Electrochemical Behavior Of Casing Steel In Cementing Cement Pores

For the study on the outside corrosion of the casing,researchers at home and abroad have made a great effort,of course,the results are remarkable.As all known,the casing is sealed by cement underground.After the hydration of cement with water,it formed a porous material.Inside the tiny pores of the cement,there is concrete pore solution with a pH value as high as 12 or more,which results in passivation of steel.The passive film has a good corrosion protection to casing surface.However,with the increasing service time,the aggressive ions transfer through pores and cracks in cement from outside and play a role on the passive film on the casing surface.As a result,it may cause the corrosion damage to the casing steel.Therefore,the main purpose of this paper is to research the formation of the passive film during the curing process of cement and its destruction effect with the corrosion medium in simulated concrete pore solution and concrete pore solution.The electrochemical properties were characterized by electrochemical impedance spectroscopy(EIS),polarization curves method and Mott-Schottky curves method during the formation of the passive film and the destruction stageby the corrosion ions.X-ray photoelectron spectroscopy(XPS)was measured to qualitatively analysis the composition of the passive film on the casing surface.Scanning electron microscope(SEM)was used to observe the micrography of the passive film on the casing surface.During the formation of the passive film,the surface passive film of P110 casing steel was more stable with the increase of time under the same temperature in simulated concrete pore solution and concrete pore solution.But the time of a stable passive film formation was different in two kinds of solution.In simulated concrete pore solution,the passive film formed completely and stably about 16 hours and the passive film formed entirely and stably about 15 days in concrete pore solution.The passive film generated by the two kinds of solution was n type semiconductor,but the micrography of the passive film was different.The composition of the passive film formed in simulated pore solution was Fe(OH)2,FeOOH and Fe2O3,and the composition was Fe3O4 and FeOOH in concrete pore solution.In order to fully realize the concrete cement production process,tap water was used in concrete pore solution.The damage experimental of the passive film carried out at the temperature of 65?.The critical concentration of Cl-for the passive film damage was 0.16 mol/L in simulated concrete pore solution,and the semiconductor characteristics of the surface passive film changed from n type to bipolar under the critical concentration,and the composition of eroded passive film was the combo of M-O-Cl and Fe2O3.The critical concentration was 0.3 mol/L in concrete pore solution,and the semiconductor characteristics of the surface passive film also changed from n type to bipolar under the critical concentration,and the composition of eroded passive film was the combo of M-O-Cl and FeOOH(Fe2O3).The critical concentration of Cl-was different and the difference was remarkable in two kinds of solution,because of the different solution composition.The different concentration of NaHCO3 was used to simulate the different concentration of CO2 in the destruction process of CO2 to the passive film.The casing steel surface was changed with the increase of NaHCO3 concentration in simulated pore solution,and the composition of changed film was FeCO3 and FeOOH,and the the semiconductor characteristics of the film was bipolar.The corrosion resistance of the film was poor in the process of film transformation.The corrosion resistance of the concrete pore solution was improved with the increase of NaHCO3 concentration.However,the corrosion resistance was worse than that of the original passive film.The composition of the film was FeCO3,Fe3O4 and FeOOH,and the semiconductor type was P type.The saturated H2S was inlet simulated pore solution,and a layer of corrosion product film was created,and the film composition was FeS2 and FeS.But the protective effect of the film was poor for the steel,and the semiconductor type was bipolar.Therefore,in the two kinds of solution,Cl-on the passive film damage belonged to direct damage,while CO2 and H2S damage to the passive film were film transformation and the corrosion resistance of the film was relatively low.