The Corrosion Behavior Of Candicate Materials Of Nuclear Waste Containers Under Deep Geological Disposal Environment

The method of deep geological disposal is to encapsulate the high radioactive nuclear waste in metal storage container,wrap the buffer material around the body of container,and stores it in an underground repository.Our country has selected the Beishan area of Gansu as the best site for deep geological repository,and selected the Gaomiaozi bentonite as the buffer material.However,the selection and design of metal materials for nuclear waste containers are still pending.Among the factors that lead to container failure,corrosion damage has the greatest impact.Therefore,it is very important to simulate the deep geological environment to study the corrosion resistance of candidate materials for containers.According to the evolution discipline of deep geological environment,the deep geological environment simulated in this paper includes:（1）early oxic simulated groundwater solution of Beishan area（25～80°C）;（2）later anoxic bentonite/groundwater solution containing 0%,20%and 30%（wt.%）bentonite.The corrosion behavior of Ti alloy TA8-1,Cu alloy B19 and Cu were investigated by short-term and long-term experiments designed in above mentioned deep geological conditions using various electrochemical measurements methods and characterization methods.（1）in this study,in the short-term experiment,electrochemical measurements,including electrochemical impedance spectroscopy（EIS）,potentiodynamic polarization and cyclic voltammetry,are used to investigate the influence of temperature（25～80°C）on the early corrosion properties of the mentioned materials.The results show that Ti alloy TA8-1 has the minimum corrosion current density(10^-7·A·cm^-2)and low pitting susceptibility;For Cu alloy B19 has the good corrosion resistance and low pitting susceptibility at high temperatures（60～80°C）,below 60°C,Cu alloy has a higher corrosion rate and occurs severe pitting;Cu T2 has the relatively high corrosion current density(～10^-5A·cm^-2)and low pitting susceptibility under the studied conditions.（2）in the early oxic（50°C）long-term immersion tests,the evolution of the long-term corrosion behavior of the mentioned materials is investigated by EIS,in combination with SEM,Raman and XPS analysis.After immersion 60 days,the results show that the superior corrosion resistance of Ti alloy TA8-1 is attributed to the formation of a thin passive film（Ti O₂）,which can increase the charge transfer resistance of the interface substantially;The corrosion resistance of the protective Cu₂O/Cu₂（OH）₃Cl deposit layer formed on the surface of Cu T2 is maintained during the two-month immersion test at 50°C;For Cu alloy B19,severe localized corrosion occurs on the surface and the corrosion resistance deteriorates obviously during the immersion test.（3）in this study the evolution of corrosion behavior of Cu T2 in anoxic bentonite/groundwater solution containing Na₂S is studied.The results showed that in anoxic bentonite/groundwater solution containing Na₂S,the resistance of the system decreases with the increasing immersion time.The fitting results show that the resistance of the system mainly comes from the charge transfer resistance of the corrosion interface,which decreases with the increase of immersion time.After adding bentonite to sulfur-containing groundwater,the resistance of the system increased,indicating that bentonite had a certain retardation effect on the diffusion of SH^-to the copper surface;the resistance of pure copper（T2）in the bentonite/groundwater system showed an early（5～30 d）is better than the later period（30～60 d）,which indicates that the retardation of bentonite on SH^-diffusion willweaken with time increasing.