Galvanic corrosion of carbon steel in anoxic conditions at 80 °C associated with a heterogeneous magnetite (Fe3O4)/mackinawite (FeS) layer

Anodic polarization of carbon steel electrodes in deaerated 0.01 mol L−1 NaCl + 0.01 mol L−1 NaHCO3 + 0.001/0.01 mol L−1 Na2S solutions (with pH ∼ 7.5) led to heterogeneous layers made of magnetite and mackinawite. A locally accelerated dissolution of the steel was observed in any case and the most corroded areas were covered with a layer enriched with mackinawite. This suggests that part of the electrode surface was blocked, covered with a magnetite rich layer where water reduction was favored. The effects of the galvanic coupling between a magnetite electrode and a bare carbon steel electrode covered with a ∼5 mm thick argillites layer were then studied in a 0.01 mol L−1 NaCl + 0.01 mol L−1 NaHCO3 solution at room temperature and 80 °C. Electrochemical measurements demonstrated that a relatively high galvanic current initially flowed between the magnetite electrode (cathode) and the steel electrode (anode). However, this current decreased with time down to negligible values. Micro-Raman spectroscopy analysis revealed that a magnetite layer was formed on the steel electrode, thus suppressing the initial difference between anode and cathode. When the same experiment was performed with the steel electrode set under an argillites layer enriched with Na2S, the galvanic current density remained constant at a significant value (∼20 μA cm−2) during the 11 days of experiment. It led to the formation of a layer composed of mackinawite (and chukanovite) on the steel electrode. This result confirms that heterogeneous magnetite/mackinawite layers can favor galvanic corrosion even in anoxic conditions.