Indoor Simulated Experiment On The Effect Of Freeze-thaw Cycle On Corrosion Behavior Of Buried Pipelines In Saline Soil

In this paper,the electrochemical corrosion behavior of NaCl contaminated sand and X70 steel/contaminated sand system under different freeze-thaw cycles and moisture content was studied by means of electrochemical impedance spectroscopy?EIS?,polarization curve and scanning electron microscopy.1.From EIS results following conclusions can be drawn out:First,when water content is 3%,Nyquist diagram of X70 steel consists of inclined line or capacitive arc in high frequency region and capacitive arc in low frequency region.Its intersection point with real axis shifts to the right with the increase of freeze-thaw cycles.The components of equivalent element solution resistance R_e and charge transfer resistance R_ctt are positive related with cycle times;When water content is 9%,Nyquist diagram is composed of capacitive arc in high frequency region and capacitive arc in low frequency region,and the intersection point of real axis moves left with the increase of freeze-thaw cycles,while solution resistance R_e and charge transfer resistance R_ctt of equivalent element are negatively correlated with freeze-thaw times;When water content is15%,Nyquist diagram is composed of capacitive arc in high frequency region,capacitive arc in medium frequency region and inductance arc in low frequency region.With the increase of freeze-thawing cycle times,the intersection point of the curve and the real axis shifts to the left which means the inductance arc decreases.The solution resistance R_e,charge transfer resistance R_ctt and inductance L of the equivalent element are negatively correlated with the number of freezing and thawing.Secondly,the equivalent elements and the their combining ways in NaCl contaminated sand with different moisture are not same,weber impedance only occurs in the system with 3%and 9%water content.The variation trend of solution resistance R_e and charge transfer resistance R_ctt of common components with freeze-thaw cycle times is the same as that of steel sheet/contaminated sand system;Thirdly,regardless of steel sheet or not,water content has a great influence on the corrosion system.Generally speaking,the larger the water content is,the smaller the overall impedance of the system turns.With the increase of water content,the solution resistance and charge transfer resistance show a significant decreasing trend.2.The results of polarization curves show that:the cathodic curve moves up with the increase of the number of freeze-thaw cycle times,which means the cathodic reaction rate increases;the increase of water content causes the anodic reaction curve to move up,which indicates the anodic reaction rate increases with the increase of water content;the influence of water content and freeze-thaw times on the polarization curve essentially changes the parameters in the corrosion process and the oxygen content of the reaction.3.Scanning electron microscopy results show that the corrosion of X70steel sheet buried in contaminated sand is mainly local corrosion,and the penetration of Cl^-and temperature stress make the surface of the steel sheet appear fine cracks.The freeze-thaw cycle accelerates the corrosion process.