The Canadian nuclear waste management concept envisages using carbon steel as a primary engineered barrier for isolating nuclear waste in a deep geological repository (DGR) located in sedimentary rock. Steel corrosion in anticipated repository environments has been studied, but was mostly focused in two main areas: (i) aerobic or oxygen containing environments (both in vapour and liquid phases); and (ii) anaerobic, solution environments. The atmospheric corrosion behaviour of steel in a humid, anaerobic or anoxic environment, is a new topic, with virtually no published data to on which to rely.
This study was undertaken to improve the existing knowledge of anaerobic, atmospheric corrosion of carbon steel. Atmospheric corrosion testing was conducted on carbon steel wires in anoxic atmospheres at various temperature and relative humidity (30-100% RH), with and without sodium chloride (NaCl) contamination of the wire surfaces. Hydrogen evolved from corrosion was monitored and converted to an estimated corrosion rate. With salt on wire surfaces, sustained final corrosion rates in the range of 0.01 to 0.8 μm·y-1 were observed over test durations of 935 to 1725 hours. Without salt contamination, the corrosion rates are very low, and can only be detected using a solid-state electrochemical hydrogen sensor. The hydrogen sensor can detect the pressure increase down to a limit of ca. 0.1 Pa, corresponding (depending on the exact procedure) to a corrosion rate as low as ca. 0.0001 μm·y-1. The estimated corrosion rates for the degreased and pickled wires were found to be < 0.01 μm·y-1.
In parallel with the corrosion experiments, corrosion product surface analyses were performed using different techniques. Oxides formed on steel surfaces consist mostly of the mixed Fe2+/Fe3+ spinel oxide Fe3O4. The experimental results of this study will be applied to assess the corrosion behaviour of carbon steel containers during the anoxic, unsaturated phase of a deep geological repository in Canadian sedimentary rock.