Corrosion Behavior Of X80 Pipeline Steel Under The Coupling Action Of Mixed Bacteria And Stress

In the actual soil media,buried pipelines are in an extremely complex service environment.Microorganisms are important factors contributing to corrosion failure,among which sulfate reducing bacteria(SRB)are the most serious and the most extensively studied.As an important component of maintaining the global nitrogen balance,nitrate reducing bacteria(NRB)is also one of the main anaerobic microorganisms causing pipeline corrosion.Buried pipelines are affected by stress in addition to microorganisms.In addition,the pipeline surface biofilm is usually composed of a variety of bacteria,and its corrosion mechanism is quite different from that of a single bacterial environment.At present,the research on the corrosion behavior of X80 pipeline steel subjected to the coupling of SRB,NRB and stress has not been reported,and the corrosion mechanism needs to be clarified.In this paper,the effect of the coupled action of SRB,NRB and stress on the corrosion behavior of X80 steel and the mechanism of stress corrosion accelerated by mixed bacteria were determined by a self-made constant load experimental device with electrochemical testing,morphology observation and surface analysis.And the hydrogen permeation pattern and mechanism of X80 steel under the coupling effect of mixed microorganisms and stress were analyzed by means of a self-made hydrogen permeation-stress experimental device combined with cathodic protection technology.The main research progresses are as follows:1.The corrosion mechanism of X80 steel under the coupling effect of SRB and NRB was investigated.The results showed that both SRB and NRB could use iron as the electron donor source to obtain energy,resulting in severe pitting corrosion on the surface of X80 steel,and the average corrosion rate of X80 steel under NRB condition was lower than that under SRB condition and mixed bacteria condition.Furthermore,the antagonism between NRB and SRB was analyzed by thermodynamic calculation.When the two microorganisms existed at the same time,NRB was in a dominant position in the process of competing with SRB for energy substances,which would inhibit the growth of SRB,thus weakening its corrosion to X80 steel.In addition,the chemical reaction between nitrite,the intermediate metabolite of NRB,and sulfide was thermodynamically favorable,so nitrite could reduce the harm of galvanic corrosion of sulfide.2.The effect and mechanism of nitrate and nitrite on corrosion of X80 steel under the coupling action of SRB and NRB were studied.The results showed that nitrite had a more significant inhibitory effect on SRB cells compared with nitrate.In the inoculation environment,the average corrosion rate of X80 steel increased with the increase of nitrate concentration,and decreased with the increase of nitrite concentration.The increase of nitrate concentration accelerated the corrosion of X80 steel,which was due to the fact that nitrate promoted the growth of NRB cells,resulting in serious NRB corrosion of the specimen.The increase of nitrite concentration inhibited the corrosion of X80 steel.The reason was that the high concentration of nitrite made the X80 steel surface form the y-Fe2O3 protective film,preventing the occurrence of corrosion.It should be noted that nitrate in the sterile environment still aggravated the corrosion of X80 steel,suggesting that nitrate was a dangerous inhibitor in this environment.3.The effects of elastic stress and yield stress on the corrosion of X80 steel under the coupling of SRB and NRB were studied.It was found that the number of microbial cells on the specimen surface changed under the external stress,and the number of sessile cells and the thickness of biofilm increased under yield stress compared with the elastic stress.In addition,the applied stress also enhanced the electrochemical activity of X80 steel,resulting in negative shift of the corrosion potential,and the stresselectrochemical effect was more obvious under yield stress.In the inoculated environment,microorganisms and yield stress synergistically accelerated the corrosion of X80 steel,and the most serious is SRB condition.SRB and yield stress together led to the formation of secondary pits on the specimen surface.However,under the condition of mixed bacteria,the pitting degree of X80 steel surface was weakened,indicating that NRB could inhibit the stress corrosion caused by SRB.4.The corrosion behavior of X80 steel under the multi-factor coupling of cathodic protection,mixed bacteria and stress were investigated.The results showed that there were no pitting pits on the surface of unstressed and stressed specimens in the sterile group,while pitting pits were still found in the inoculated group under cathodic protection.Moreover,with the negative shift of the cathodic potential,uniform corrosion degree of X80 steel was reduced,but the pitting degree was intensified.5.The coupling effects of cathode polarization,mixed bacteria and stress on hydrogen permeation behavior of X80 steel were studied.The results showed that both cathodic polarization and SRB physiological activity could accelerate the hydrogen permeation process of steel,leading to the increase of apparent hydrogen concentration and steady-state current density.At the same cathodic polarization potential,SRB could promote the hydrogen permeation process of X80 steel in the following two ways:sulfide,the metabolite of SRB,could promote the formation of hydrogen atoms and prevent hydrogen atoms from combining into hydrogen molecules,resulting in the increase of the surface effective hydrogen concentration;the acidic substances secreted by SRB lowered the local pH of the biofilm and accelerated the formation of hydrogen atoms.Under the effect of yield stress,X80 steel would produce plastic strain,resulting in a large number of reversible hydrogen traps,so the steady-state current density and apparent hydrogen concentration of the steel were significantly lower than the unstressed specimens.