Theoretical and experimental bases for the equivalent circuit model for interpretation of silty soil at different temperatures.

The exploitation of underground space is accompanied by complex geotechnical problems. The development of electromagnetic exploration technology provides a new perspective for preventing and avoiding these problems. In this work, electrochemical impedance spectroscopy (EIS) was used to test the single-phase and mixed-phase medium. Based on the unsaturated soil theory and the dual-water conductivity theory, an equivalent circuit model to describe the electrochemical characteristics and microstructure of silty soil with temperature changes through comparative research. The results indicate that the resistance of near-water layer is not affected by temperature, the resistance of silty soil increases mainly results from the influence of the far-water layer until which increases significantly after freezing. The capacitance change of silty soil is mainly affected by the slowing down of the orientation movement of polar molecules in the far-water layer. Based on the fitting data, a mathematical model for calculating the unfrozen water content of frozen soil was proposed, which reasonably verified the relationship between the unfrozen water content and electrical resistance. By improving the testing conditions of electrochemical impedance spectroscopy, this method may provide new insights for future research of soil electromagnetic testing technology.

Electrochemical response of solidification Cu2+ contaminated soil influenced by red mud/fly ash ratio.

The main purpose of this work was to study a new method for evaluating the solidification of contaminated soil based on electrochemical impedance spectroscopy (EIS). To explore how the EIS parameters were affected by the pore structure and mesostructure of the cured system, the physical and mechanical properties, leaching toxicity, microstructure, and EIS of the stabilized contaminated soil were tested after 7, 28, 60, and 90 days of curing. Based on the EIS results, a physical and equivalent circuit model of the stabilized contaminated soil's impedance response was established to reveal the mechanism of binder-heavy metal ion-soil interaction. The results showed that as the red mud (RM)-fly ash (FA) mass ratio and curing age increased, the strength and structural compactness of the solidified body also increased. The best curing effect was achieved with an RM-FA mass ratio of 7:3 after curing for 90 days. The equivalent circuit model of the solidified body obtained by EIS was Rs (Q1 (Rct1W) Q2Rct2). The pore solution resistance Rs, solid-liquid interface ion transfer resistance Rct 1, and unconfined compressive strength (UCS) qu all showed an increasing trend with increasing RM-FA mass ratio and increasing curing time. Fitting the model demonstrated that both Rs and Rct1 were closely correlated with the strength of the solidified bodies. These conclusions were further verified by scanning electron microscope (SEM) experiments. Overall, this work demonstrates that the strength characteristics of solidified bodies can be evaluated by EIS and reveals the microscopic mechanism of the solidification of Cu2+-contaminated soil.

EIS analysis of the electrochemical characteristics of the metal-water interface under the effect of temperature.

The corrosion performance of metals is closely related to their durability. Available studies on metal corrosion have seldom focused on the interfacial reaction behaviour influenced by a conductive medium under different temperatures. In this work, a laboratory corrosion simulation environment has been designed for EIS measurements to investigate the electrochemical behaviour of copper immersed in distilled water in different temperature environments. The relationship between the mathematical model of impedance response and the equivalent circuit model is determined based on electrochemical kinetics theory. The complex process of the dielectric properties of distilled water affected by temperature is analysed, and a simple method for calculating the kinetic parameters is presented. The experimental and model results have a good fit, and the analysis results indicate that the semicircle in the high-frequency region of the complex impedance curve represents the charge transfer process of the conductive medium. The decrease in temperature is the major factor that inhibits the rate of dissolution and passivation, resulting in the change rate of surface coverage slowing down, until the attenuation of the mass transfer process of the conductive medium dominates the full range of AC frequencies. This model provides an improved approach for determining physical parameters based on electrochemical impedance spectroscopy to characterize the electrochemical properties of materials.

Corrosion behavior and mechanism of X80 steel in silty soil under the combined effect of salt and temperature.

In this study, X80 pipeline steel was embedded in silty soil with different salinities and subjected to corrosion at a constant temperature for 24 h before electrochemical testing. The effect of soil medium, temperature, and salt content on the kinetics of corrosion behavior of X80 steel was analyzed. Furthermore, the compositions and structures of the corrosion products were analyzed by X-ray photoelectron spectroscopy and scanning electron microscopy. Based on the results, the anodic dissolution reaction mechanism of X80 steel in silty soil was determined, the differences in the corrosion process caused by different soil systems were comprehensively contrasted, and the impact of the migration process of heterogeneous silty soil on corrosion behavior under different conditions was systematically explored. Comparative analysis revealed that chloride ions possess strong adsorption ability at temperatures above freezing point and that more oxidized substances are present in the deposited layer on the surface of corroded steel, which facilitates the occurrence of corrosion under deposition. At temperatures below freezing point, the sulfate ions present in the pore solution contribute to crystallization-induced expansion and lead to swelling and deformation of the soil, rendering the X80 steel more prone to corrosion in sulfate corrosion environments.