Mechanism Of Stress Corrosion Of X80 Steel In Presence Of Alternating Current And Sulfate-reducing Bacteria

Due to the space geography constraints,an increasing number of pipelines are installed across or in parallel to high-voltage transmission lines or electrified railways.It is often inevitable that buried pipelines are subjected to interference from the alternating current(AC)induced by the neighboring power facilities.AC-induced corrosion,microbiological induced corrosion(MIC),and stress corrosion cracking(SCC)are important forms of corrosion failure for buried pipelines,which have received growing concerns from scholars throughout the world.However,the interaction between multifactor is often ignored by the researchers.And the study related to the stress corrosion behavior of pipeline steel under the combined effect of AC and microorganisms is still rarely reported.In this work,a simulated device for AC corrosion was established,and weight-loss test,electrochemical test,constant load tensile,slow strain rate tensile test(SSRT),and surface analysis technology were used to investigate the effect of AC frequency on the corrosion behavior of X80 pipeline steel in Dagang coastal saline soil,the corrosion behavior of X80 pipeline steel under the combined action of AC and sulfate-reducing bacteria(SRB),the corrosion behavior of X80 pipeline steel under the combined action of external stress,AC and SRB,and the stress corrosion behavior of pipeline steel under the combined action of cathodic protection,AC and SRB.The main results are as follows:1.AC inhibited the metabolism and growth of SRB,and the count of sessile SRB cells decreased with the AC density increasing.The cathodic protection potential had an effect on the number of sessile SRB cells,and the effect was slight when the cathodic protection potential was-850 mV.With the negative shift of the cathodic protection potential,the number of sessile SRB cells decreased significantly.The number of sessile SRB cells decreased sharply under the combined effect of AC and cathodic protection potential.2.The AC accelerated the corrosion of X80 pipeline steel in the saline soil,and the corrosion rate decreased with the AC frequency increasing.There is a power function relationship between the corrosion rate and the AC frequency.The corrosion product films formed on the surface of the specimen at low AC frequency were easier to breakdown than that at high AC frequency,which induced more severe pitting corrosion.The corrosion products of X80 pipeline steel under conditions with different AC frequencies mainly consisted of SiO2,FeOOH,Fe2O3,and Fe3O4.3.The corrosion of the X80 steel affected by AC was much more severe than that affected by SRB,and applying AC caused much severe pitting corrosion with a maximum pit depth of 25.0 μm.A thick layer of corrosion-product film was formed on the specimen surface when AC was applied,thus creating an environment with organic carbon starvation.The SRB sessile cells were forced to reduce sulfate with iron as the electron donor through the extracellular electron transfer(EET)process.As a result,the SRB and AC synergistically exacerbate the corrosion of the steel specimen,with the maximum pit depth increasing to 76.2 μm.4.SRB cells and AC can facilitate the pitting corrosion of X80 pipeline steel.These formed pitting holes can be used as the source of crack initiation,and the cracks formed begin to propagate once the stress is applied.AC,SRB and tensile stress can produce synergistic effects to aggravate the corrosion of X80 pipeline steel.The corrosion rate of X80 pipeline steel did not increase with the increase of AC density under the combined action of AC,SRB and elastic stress.When the AC density was 200 A·m-2,the count of sessile SRB cells was reduced by five orders of magnitude compared with that without AC interference,and the synergistic effect of AC,SRB and elastic stress was weakened.When the AC density was 100 A·m-2,the count of sessile SRB cells still at a high level.The synergistic effect of AC,SRB and elastic stress exacerbate the X80 pipeline steel corrosion.Therefore,within the AC density range set in the experiment,the corrosion rate of X80 pipeline steel was the highest when the AC current density was 100 A·m-2.The corrosion products of X80 pipeline steel in conditions with different AC current densities were mainly composed of SiO2,Fe3O4,FeOOH,and FeS.5.When the cathodic protection potential was-850 mV,the SCC susceptibility of X80 pipeline steel decreased compared with that under open circuit potential.As the cathodic protection potential moved to a more negative value,the SCC susceptibility of X80 pipeline steel increased.Under the same cathodic protection potential,the X80 pipeline steel with AC interference had higher SCC susceptibility than that without AC interference.