Galvanic corrosion of carbon steel under argillite layers in carbonated media

摘要

This study relates to the problem of long-term interim storage of nuclear wastes. In France, it is envisaged that high-level radioactive wastes will be confined in a glass matrix, stored in a stainless steel canister, itself placed in a carbon steel overpack. The wastes will then be stored at a depth of ～450 m in a deep geological disposal, drilled in a very stiff (indurated) clay (argillite) formation. Because of the intense radioactivity, a temperature as high as 90℃ is expected at the steel surface. In these conditions, magnetite, siderite and chukanovite are the main expected corrosion products of steel .One of the possible risks of localized corrosion of the carbon steel overpack is the galvanic coupling that might happen between two zones of the metal covered with different corrosion products or in contact with different electrolytes. This phenomenon is also known in the oil and gas industry and often described as mesa-corrosion.rnTo address this problem, the effects of the galvanic coupling between a magnetite electrode and a bare carbon steel electrode were investigated at 25℃ and 80℃. Both electrodes were covered by a ～5 mm thick argillite layer that strongly hinders the transport of oxygen and more generally the transport of matter [1-2], and immersed in a NaHCO3 (0.01 mol L~(-1)) + NaCl (0.01 mol L~(-1)) solution. The current density between the two electrodes was monitored thanks to a ZRA (zero resistance ammeter) method for different surfaces ratios (S_(anode)/S_(cathode)). To simulate a local source of sulfide species (sulfate reducing bacteria or pyrite FeS2, a mineral present in argillite), the galvanic coupling between a first carbon steel electrode immersed in a sulfide-containing solution and another immersed in a NaHCO_3 + NaCl solution was studied at 80℃, the electrodes being covered by a ~5 mm thick argillite layer. In both kinds of experiment, μ-Raman spectroscopy was used to identify the compounds formed on the electrodes at the end of the experiment.rnIn both cases, galvanic corrosion could be observed, associated with significant galvanic current densities (10-60 μA/cm~(-2)). The galvanic current proved more important between magnetite (cathode) and steel (anode) at 80℃. In most cases, the galvanic current tended to decrease with time. Raman analysis revealed that the electrodes coupled with magnetite were covered by a corrosion products layer made of an inner stratum of magnetite and an outer stratum of siderite and chukanovite. The magnetite stratum was not homogeneous, implying that anodic areas were still remaining on the carbon steel surface.