Evolution Study Of Local Corrosion Of X80 Pipeline Steel In Coastal Soil Environment

With our increasing dependence on foreign oil and gas and the development of offshore oil and gas field,a series of coastal storage devices have been established,followed by the laying of a large number of pipes.There will be serious corrosion problems for the pipelines in coastal soil environment which has the characteristics of high salt content,high water content and so on.In the coastal soil environment,coating protection and cathodic protection are always used together to suppress the corrosion of pipelines.But,the place,where the protective coating fails and disbonds,will become the primary location of the initiation of local corrosion and stress corrosion of the pipelines.In order to clarify the evolution of local corrosion of X80 pipeline steel and the characteristics of stress corrosion under disbonded coating in coastal soil environment,in the paper,the ability of corrosion of X80 pipeline steel in coastal soil environment was evaluated by potentiodynamic polarization(Tafel),electrochemical impedance spectroscopy(EIS)and Linear polarization,the local corrosion of X80 pipeline steel welded joints was studied by scanning vibrating electrode technique(SVET),the evolutions of local corrosion of X80 pipeline steel in coastal soils and under disbonded coating were studied by electrode array(EA),the stress corrosion behaviors of X80 pipeline steel under disbonded coating in coastal soil leaching solutions and NS4 solution were researched on by slow strain rate test(SSRT).The results show as follow:1,The law of corrosion strength the three coastal soil leaching solutions to X80 pipeline steel is Qingdao Port > Youkuqu > Dongjiakou,and the law of the three coastal soils is Dongjiakou > Qingdao Port > Youkuqu.2,The corrosion characteristics of X80 pipeline steel welded joint in the three coastal soil leaching solutions are similar.The characteristic is that there is relatively larger localized corrosion current density in the local area near the welded fusion line,than the central area of the weld and the area near base metal,which makes the area near the welded fusion line weakest in coastal soil leaching solutions.Localized corrosion took place easily in Qingdao Port sandy soil in the beginning of immersion time,as time went on,the corrosion form changed from localized corrosion to general corrosion.While in the initial stage buried in Dongjiakou clayey soil,general corrosion occurred firstly,with time went by,the corrosion form turned from general corrosion to localized corrosion.In the thin layer of coastal soil leaching solution under disbonded coating,corrosion happened first at the bottom of the crevice and the area near the mouth of the crevice,leading to significant localized corrosion,and with time went by,the difference increased in the first several days and then became smaller in different positions in the crevice.3,X80 pipeline steel showed relatively high stress corrosion sensitivity under disbonded coating in NS4 solution when no potential was applied.As the applied potential turned negative,the sensitivity increased first and then fell down,when the applied potential turned to-900 mV,fracture morphology showed significant brittle fracture characteristics,stress corrosion sensitivity was relatively high,while when the applied potential turned to-1000 mV,the stress corrosion sensitivity significantly reduced,so-1000 mV was the best cathodic protection potential.The stress corrosion sensitivity was much lower,in Qingdao port and Dongjiakou coastal soil leaching solutions under disbonded coating without applied potential,than that in NS4 solution.However the changes of SCC sensitivity with applied potential in the two coastal soil leaching solutions were different,the former declined first and then rise,when applied potential turned to-900 mV,the plasticity index values were the maximum,and plastic fracture characteristics were obvious,best protection was brought in.Then the later rise first,then dropped down and rise again at last,when the applied potential turned to-1100 mV,the X80 pipeline steel inside the crevice was fully protected,and the role of hydrogen evolution was not obvious,it showed best protection.