Influence Of Elemental Sulfur On The Structure And Protective Performance Of Fe S And FeCO₃ Film

When the velocity of gas and liquid was not big enough, sulfur in pipelines would precipitate on the surface of steels, while the speed increased, sulfur existed in the solution in the shape of suspension, and pipelines were corroded by suspended sulfur. So the exploitation of oil and gas was seriously influenced by metallic corrosion, which occurred in sulfur bearing system, and it became the concern of people. Although corrosion products generated by corroding in H2S/CO2 environment had good protective performance, it was easily affected by many factors, such as temperature, press, sulfur and acidity, so it is meaningful to study the influence of elemental sulfur on corrosion products in H2S/CO2 environment. To this question, we studied the destructive process of FeCO3 and FeS film in the solution contained sulfur, the mechanism of inhibitor under films was also researched.In this work, a FeCO3 film was prepared by using anodic electrolysis of steel in NaHCO3 solution at atmospheric pressure and 80?, and a FeS film was formed by immersing steel in the Na2 S solution at the same conditions. Then, the protective performance, destructive processes and effect of inhibitor on these FeCO3 and FeS films in sulphuric system were studied by immersion test, electrochemical impedance spectroscopy(EIS), potentiodynamic polarization, environmental scanning electron microscope(ESEM) and Three-dimensional microscope. Results were as following:In the simvlative solution, FeCO3 and FeS film had good protective performance in the absence of elemental sulfur, and the FeCO3 film had better protective performance, also the protective performance of these two films weakened with the decrease of pH value. However, the protective performance disappeared with the presence of sulphur, and FeCO3 film was better than FeS film to some extent. In the neutral system, compared to FeS film, FeCO3 film could accelerate local corrosion seriously; while the pH of solution was equal to 3, extent of local corrosion of the steels with or without films decreased greatly. Because of the occurrence of galvanic corrosion caused by the different potential between the covered and uncovered areas by sulfur, local corrosion is the important corrosion formation generated by immersing carbon steel in the solution contained sulfur-deposit and suspended sulfur.In the solution contained inhibitor, with the concentration of inhibitor increased, the corrosion rate decreased first and then increased in the absence of elemental sulfur. The corrosion rate of steels with films was lower than the steels without film, however, the inhibitive efficiency of bare steels was the highest, and the steels covered by FeS film was the better one. With the acidity of medium enhanced, inhibitive efficiency enlarged monotonously. The inhibitive effect would disappear with the participation of sulfur, local corrosion was also accelerated by inhibitor, and samples with FeCO3 film corroded most severely. In addition, the corrosiveness of carbon steel corroded under sulfur deposition was more serious than corroded in the environment with suspended sulfur.