcathodic potential moved negatively. In addition, X70steel SCCsensitivity was highest in solution and the crack mechanism of fromductile turned to brittle fatigue with SRB.