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 (EIS) and potentiodynamic polarization curves were employed to quantitatively assess the electrochemical properties of Q235 steel. The results showed that the surface of the 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 (4.92mol/LNaCl+0.165mol/L CaCl₂+0.054mol/L MgCl₂·6H₂O), Q235 steel performs optimality-corrosion property. XRD 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 do 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 anti-corrosion performance of Q235 steel, and also responsible for the active-passive corrosion transition behavior.