The influence of groundwater anions on the impedance behaviour of carbon steel corroding under anoxic conditions

The anoxic corrosion of carbon steel liners to produce Fe2+ and H2 within copper nuclear waste containers could lead to the scavenging of the radiolytic oxidants (H2O2, O2) required to drive nuclear fuel (UO2) corrosion and the release of radionuclides. Steel corrosion has been studied in solutions containing various concentrations of Na2CO3/NaHCO3, Na2SO4 and NaCl to simulate an anticipated change in environment from carbonate-dominated to chloride-dominated groundwater composition over time. A combination of electrochemical techniques, scanning electron microscopy, and in situ Raman spectroscopy has been used. In concentrated carbonate solutions, corrosion is maintained within pores in a siderite (FeCO3·H2O) deposit by the complexation of Fe2+ by HCO3− and the reduction of protons supplied by HCO3− dissociation. In mixed-anion solutions at lower CO32−/HCO3− concentrations, a compact green rust deposit covers the surface and the corrosion rate is suppressed. In concentrated NaCl solutions, the absence of CO32−/HCO3− buffering leads to a rapid shut down of the corrosion process which may be aided by the formation of FeIII surface phases if traces of dissolved O2 are available.