Analysis of Corrosion Behavior Transition of Q235 Mild Steel with the Ionic Concentration in Oil Production Water Simulation Solutions

To investigate the influence of the concentrations of metal cations Ca2+, Mg2+, and Cl− in oil production water on the corrosion of Q235 steel, electrochemical impedance spectroscopy, and potentiodynamic polarization curves were used to assess the electrochemical properties of Q235 steel quantitatively. The results showed that the surface’s sample gradually changed from active dissolution to passivation state with the increase of solution concentration, and the corrosion was significantly restrained at high ion content. When the solution medium concentration is 4.92 mol/LNaCl + 0.165 mol/L CaCl2 + 0.054 mol/L MgCl2·6H2O, Q235 steel performs optimally in terms of the corrosion property. X-ray diffraction and Raman analysis reveal that the corrosion products in the solution are mainly composed of calcium and magnesium ion complex carbonate (Mg3Ca(CO3)4), γ-FeOOH, as well as iron oxides Fe2O3 and Fe3O4. Among them, Mg3Ca(CO3)4 and γ-FeOOH, as a sedimentary layer, have excellent corrosion resistance, which does not exist at low ionic concentration (where there were iron oxides only) and gradually emerge with the rise of Mg2+ and Ca2+. This is crucial for the anticorrosion performance of Q235 steel and responsible for the active-passive corrosion transition behavior.