Corrosion Mechanism Of Coastal Landing Pipeline Induced By Sulfate Reducing Bacteria And Protection Method

The corrosion phenomenon of pipeline steel in the marine engineering environment poses a serious threat to the safe operation of pipelines,the service conditions of the landing section pipeline are particularly complex and variable.The landing section pipelines often cross sandy sea mud and seawater area,suffer from crevice environment caused by riveted structure or disbanding coatings and stress loading caused by self-weight or wave impact.Microbiologically influenced corrosion（MIC）induced by sulfate reducing bacteria（SRB）is one of the main causes of pipeline failure in the marine environment.To ensure the safe operation of marine pipelines,it is of vital importance to reveal the MIC mechanism of pipeline steel under various engineering conditions,evaluate the effectiveness of the existing corrosion protection methods,and explore effective pipeline MIC protection methods.This thesis focuses on the MIC of pipeline steel caused by SRB,the main research conclusions are as follows:1.The corrosion promotion mechanism of SRB on X80 carbon steel in sandy sea mud:In the presence of SRB,X80 carbon steel suffered more severe MIC in sea mud than in seawater,which was related to the bacteria-induced galvanic effect and promoted conductivity of the biotic sea mud.Significant galvanic effect and galvanic corrosion formed between the carbon steel in seawater and sea mud because of the sessile cells and biofilm difference,during which the carbon steel in sea mud became the anode and its corrosion was accelerated.In addition,some substance such as Fe S enhanced the electrical conductivity of the biotic sandy mud.The highly conductive SRB-containing sea mud could transfer electrons from the carbon steel to the SRB which was not in contact with the carbon steel over long distances,causing more MIC of X80 carbon steel in sea mud.2.Crevice corrosion of X80 carbon steel induced by SRB:SRB caused severe crevice corrosion of X80 carbon steel in the crevice environments with widths of 0.1 mm,0.3 mm,and 1.0 mm.SRB induced the galvanic effect between the carbon steel inside and outside the crevice,and the carbon steel inside the crevice became the anode of galvanic corrosion,and its corrosion was accelerated.The growth of SRB inside the crevice was limited by the narrow crevice environment,and the number of sessile bacteria cells and biofilm densities on the carbon steel outside the crevice were higher than that inside the crevice,resulting in accelerated cathodic process on the carbon steel surface outside the crevice.As the crevice width increased,the difference between the amount of sessile bacteria cells inside and outside the crevice decreased,the galvanic effect and galvanic current became lower,and the crevice corrosion of X80 carbon steel decreased.However,SRB still induced obvious crevice corrosion of X80 carbon steel in a crevice with a width of 1.0 mm.3.Effect of the tensile stress on X80 carbon steel corrosion by SRB:A constant tensile stress loading device was designed,and the effects of different tensile stress on the MIC behavior of X80 carbon steel were investigated by finite element simulation and slow strain rate stretching（SSRT）.Tensile stresses below 400 MPa showed a slight promotion on SRB carbon steel corrosion,while 550 MPa tensile stresses near the yield strength significantly accelerated MIC of X80 carbon steel.Under the high elastic stress load,the stress concentration area at the bottom of the pit occurred plastic deformation.Meanwhile,the mechanical-chemical effect caused the metal potential inside the pit to shift negatively,forming a large cathode and small anode structure with the metal around the pit,metal dissolution inside the pit was therefore promoted.In addition,the metabolic activity of SRB could produce a large amount of H₂,which increased the risk of brittle fracture of X80 carbon steel under the condition of coexistence of high elastic stress load and SRB.4.Protective behavior of Al-Zn-In-Mg-Si sacrificial anodes for X80 carbon steel in the presence of SRB:When X80 carbon steel was coupled with aluminum anode,SRB preferentially adhered on the carbon steel and promoted the cathodic process,significantly increased the galvanic current density between carbon steel and aluminum anode.SRB biofilm on carbon steel not only accelerated the carbon steel corrosion unfer cathodic protection,but also caused dramatic consumption of sacrificial anode.The time-frequency conversion based on Hilbert-Huang transform（HHT）showed that SRB enhanced the electrode reaction kinetics during the galvanic corrosion.The enhanced cathodic process in the galvanic corrosion was attributed to the electron uptake by living SRB and the catalytic proton reduction by certain substances such as Fe S in the biofilm.The corrosion of the coupled aluminum anode in service was mainly caused by the SRB enhanced galvanic effect rather than direct corrosion promotion.5.Marine anticorrosive and antifouling epoxy coating:Polyhexamethylene guanidine molybdate（PHMG-Mo O₄）with low water solubility and high antimicrobial properties was synthesized.Anticorrosive and antibacterial epoxy coating was prepared based on PHMG-Mo O₄.PHMG-Mo O₄ not only retained the antimicrobial polymer cation structure of polyhexamethylene guanidine hydrochloride（PGMG-Cl）,but also possessed lower water solubility.Therefore PHMG-Mo O₄ can remain in the epoxy coating for a long time,kill sessile bacteria and inhibit SRB biofilm formation on epoxy coating within 120days.The PHMG-Mo O₄/epoxy coated carbon steel exhibited higher integrity,adhesion and impedance values in enriched artificial seawater containing SRB.PHMG-Mo O₄/epoxy coating effectively inhibited the adhesion of marine fouling organisms in a 4-month real sea immersion test and showed great application value.