The Influence Of Particle Size On The Electrochemical Corrosion Behavior Of The Contaminated System And X70 Steel In NaCl Contaminated Sandy Environment

Buried pipelines have been commonly used for energy delivery purposes. However, they are prone to failures when exposed to a corrosive soil environment, causing a huge loss of human’s property and life. The evaluation and treatment of contaminated corrosive soil are the important issues that have not been solved completely and deserve a further study, particularly the influence of corrosive soil environment on the corrosion behavior of buried pipeline as it is directly related to construction economy and energy transportation security. Traditionally, the evaluation method and the resistivity method are the two major methods for evaluating the corrosion of contaminated soils. However, they require more parameters and longer test time, thus relatively larger errors. Although the resistivity method can precisely evaluate the corrosion of contaminated soils, only the single resistivity parameter is considered. Furthermore, it cannot completely reflect the dynamic corrosion process, double layer, electrode materials, solid electrolyte, and protection mechanisms of corrosion. Therefore, it is necessary to explore a new method to the corrosion of contaminated soils.Corroded soil is a complex material consisting of soil partic les, pore fluid, and pore gas. The behavior of buried pipelines in a corrosive soil environment is influenced by many factors. However, the effect of soil particle s ize on the electrochemical corrosion behavior of steel pipelines and the behavior of steel pipelines in a corrosive soil system with different partic le sizes remain unknown.The effects of different influencing factors considered by others plus the soil particle size to the electrochemical corrosion behavior of X70 steel are studied systematically by using the electrochemical impedance spectroscopy theory and experimental simulation test. The main findings from this study are summarized as follows.(1) The resistivity of NaCl contaminated sandy soil with different water content decreases linearly with increasing partic le size. A statistical analys is of laboratory test results was performed followed by the following derived formula for predicting the variation of the contaminated soil res istivity with water content and partic le s ize of sandy soil ρ= [(3-2w)(3w-35)] [(w+3) +D] where w, D, and ρ represent sandy soil water content, partic le size, and resistivity respectively. Based on the Archie resistivity model and the introductive concept of tortuosity, the effect of particle size on the tortuosity was analyzed and then the resistivity model affected by tortuosity was established through the introductive concept, resulting in the effect of particle size on the tortuosity,where ? and represent porosity and the pore water res istivity, respectively and are related to the soil particle size.(2) Based on the theory of electrochemical impedance spectroscopy, a new method for evaluating the corrosion of contaminated sandy soil is proposed. Moreover, the effects of various factors including water content, NaCl content, partic le size, and pollution age on the electrochemical impedance spectra of sodium chloride contaminated sandy soil were tested in laboratory. Then, the application of electrochemical impedance spectroscopy for evaluation of the corrosion of contaminated sandy soil was verified.(3) The results of the effect of sandy soil partic le size on corrosion behavior of X70 steel show that the corrosion behavior of X70 steel is affected by the partic le size. The corrosion rate of X70 steel in sodium chloride contaminated soils increases with increasing buried time. The corrosion rate decreases with the increasing soil partic le size from 0.15 mm to 0.8mm. However, it increases with the further increasing particle size from 0.8mm to 1.5mm.(4) The results of the two typical soil partic le-s ize distributions on corrosion behavior of X70 steel show that the corrosion behavior of X70 steel is affected by the soil partic le-size distribution. The corrosion rate of X70 steel in the gap gradation sandy soil environment is significantly larger than that in the continuous gradation sandy soil environment.(5) The experimental results show that the electrochemical corrosion behavior of metal materials in contaminated soil is an effective way to evaluate the corrosion of soil environment.