Corrosion Behaviors Of X100 Pipeline Steel In Simulated Solution Of Alkaline Soil

Saline soil is rich in chloride,sulfate and carbonate which can easily cause the soil corrosion of underground metal pipelines and effect the safe operation of oil and gas transport.X100 pipeline steel produced by China with high strength,good resistance to pressure and low cost is expected to be applied in the west-east gas pipeline project.Therefore,the laboratory corrosion experiments carried out using the typical simulation soil solution of the west-east gas pipeline is necessary and that hopes to obtain basic data for the engineering application and service life prediction of pipeline steel,which would have much engineering significance.The korla soil of Xinjiang contains chloride,sulfate and carbonate,is one of the strong corrosive alkali soil types in China.In this paper,the simulated solution of Korla soil was selected as the experimental corrosion solution and X100 pipeline steel as the research material.The corrosion behavior of X100 pipeline steel in the simulated solution of Korla soil was studied using X-ray diffractometer?XRD?,optical microscope?OM?,scanning electron microscope?SEM?,energy spectrometer?EDS?,M370 scanning electrochemical workstation and M398 electrochemical synthesis test system.Synergistic effects of SO₄^2-and Cl^-on the corrosion of X100 pipeline steel,as well as the effects of HCO₃^-and Cl^-on the corrosion were discussed and the corrosion mechanism models were also established.The results show that,X100 pipeline steel is characterized by a typical active dissolution,which there is no obvious passive region in the simulated solution of Korla soil containing various concentrations of SO₄^2-and Cl^-.While there is an obvious passive region in the HCO₃^-simulated solution.With the extension of corrosion time,the average corrosion rate and instantaneous corrosion rate of X100 pipeline steel in the simulated solution of Korla are all increase firstly,then decrease afterwards,and increase again finally,which is mainly due to the formation of corrosion product layers and the existence of Cl^-.In the simulated soil solution of Korla with co-existing of SO₄^2-and Cl^-,the matching circuit is R?C?R?CR???.With the increase of SO₄^2-mass fraction,the polarization resistance Rp increases significantly and the corrosion current density icorr decreases,which indicates that X100 pipeline steel decreases the pitting corrosion propensity as the concentration of SO₄^2-increasing,and that means SO₄^2-in Korla soil belongs to inhibit corrosion anion.The corrosion mechanism is that SO₄^2-pushes the unreacted Cl^-out to the hole by competing adsorption with Cl^-,while SO42-is also involved in the reaction and reacts with water to produce OH-,and the corrosion product FeS generates simultaneously then covers the surface of corrosion holes,thereby reduces the corrosion rate.In the simulated solution of carbonate,a white corrosion product FeCO3 film forms on the surface of X100 pipeline steel,thus the corrosion rate is inhibited,which means HCO₃^-in Korla soil belongs to inhibit corrosion anion.Experiments show that,the corrosion behavior of X100 steel in Korla soil simulated solution is more serious than in the solution of carbonate,indicating that Cl^-plays a dominant role in the corrosion reaction.In the simulated soil solution of Korla containing different concentrations of Cl^-,the electrochemical fitting circuit model is R?C?R?CR???,and the polarization resistance Rp decreases gradually with the increase of Cl^-concentration while the corrosion current density icorr increases,which means Cl^-can promote the corrosion process in the simulated soil solution of Korla,and lead to the increasing of corrosion rate of X100 steel with the increase of Cl^-concentration.The main corrosion mechanism is that Cl^-replaces the O2-to form H+ by competing adsorption in the initial stage of the reaction,causing pitting nucleation.As the reaction progressing,although corrosion products fill in corrosion orifice,Cl^-still can enter the corrosion hole as a result of its high penetration ability and thus promote ongoing reaction.