Electrochemical Behavior Of X70 Steel In Ku'erle Soil During Initial Corrosion Period

Few researches were made on the corrosion behaviors of the pipeline steel X70 in western soils of our country. As more and more X70 pipelines are used, it's necessary to make the researches on the corrosion mechanics of X70 steel in the western soils. In this thesis the electrochemical behaviors of X70 steel in Kuer'le soil of different moisture content and different temperature were studied by some electrochemical test techniques, such as Polarization Curve, Electrochemical Impedance Spectroscopy (EIS) and Electrochemical Noise (EN).From the obtained polarization curves, we can find that the anodic diffusion is not obvious in soil with various moisture content, and corrosion of X70 in the original soil and the soil with 2% moisture content is controlled by the cathodic diffusion process.The corrosion potential drifted in the negative direction and polarization current density increased with the increase of the corrosion time and the temperature.The EIS results show that corrosion of X70 in the original soil and the soil with 2% moisture content is controlled by the cathodic diffusion process.The characteristics of EIS for X70/soil system related to soil corrosion morphology. EIS of low moisture content included diffusion impedance in low frequency region, two capative semi-circles in high and medium frequency region, and the corrosion type was local corrosion. The capative semi-circles in high \medium \low frequency region and the impedance of the whole system decreased obviously with increase of corrosion time (t).The EIS fitting results show that polarized impedance (R1)\transmition impedance (Rt) \diffusion impedance (Ws) all decrease obviously with increase of t. EIS of high moisture content only contained capative semi-circles, and the corrosion type in soil with moisture content of 14.5% was general corrosion. R1 and Rt also decreased obviously with increase of t. The increase of the temperature (T) didn't influence the soil corrosion morphology, and R1 and Rt decreased obviously with the increase of T.The Electrochemical Noise results show that the obvious characteristic peak appeared in different time. The characteristic peak appeared earlier with the increase of moisture content and temperature. The corrosion potential drifted to the negative direction, the maxium of the potential fluctuatuatin decreased and the current variated irregularly with the increase of moisture content and temperature. The maxium of the fluctuating current decreased with the increase of the temperature and increased with the increase of moisture content.The method to treat the EN data in soil corrosion system needs further study.