Study On The Electrochemical Behavior Of X80 Steel In Red Soil Simulated Solution And Thin Electrolyte Layer

The length of the "West-East pipeline" is about 1/5 across the acid red soil region,The corrosion failure of X80 pipeline steel in red soil environment is an important problem for its service life and safety.At present,there is no deep research on the corrosion of pipeline steel,such as the effect of corrosive ions,and the influence of corrosion product film on the corrosion rate,and the corrosion of X80 steel in the thin electrolyte layer?TEL?of the disbonded coating interface.Therefore,it is necessary to study the corrosion electrochemical behavior of pipeline steel in the red soil simulated solution and its TEL.In this paper,the effect of SO₄^2-in the simulated solution and the corrosion product film on corrosion behavior of X80 steel were studied by potentiodynamic polarization,electrochemical impedance spectroscopy?EIS?,three-dimensional video microscopy and scanning electron microscopy?SEM?.Besides,the effect law of pH and TEL thickness on the electrochemical corrosion behavior of X80 steel under TEL were analyzed.The results showed that in the acid soil simulated solution,the corrosion rate of X80 steel has obvious changes with diversification of the concentration of SO₄^2-.When the content of SO₄^2-in the simulated solution is 1%⁵%,SO₄^2-can inhibit the adsorption of Cl-on the electrode surface,and thus slow down the corrosion,the corrosion rate of X80 steel decreases with the increase of the concentration of SO₄^2-.When SO₄^2-is higher than 5 wt.%,high concentration of SO₄^2-can damage the formation of surface film on the electrode and accelerate the corrosion,the corrosion rate increased.In the simulated solution with 1%¹0%SO₄^2-,all EIS of X80 steel showed a capacitive arc in the high frequency and a inductive arc in the low frequency.Both the analysis results of corrosion weight loss and corrosion polarization curve showed that the thickness and micro-structure of the corrosion product film had the significant effect on the corrosion rate of X80 steel in red soil simulated solution.When the X80 steel was immersed in the simulated solution for 4 days,the protective initial corrosion product film was formed on the electrode surface,and at now the corrosion rate of the sample is the lowest.With the increase of immersion time,the X80 steel surface was coated with the medium corrosion product film,the film is divided into the thinner black inner and the loose yellow outer layer,then the protective effect of corrosion product film was very bad,and so the corrosion rate increased significantly.When the immersing time is 30 days,the sample surface is coated with a thicker but more loose late corrosion product film.Meanwhile,the corrosive ions in the solution are easy to react with the metal substrate through the defects of product film,and the corrosion rate is accelerated to some extent.In addition,the electrochemical data and corrosion morphology of X80 steel under TEL with various pH and thicknesses were obtained.The comprehensive results showed that the corrosion of X80 steel is affected by the corrosion product film and oxygen diffusion in the 80 ?m TEL with pH of 4.0,the corrosion rate of X80 steel is the lowest.When the pH value of 80 ?m TEL increased to 5⁷,the oxygen reduction reaction controlled the corrosion process,and the corrosion rate of X80 steel increased slightly.When TEL thickness increased to 110?m,280?m and 1000?m,the corrosion rate of X80 steel decreases with the increase of pH value,which is consistent with the results obtained in the bulk solution.