Galvanic corrosion of carbon steel in anoxic conditions at 80°C associated with a heterogeneous magnetite (Fe_3O_4) / mackinawite (FeS) layer

摘要

This study relates to the problem of long-term interim storage of nuclear wastes. In France, it is envisaged to store high-level radioactive wastes at a depth of ～500 m in a deep geological disposal, drilled in a very stiff (indurated) clay (argillite) formation. Carbon steel sleeves will be inserted inside the horizontal galleries and a specific cement grout (pH ～10) will be used to fill the gap between the carbon steel sleeve and the argillite. A temperature as high as 90°C is expected at the steel surface because of the intense radioactivity of the wastes and anodic conditions will rapidly prevail. One of the possible risks of localized conosion of the carbon steel sleeve is the galvanic coupling that might happen between two zones of the metal in contact with different environments, i.e. argillite and cement grout. Such a galvanic effect could persist if different corrosion products were to form on the two different zones. To address this problem the influence of a heterogeneous corrosion product layer made of magnetite and mackinawite was studied. Magnetite is expected to form in alkaline conditions (e.g. metal in contact with cement grout) while mackinawite is expected to form locally as a consequence of the presence of pyrite FeS_2 in argillite or the activity of sulfate reducing bacteria. Anodic polarization of carbon steel electrodes in deaerated 0.01 M NaCl + 0.01 M NaHCO_3 + 0.001/0.01 M Na_2S solutions of pH = 7-7.5 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 in mackinawite. This suggests that part of the electrode surface was blocked, covered by 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 by a ～5 mm thick argillite layer enriched with Na_2S were then studied in a 0.01 M NaCl + 0.01 M NaHCO_3 solution at 80°C. A galvanic corrosion could be observed, associated with a significant galvanic current density (～20 μA cm~(-2)) that remained constant during the 11 days of experiment. It led to the formation of a layer made up of mackinawite (and chukanovite) on the steel electrode. This result confirms that heterogeneous magnetite/mackinawite layers can favor galvanic corrosion even in anoxic conditions.