Crevice Corrosion Behavior And Mechanism Of X70 Pipeline Steel In NaHCO₃+NaCl Solutions

Crevice corrosion,a common localized corrosion for metal materials in service,could cause serious damage to metal materials.X70 pipeline steel,which is widely used in the first phase of West-East Gas Pipeline project,suffered crevice corrosion inevitably.Currently,there is some controversy on the crevice corrosion mechanism due to its numerous factors and complicated corrosion process.In this dissertation,the crevice corrosion behavior and mechanism for X70 pipeline steel in NaHCO₃ + NaCl solutions were investigated by potentiodynamic scanning,potentiostatic polarization,electrochemical impedance spectroscopy?EIS?and scanning electron microscopy?SEM?.The effects of Cl-,HCO₃-and the potential of electrode outside the crevice were discussed.The influence of corrosion products including both solid product in anode and gas product in cathode was also concerned.The main results are as follows:?1?The X70 steel inside the crevice could suffer localized corrosion in NaHCO₃ solution at-0.4V.The acidification initiated at the crevice opening and then extended to the bottom.The hydrogen evolution could be observed with the development of corrosion and acidification.The cathodic reaction changed from oxygen reduction to hydrion reduction.HCO₃-could promote passivation therefore the increase of the HCO₃-concentration inhibits the development of crevice corrosion obviously.The presence of Cl-did not change the crevice corrosion mechanism.With the increase of Clconcentration,however,the crevice corrosion rate increased.?2?The crevice corrosion initiated later and developed slower as the increase of polarized potential.The corrosion region moved crevice opening to bottom gradually and pitting corrosion occurred as polarized potential increasing.The initiation of crevice corrosion was determined by polarized potential.?3?Solid corrosion products have great influences on the crevice corrosionbehavior of X70 steel.At the beginning of corrosion,corrosion product film covered completely but not tight,the surface of film contains many pores that accelerate the corrosion instead of protection.Later,the film worked to protect the metal.The corrosion product changes from Fe?OH?₂ to FeCO₃ gradually when the potential moves positively.And the different composition of products accumulated in different shape.?4?The influence of hydrogen evolution on crevice corrosion process of X70 pipeline steel depended on the orientation of electrode.If the electrode surface is upturned,small hydrogen bubbles accelerate the localized corrosion at the beginning.Latterly hydrogen bubbles could protect the electrode.If the electrode surface is down,the hydrogen mainly affects the formation of corrosion produce film by a physical way that depends on the tilt angle.