The anoxic corrosion behaviour of carbon steel in anoxic alkaline environments simulating a Swiss L/ILW repository environment

Abstract

The Swiss waste management programme foresees that low‐ and intermediate‐level radioactive waste will be disposed of in a deep geological repository constructed in Opalinus Clay. Gas generation is expected 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 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 at 50°C. The corrosion rate was measured indirectly through the hydrogen analysis using a solid‐state probe. The hydrogen evolution behaviour of grout was also monitored to more accurately determine the hydrogen generating from the corrosion of the embedded steel. For steel in water vapour or in alkaline environments, embedded in cementitious material or immersed in simulated aged cement pore water, corrosion rates were invariably <1 nm/year after several years of analysis.

Release and speciation of14C during the corrosion of activated steel in deep geological repositories for the disposal of radioactive waste

“…This indicates that 14 C present within the matrix of irradiated stainless steels will potentially be released at a very slow rate. Likewise long-term uniform corrosion rates for carbon steels are generally only a few 10s nm/a, and for carbon steel embedded in unsaturated, anoxic cement corrosion rates as low as 2–4 nm/a have been reported recently (Senior 2017).…”

“…In absence of early water ingress, as would be expected in clay-based repositories, oxic unsaturated conditions are expected to gradually evolve to anoxic unsaturated conditions as oxygen encased in the cement pore space is consumed by corrosion and other processes. While there is no experimental information on the release and speciation of 14 C under such conditions, corrosion rates for carbon steel embedded in unsaturated, anoxic cement have been reported to be as low as 2–4 nm/a (Senior 2017). The released dissolved 14 C species can only slowly diffuse away in the thin water film of the unsaturated cement backfill and accumulate near the metal/cement interface, which can lead to precipitation for some solutes, such as carbonate that forms calcite.…”

“Impact of γ-radiation on carbon steel anaerobic corrosion and H2 production.”

“…6), which passivate and protect the carbon steel coupons surface. The identification of magnetite as predominant corrosion product has previously been observed in several carbon steel anoxic corrosion studies performed either under un-irradiated conditions [6,8,9,[31][32][33][34] or under irradiation [14,23,[35][36][37]. In previous studies dealing with long term anoxic corrosion under irradiation [14,38] such progressive magnetite formation has been associated to corrosion rate decrease.…”

Experimental factors and their impact on understanding corrosion in anoxic environments

“…For a DGR, the length of time between construction, emplacement and sealing is of the order of many years and involves atmospheric exposure which is understood to rapidly consume metallic particles within the cement due to their high surface area to volume ratio. [ 6 ]…”