The Swiss waste management program foresees that low- and intermediate-level radioactive waste will be disposed of in a deep geological repository constructed in Opalinus Clay. Gas is expected to be generated in the repository due to the decomposition of organic materials and the corrosion of metals, with carbon steel being the primary source.
The corrosion behaviour of mild steel under anoxic conditions has been studied over the course of several years in order to better understand the long-term hydrogen evolution profile under anticipated repository conditions. Steel, either bare or encased within mortar, was tested in water vapour or immersed in electrolytes representative of aged cement waters. Tests were primarily conducted at 50 °C as the temperature in the repository will be 35-45 °C depending on the selected site.
The corrosion rate was measured indirectly through hydrogen analysis using a solid-state probe, which yields good sensitivity, permitting corrosion rates as low as 0.01 nm/year to be measured if hydrogen is accumulated over a three month period.
As metallic specimens were clean prior to incorporation into the test cells under anoxic conditions, a period of time, typically less than one year, was required for the formation of corrosion products and the stabilization of the corrosion rate. Thereafter, the corrosion rates decreased steadily. For steel in water vapour, the corrosion rate reduced from a peak of 100 nm/year to less than 1 nm/year after four years. Similarly, for all steels in alkaline environments (either embedded in mortar in water vapour, or immersed) corrosion rates were invariably less than 1 nm/year after several years of analysis.
For these analyses, it was necessary to take into account the hydrogen evolved by the cement. Additional steps were taken to minimize debris within the cement, by attritor-milling clinker with ceramic grinding media, but the hydrogen generation was still found to be significant, albeit at a fraction of that observed for commercial cements cured in oxygen-free environments [1]. Additional experiments were conducted, where attritor-milled mortar-encased steel was left to cure in air before being transferred to an anoxic environment. For these cells, cement detritus appears to be fully consumed within a matter of weeks and the corrosion rates of the steel were less than 0.5 nm/year. Similarly, hydrogen evolution from two air-cured commercial cements was tested and found to be comparable to attritor-milled cements cured in an anoxic environment. Hydrogen evolution from cement is thus likely to be a concern only for the interpretation of experiments.
[1] N. Senior, R. Newman, S. Wang, N. Diomidis, “Understanding and quantifying the anoxic corrosion of carbon steel in a Swiss L/ILW repository environment,” Corros. Eng. Sci. Technol. 52 (2017) 78–83. doi:10.1080/1478422X.2017.1303102.
steel, anoxic, corrosion, low and intermediate level waste, hydrogen evolution
