The Study Of The Corrosion Resistance Of X70 And The Protection Mechanism Of Iron Oxidation

The present work is aimed to service the Transmitting from west to east construction project. Potentiodynamic curves and E-pH diagram analysis have been used to study the corrosion effect of aggressive ions such as CO₃^2- HCO₃^- SO₄^2- and Cl^- in the soil in the west part of China on X70 pipeline steel. The results show that there is no obvious difference between the corrosion effect of SO₄^2- and Cl^- on X70 steel in acidic solutions. X70 steel behaves active dissolution in both solutions; X70 steel has a more corrosion resistance due to its homogeneous microstructure and its chemical composition containing many passivated elements in comparison with A3 steel. However, in the alkaline solutions containing CO₃^2- and HCO₃^- respectively, the corrosion resistance of X70 steel varies with pH value of solutions. In the solutions containing HC₃^- with 8 pH value, X70 steel begins to appear passive behavior and it has a worse corrosion resistance in comparison with A3 steel. As the pH rises to 10, the corrosion resistance of X70 steel becomes better than that of A3 steel. But when the pH rise to 13, the corrosion resistance of X70 becomes worse again than that of A3 steel.The constitute, structure of corrosion product will greatly affect the corrosion behaviors of metal after it was corroded, the corrosion product formed in nature is very complex, so the effect only Fe₂O₃ film on metal was studied in order to facilitate the investigation. This part of the present work was aimed to study the mechanism that Cl^-penetrated Fe₂O₃ oxide scale which was formed on pure Fe at 1100℃ for 5 hours. The potentiodynamic measurements showed that the Fe₂O₃ oxide scale has certain protection role from corrosive ions. EIS and oxide film ion-selected measurements indicated the oxide scale has a property of anion-selected and anions such as Cl^- can penetrate the film and degenerate the corrosion protection of the film. Mott-Shottky plot analysis further demonstrated that the oxide film was a n-type semiconductor and the higher donor density under high potential further indicated the existence of Cl^- in the film.