Corrosion Mechanisms Of X80 Steel Under Disbonded Coating Under The AC,Microorganisms,and Stress

Acidic red soil is one of the typical highly corrosive soils in China,and the pipeline corrosion mechanism in this type of soil is different from that of in the neutral/alkaline soils.The service environment of high-strength pipeline steel is complex,in which alternating current(AC),sulfate-reducing bacteria(SRB)and stress are the important factors affecting the corrosion of buried pipelines.Actually,there were synergistic effects between these factors in the field,accelerating the corrosion of pipeline steel.In addition,pipeline failure in soil environment often occurs under disonded coating,and the corrosion behaviors are quite different from those of bare steel.So far,the corrosion behaviors and mechanisms of pipeline steel under diabonded coating subjected to multiple factors are seldom reported.Therefore,it is of great significance and high engineering practice to explore the corrosion mechanism of pipeline steel under disbonded coating subjected to the coupling of multiple factors(AC,SRB,and stress).The research results will provide scientific basis for high-strength pipeline corrosion and stress corrosion mechanism,safety evaluation methods and prevention in the acid red soil environment.The AC corrosion mechanism of X80 steel in the acid red soil was studied.The results showed that AC changed the electrochemical corrosion process of X80 steel in acidic red soil environment,and further changed the composition of corrosion products on the surface of X80 steel.The content of Fe3O4 in corrosion products decreased with the increase of AC densities,while the content of FeOOH increased gradually.The AC corrosion mechanism of X80 steel in the acid red soil environment was the rectification effect mechanism.In the whole process of AC,the anodic oxidation during the anodic half-cycle of the AC signal was much greater than the cathodic reduction during the cathodic half-cycle of the AC signal.The corrosion rates of X80 pipeline steel increased gradually with the increase of AC densities.In the acidic red soil environment,the critical threshold of AC density of high AC corrosion risk region was further reduced compared with the standards.Therefore,the change of soil environment should be considered in order to effectively judge and prevent the AC corrosion risk of pipes.The SRB corrosion mechanism of X80 steel under disbonded coating in the acid red soil environment was studied by a homemade in-situ monitoring array electrode experimental device to simulate the case of coating disbondement.The results showed that SRB was physiologically metabolized in the acid red soil extracted solution and formed a composite film containing biofilm and corrosion products on the surface of steel.The number of SRB distributed gradiently under disbonded coating.The number of planktonic and sessile SRB were the most at the holiday,and gradually decreased with the increase of the distance from holiday.SRB changed the corrosion morphologies of pipeline steels under disbonded coating and induced localized pitting corrosion.The pitting corrosion at the holiday was the most serious,and the degree of pitting corrosion gradually decreased with the increase of the distance from the holiday.In acidic red soil environment,SRB used steel as electron donor and high valence Fe(Ⅲ)as terminal electron acceptor to built electron transfer chain and obtain the energy required for life activities,further accelerated the corrosion of pipeline steel.The coupling effect of elastic stress and SRB on the X80 steel corrosion under disbonded coating in the acid red soil environment was studied,and the main ways of stress-assisted SRB corrosion was clarified.The applied elastic stress affected the microstructure of X80 steel,which promoted the grain deformation,introduced high residual stress,and resulted in an increase in the proportion of low angle grain boundaries.After applying elastic stress,micro-cracks appeared on the surface of corrosion products.The number of SRB under the disbonded coating further increased after applying stress.SRB and stress promoted the corrosion of pipeline steel jointly.Under the elastic stress,the pitting morphologies of pipeline steel under disbonded coating were changed,the stress concentration occurred at the bottom of the pit,and the local corrosion cracking sensitivity of pipeline steel was improved.The corrosion behavior of pipeline steel under disbonded coating under the coupling of AC,SRB and external elastic stress was studied.There was AC current gradient distribution under disbonded coating.Under the coupling effect of multiple factors,the most serious corrosion of coupon was observed at the holiday,and the corrosion degree gradually decreased with the increasing depth of coating disbondment.Large number of pit holes were formed on the surface of the steel.Ulcer-shaped pit holes were observed on the coupon at the holiday,and the pits on the coupons in the crevice region were irregular clusters.Cracks were obviously observed on the surface of pipeline steel at the holiday,and SRB participated in the crack propagation of pipeline.AC interference,SRB physiological activities and elastic stress synergistically increased the driving force,the mechanical-bioelectrochemical activity of pipeline steel corrosion,and the driving force of anodic and cathodic reaction process.The corrosion mechanism of pipeline steel under disbonded was under a combined action of anodic dissolution and hydrogen embrittlement.