Electrochemical Corrosive Behavior Of X70 Steel In Different PH Value Contaminated Silts

In this paper,the resistivity,electrochemical impedance spectroscopy（EIS）,polarization curve（PC）,scanning electron microscopy（SEM）and energy spectrum analysis（EDS）were used to study the differences of electrochemical corrosion behavior of X70 steel in different pH contaminated silt（H₂SO₄ and NaOH changed the pH value of silt）and its corrosion mechanism.Through the analysis of the test results of the resistivity of different pH contaminated silt,it is found that the resistivity of the contaminated silt increases with the increase of the pH value under the acidic condition,and the alkaline condition decreases with the increase of the pH,and the correlation between the resistivity and the corrosivity of contaminated soil can be preliminarily determined:the strong alkaline contaminated silt（pH=11.98 and 13.07）is the strongest,The corrosiveness of the acid（pH=1.50,3.78 and 5.03）and the weakly alkaline（pH=9.09）contaminated silt has been greatly reduced with time,and the corrosion of neutral silt（pH=7.07）is rather weak.The macro and micro morphology of X70 steel and the results of energy spectrum analysis show that X70 steel has different degree of local pitting in different pH contaminated silt,and the corrosion products are mainly Fe₂O₃ and Fe₃O₄.But in strong alkaline（pH=11.98 and 13.07）contaminated silt,there is only slight corrosion on the surface of X70 steel.Through the analysis of the test results of polarization curve（PC）,it is found that the instantaneous corrosion rate of X70 steel in different pH contaminated silt decreases with the increase of pH value,but as the corrosion time goes on,the deposition of passivation film formed on the surface of X70 steel or the accumulation of different corrosion products will produce different degrees of influence on the instantaneous corrosion rate of the X70 steel.X70 steel appears passivating in strong acid（pH=1.50）and strongly alkaline（p H=11.98 and 13.07）contaminated silt,which makes the surface of X70 steel slightly corroded.The analysis of the test results of electrochemical impedance spectroscopy（EIS）shows that the Nyquist diagrams of X70 steel buried in the contaminated silt with different pH values are composed of the reactance arc and the diffusion arc in the middle and low frequency regions of the high frequency region.The corrosion process is mainly controlled by the charge transfer process and the diffusion process,that is,the electrochemical polarization and the concentration polarization occur simultaneously.When the buried time is 7d and 14d,the charge transfer resistance Rct decreases with the increase of the pH value of the contaminated silt.When the embedding time reaches 21d,the charge transfer resistance Rct is the largest in the pollution-free silt environment with pH 7.07,that is,the weakest corrosion.