Experimental Study On The Electrochemical Corrosion Behavior Of X70Steel In The Sandy Soil Under DC Conditions

Through a simulation experimental, the corrosion behavior of X70steel insodium chloride (NaCl) contaminated sandy soil environments with differentNaCl and water content were investigated using the methods of electrochemicalmeasurements, scanning electron microscopy (SEM), energy dispersivespectrometer (EDS). Additionally, the corrosion behavior and mechanism ofX70steel in the soils fore-mentioned when applied direct current (DC) on theboth end cross-section of samples was also studied.Using the CS350electrochemical test system, the corrosion behavior of X70steel in sandy soil and the corrosive of soil with different water content werestudied in order to reveal the relationship between the X70steel corrosion rateand the water content of soil environments. The results showed that thecorrosion current density of X70steel increased, the negative shift of thecorrosion potential and the anodic process accelerated with increasing the watercontent, which leading to the corrosion rate increased. The water content acts asthe control factor in the lower moisture of soil environment and thus there waslittle corrosion rate of X70steel variation. However, in the higher moisture ofsoil environment, the ions concentration increases result from the salt contentincreased and then the number of effective charge provided by ions increase. Thus, the polarity of the solution increased while the solubility of non-polarmolecules of oxygen decreased result in the decrease of oxygen content and thecorrosion rate decrease.A direct current (DC) applied on the both end cross-section of samples inorder to simulate the acceleration experimental the corrosion process of X70steel in sandy soil environment. The electrochemical corrosion behavior of X70steel in the soil water content of6%and8%were studied by thepotentiodynamic polarization curves. The results indicated that the corrosionrate of X70steel is smaller in the6%water content sandy soil than that in NaClcontaminated sandy soil. But, the corrosion rate of X70steel didn’t appear to bemuch different in sandy soil with18%water content and NaCl contaminatedsandy soil. In these two types of sandy soil environments, the corrosion rate ofX70steel increased in the initial stage, then decreased and slowly increased inthe final stage with the increasing of conduction time, which associated with thecompactness of corrosion product film formed on the X70steel surface and thesalt content of soil environments.By analyzing the electrochemical impedance spectroscopy of the corrosionprocess of X70steel in NaCl contaminated sandy soil environments, it can befound that the Nyquist plots exist the characterization of two capacitivesemicircles, of which the high-frequency arc should be related to the corrosionproduct film formed on the X70specimen surface, in this case, the radius of thissemicircle can reflect the resistance and the protective property of corrosionproduct film. However, the capacitive semicircle at low-frequency arc shouldcorrespond to the double layer capacitive, which associated with the chargetransfer resistance. In this case, the radius of semicircle increase with the soilwater content decreasing, leading to the charge transfer resistance increased andthus the corrosion rate of X70decrease.It can be seen from the observations of the morphology and microgram ofX70steel corrosion that the formation of corrosion product film, pitting corrosion induced phase and development phase compose the corrosion processof X70steel. The characteristics of X70steel corrosion is the uneven corrosion,in the initial stage, and there is less corrosion products on the metal surfaceindicating the feature of corrosion is not obvious. The number and size ofcorrosion pits is increase with the corrosion time increasing. Furthermore, thedepth of corrosion pits increase result in the severe corrosion feature on themetal surface.Analysising X70steel corrosion products by EDS,and using the chemicalreaction to analysis the corrosion behavior and mechanism,obtain: corrosionproducts of X70steel initially is iron hydroxides, the product slowly oxidized tostable and protective product of Fe2O3and Fe3O4with the increase of power-ontime, mainly process is oxygen corrosion.