| The safe disposal of high-level radioactive waste(HLW)has become the key to restrict the development of nuclear industry.Deep geological disposal is generally accepted internationally as the only feasible scheme,which uses multi-barrier system to isolate HLW from the living environment.In the multi-barrier system,the corrosion resistance of metal containers is the primary factor to meet the stability of the repository system.In view of its mechanical properties and cost-effectiveness,low carbon steel has been considered as the ideal candidate material for metal containers by many countries.However,recent studies have found that the corrosion rate of low carbon steel in simulated groundwater environment is too fast,and there is a tendency of local corrosion,so it is difficult to meet the long-term service life requirements of metal containers for HLW disposal.In order to improve its corrosion resistance,the most feasible method is alloying.In this study,NiCu low alloy steel was selected as the candidate material for metal disposal container.Combined with the disposal scheme of HLW in China and the characteristics of groundwater environment in Beishan area(pre-selected site of repository)of Gansu Province,the corrosion behavior and evolution law of NiCu steel in simulated geological disposal environment was discussed by corrosion weight loss test,corrosion product/rust layer characterization,thermodynamic analysis and electrochemical measurement.Meanwhile,the interaction and mechanism between the corrosion behavior of disposal containers and the properties,structure and buffering properties of bentonite were investigated.The research results provided an important theoretical basis for material selection and service life prediction of metal containers for HLW disposal in China.In the simulated Beishan groundwater solution,the addition of Ni and Cu alloy elements obviously inhibited the cathodic and anodic processes of steel corrosion.Ni and Cu significantly enriched in the inner rust layer of NiCu steel,the density of the rust layer was enhanced,and the content of α-FeOOH in the rust layer was obviously higher than that of Q235 steel.Ni(Ⅱ)and Cu(Ⅱ)transformed from Ni and Cu alloy elements could isomorphic substitute with Fe(Ⅱ)of Fe6(OH)12SO4 to improve the latter oxidation resistance.Meanwhile,Ni(Ⅱ)and Cu(Ⅰ)could made Fe3O4 negatively charged by doping,and effectively repelled aggressive anions in the simulated solution,thus improving the corrosion resistance of steel.With the increase of immersion time,the cathodic process of steel corrosion transformed from hydrogen evolution reaction(HER)to rust reduction,while the anodic process always showed the active dissolution characteristics of iron.After long-term immersion,the corrosion mass loss of NiCu steel was obviously lower than that of Q235 steel,which further proved that the alloying of Ni and Cu is beneficial to improving the corrosion resistance of steel.In the simulated groundwater/bentonite environment,with the increase of ion concentration in groundwater,the buffer performance of bentonite decreased,the bentonite particles significantly aggregated,and the diffraction peak intensity and interlayer spacing of montmorillonite(Mt)in bentonite decreased.Meanwhile,the relative contents of Fe6(OH)12SO4 and Fe3O4 in the rust layer of NiCu steel gradually increased,the cathode resistance and anode resistance decreased,and the corrosion rate(Vcorr)increased.With time,the cathodic process of NiCu steel corrosion transformed from oxygen reduction to rust reduction.In addition,the corrosion mass loss of NiCu steel in the simulated Beishan groundwater/bentonite system was obviously lower than that in the simulated Beishan groundwater system,which suggested that bentonite can significantly inhibit the corrosion of NiCu steel.In the simulated corrosion process of the disposal container,Fe2+/Fe3+(mainly Fe3+)produced by iron corrosion could be adsorbed on the surface of Mt laminate in bentonite,and substituted part of Na+in Mt layers,resulting in the decease of Mt volume and interlayer spacing,and the structural integrity of Mt was destroyed.Meanwhile,the cation exchange capacity and water swelling capacity of bentonite significantly decreased.This shows that the corrosion products of iron-based containers have a strong deterioration effect on the properties,structure and buffering performance of bentonite.Compared with the original bentonite,the cathodic resistance and anodic resistance of NiCu steel in simulated bentonite deterioration environment(100:1 and 10:1 bentonite/iron powder)decreased with the increase of bentonite deterioration degree,and the corrosion rate gradually increased.In the simulated nuclide leakage stage,the concentration of H2O2(radiolysis product)obviously affected the corrosion behavior of NiCu steel in the simulated Beishan groundwater.As[H2O2]≤0.001%,the formation of γ-FeOOH was inhibited,and then the cathodic reduction rate of steel was slowed down.Meanwhile,the relative content of Fe3O4 in the rust layer of NiCu steel increased,and the compactnes of rust layer was enhanced,which can effectively prevent the migration of aggressive anions to the steel matrix,thus improving the corrosion resistance of steel.With the increase of[H2O2](0.01%→1%),the dissolved oxygen content involved in the cathodic depolarization process of steel in the simulated solution increased,the relative contents of Fe6(OH)12SO4 and Fe6(OH)12CO3 in the rust layer ofNiCu steel gradually decreased,and the cross-section rust layer became loose and porous,which reduced the protection to the matrix,resulting in the decrease of cathodic resistance and anodic resistance of NiCu steel,and the corrosion rate of steel increased.The comprehensive analysis showed that NiCu low alloy steel has excellent corrosion resistance in simulated geological disposal environment of HLW,and it is promising to substitute low carbon steel as the ideal material for metal containers for HLW disposal in China. |
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