The proposed disposal scenario for high-level nuclear waste (spent fuel) in Canada is emplacement within a sealed, deep geological repository (DGR) located in either granitic rock or sedimentary clay. Disposal is based on a multi-barrier approach, with the primary barrier being a sealed container which could be either dual-walled with a copper shell over an inner carbon steel vessel for granitic rock or a single thick-walled steel container for sedimentary clay. This study focuses on the corrosion behaviour of A516 Gr70 carbon steel as well as the corrosion products formed in a variety of groundwater compositions and concentrations expected within a sedimentary clay DGR environment. In particular, the effects of groundwater anions such as Cl-, HCO3-/CO32-, and SO42- on the corrosion behaviour and corrosion product compositions and morphologies were studied. Several electrochemical and surface characterization techniques were employed to investigate the corrosion behaviour of the steel as well as the identities and morphologies of the subsequent corrosion products.
It was shown that in the presence of trace levels of O2, Cl- is able to induce passivation of the steel surface by the catalytic conversion of Fe2+ to Fe3+ with passivation induced in this manner then leading to the initiation of breakdown sites. The addition of HCO3-/CO32- to highly concentrated Cl- solutions led to a competition between the catalytic formation of FeIII oxides and the stabilization of soluble Fe2+ by complexation with HCO3-/CO32-. In addition, an increase in the total carbonate concentration increased the breakdown potential by preventing the stabilization of pits by buffering the development of acidity required for propagation. In contrast, SO42- was shown not to interfere with the Cl--catalyzed oxidation to FeIII oxides in the presence of trace O2 but to have a significant effect on the breakdown potential, possibly due to its ability to be more strongly adsorbed to the FeIII surface.
Electrochemical experiments performed under totally anaerobic conditions showed that an increase in [Cl-] promoted corrosion leading to an increased roughening of the steel surface. This was attributed to an acceleration of the cathodic reaction on exposed Fe3C bands from the pearlite structure. The addition of groundwater ions led to a suppression of the anodic kinetics due to the accumulation of CaCO3 crystals. Addition of HCO3-/CO32- to buffer the pH to 8.85 led to a significant decrease in the corrosion rate. This was attributed to the growth of a Fe3O4 barrier layer with additional protection provided by an outer layer of Fe2(OH)2CO3.
Complementary long-term corrosion studies showed that an initial period of humid air exposure led to the formation of a γ-Fe2O3 layer which was subsequently reduced to Fe3O4 by galvanic coupling to steel dissolution over approximately the first 100 days of exposure. Corrosion occurred preferentially at pearlite grains due to the lower cathodic overpotential on the Fe3C lamellae. Addition of groundwater ions suppressed steel corrosion due to the rapid deposition of CaSO4 and CaCO3 crystals. High levels of Mg2+ were shown to promote the formation of aragonite, a polymorph of CaCO3 known to cause a reduction in steel corrosion rates. Finally, the addition of HCO3-/CO32- led to the rapid formation of Fe2(OH)2CO3, attributed to the initial γ-Fe2O3 layer whose reduction led to high [Fe2+] and the promotion of Fe2(OH)2CO3 deposition. However, thermodynamic transformation of Fe2(OH)2CO3 to FeCO3 appeared to induce some localized corrosion/pitting processes.
The influence of H2O2 on steel corrosion under deaerated and totally anaerobic conditions was studied to determine whether radiolytic oxidants produced by the radiation fields in the fuel waste form would influence corrosion of the inside of a failed waste container. The interaction of the H2O2 with the steel was confirmed by the presence of FeIII-containing corrosion products. The results showed that continuous steel corrosion can be expected in an anaerobic environment but that passivation occurred in the deaerated experiment. However, passivation was attributed to the higher levels of dissolved O2 present and not the addition of H2O2 used as a surrogate for radiolytic oxidants. As such, active steel corrosion should be maintained inside a failed container and the soluble corrosion products (Fe2+ and H2) will be available to suppress fuel corrosion and radionuclide release
