On corrosion of carbon steels in Callovo-Oxfordian clay: complementary EIS, gravimetric and structural study providing insights on long term behaviour in French geological disposal conditions

Abstract

Dedicated integrated experiments have been used to determine in situ the corrosion rate of carbon steels in geological repository mock-ups (90uC, anaerobic, slow synthetic water flux…). Electrochemical impedance spectroscopy (EIS) measurements were performed, allowing for the extraction of instantaneous corrosion rate. In a separate but comparable set-up, occasional sampling among a batch of fifteen equivalent cells was performed, allowing for both gravimetric measurements and corrosion products characterisation. In both cases, the corrosion rate decreases continuously but the stationary state seems not to be reached within the duration of the EIS experiment. The slow kinetics and pseudo steady state of the corrosion process are confirmed by post-exposure characterisation of the corrosion products by SEM, micro-Raman spectroscopy and micro-XRD, revealing the rapid formation of an iron phyllosilicate layer that may act as a barrier layer for diffusion of the species between the clay environment and the alloy. This analysis provides good confidence on long term (over a millennium) extrapolations made from the experimental data fits, leading to an estimation of reasonable cumulated damages in the [40;120] mm range after 1000 years exposure.

Corrosion of carbon steel in clay compact environments at 90 °C: Effect of confined conditions

“…These specimens were reacted for more than six years at 90 °C (the peak temperature expected for the repository) under anoxic conditions in water-saturated Cox in a dedicated setup [38], under constant slow flux of water in equilibrium with clay. These experiments extend and complete previous observations made on shorter time scales [39]. Corrosion damage was assessed by weight loss, and the nature and distribution of corrosion products (CPs) by microscale investigations.…”

Characterization of Corrosion Behavior of Irradiated X65 Low Carbon Steel in Aerobic and Unsaturated Gaomiaozi Bentonite

“…The average aerobic corrosion rate of X65 low carbon steel in Gaomiaozi bentonite containing 17 wt% Beishan groundwater exposed to gamma irradiated and thermal aging is (45.16 ± 1.53) lm/year. It is timely higher than the corrosion rate of carbon steel in anaerobic MX-80 and Callovo-Oxfordian clay as 3-20 lm/year [15][16][17][18][19]. King et al [28] claimed that O 2 diffusion controlled the corrosion rates of the carbon steel in bentonite.…”

“…However, most of the simulating investigations on the carbon steel canister corrosion behavior have been focused on the latter anaerobic period. Martin et al [15][16][17] and Schlegel et al [18,19] investigated the corrosion behavior of carbon steel in anaerobic and saturated MX-80 and Callovo-Oxfordian clay at 90°C for up to 40 months. They found that the corrosion rate of carbon steel was more than 20 lm/year at the beginning and decreased to 3 lm/year after the protective corrosion product layers being generated.…”

Corrosion of carbon steel components in the French high-level waste programme: evolution of disposal concept and selection of materials

“…cells for studying the evolution of the instantaneous corrosion rate in clay porewater, argillaceous suspensions or claystone, by electrochemical impedance spectroscopy [6] (Figure 5); . for the long-term tests, banks of cells containing steel specimens coated in claystone in anoxic conditions [7]; . centimetric mock-ups representing an overpack inside a tube closed at its ends and drilled with calibrated holes to simulate penetrating defects in the lining.…”