Corrosion Behavior Of Q235 Low Carbon Steel In Simulated Geological Disposal Environment Of HLW

High level radioactive nuclear wastes（HLW）,as the inevitable products of the nuclear industry,would tnreaten the security of the biosphere without proper ultimate disposal due to the high contents of nuclides and their long half-lives.Nowadays,geological disposal is widely accepted as the best way to ultimately dispose HLW,namely,HLW is encapsulated in a metallic canister which is buried in the underground at a depth of 500 to 1000 m for its permanent isolation from the biosphere.During the long-term disposal,corrosion failure is believed to be the important factor to cause the leakage of HLW,thus,the corrosion performance of the metallic canister is the most important property when choosing the candidate material for manufacturing it.Low carbon steel,as a kind of qusi-corrosion material,is selected as the candidate material by many countries since it has a low tendency to be attacked by localized corrosion,has high strength and is low-cost.During the long-term disposal of HLW,the main environmental factors that affect the corrosion behavior of low carbon steel include the oxyen contents,the compositions of groundwater and the temperature.This paper investigates the influence of these three environmental factors on the corrosion evolution of low carbon steel in simulated geological disposal environments with electrochemical measurements including the meansurements of potentiodynamic/potentiostatic polarization curves,open circuit potential and electrochemical impedance spectroscopy,corrosion mass loss test and surface analysis methods including X-ray diffraction,scanning electron microscope and energy dispersive spectrometer.Results show that in 0.1 M NaHCO₃ aerobic solution,the carbon steel initially tends to be in an oxidized state of pseudo-passivation which relates to the depolarization of oxygen,while in the deaerated solution,the carbon steel is in a reductive state of acitive dissolution.At the final immersion stage,the carbon steels are all in a stable state with α-FeOOH.Thus,the cathodic process in deaerated solution is dominanted by the reduction of α-FeOOH,while that in aerobic solution simultaneously includes the reduction of oxygen and α-FeOOH.With the increase of dissolved oxygen,the proportion of α-FeOOH and Fe₆（OH）₁₂CO₃ increases.After the addition of 0.1 M NaCl in 0.1 M NaHCO₃ solution,the steel quickly comes into the state of active dissolution initially in both aerobic and anaerobic solution.After the long-term immersion,the corrosion film on the steel is relatively thin and general in anaerobic solution,while that is relatively thick and porous in aerobic solution with the steel attacked by localized corrosion.Compared with the former bicarbonate solution,the reductive resistance of oxygen in the aerobic solution with Cl^-is apparently decreased,which could be attributed to the porous rust layer providing convenient channels for the diffusion of oxygen.In the（0.01 M NaHCO₃ + 0.1 M Na₂SO₄ + 0.1 M NaCl）solution/bentonite with different oxygen contents,the steels are all in a state of active dissolution.The corrosion rate of the steel is promoted by increasing the dissolved oxygen in both simulated solutions and simulated solution/bentonite.In bicarbonate solution/bentonite,the steels are in a state of active dissolution when the concentration of HCO₃^-is below 0.01 M.The corrosion rate of the steel firstly increases and then decreases with immersion.The initial decrease of the corrosion rate is correlated with the protective effect of corrosion products while the later increase is correlated with the deteroirating of the barrier effect of bentonite.In 0.01 M NaHCO₃ solution/bentonite,when the concentration of Cl^-equals to 0.05 M,the initial OCP of the steel increases with fluctuation and then slightly decreases.While when the concentration of Cl^-increases to 0.1 M,the OCP of the steel increases with immersion steadily and then decreases slightly.The addition of chloride decreases the value of charge transfer resistance,especially when the chloride equals to 0.1 M,the steel has a tendency to be attacked by localized corrosion as the determined step of corrosion rate transforms from electron transfer process to diffusion process.In 0.01 M NaHCO₃ solution/bentonite,when the concentration of SO₄^2- is 0.05 M,the initial OCP of the steel increases with fluctuations and then becomes stable.While when the concentration of SO₄^2- is 0.1 M,the OCP of steel initially increases steadily,then fluctuates in a way of square wave and finally decreases a little.SO₄^2- with a low concentration could effectively decrease the corrosion of carbon steel while that with high a concentration only decreases the corrosion rate of the steel in the initial immersion stage.When 0.05 M Cl^-and 0.05 M SO₄^2- are simultaneously added into 0.01 M NaHCO₃ solution/bentonite,the initial OCP of the steel increases with fluctuations and then becomes stable;while when the concentration of Cl^-and SO₄^2- both increases to 0.1 M,the initial OCP of the steel steadily increases and then slightly decreses.The interaction of Cl^-and SO₄^2- could decrease the long-term average corrosion rate of the steel when comparing with that in 0.01 M NaHCO₃ solution/bentonite.In（0.01 M NaHCO₃ + 0.1 M NaCl + 0.1 M Na₂SO₄）solution/bentonite,the promotion of temperature could increase the long-term corrosion rate of the steel.When the temperature keeps at 50℃,the initial state of the steel is unstable with inductive resistance appearing.With immersion,the determining step of the corrosion reactions transforms from electron transfer process to diffusion process.While when the temperature increases to 80℃,the state of the steel is relatively stable with the corrosion process controlled by electron transfer process.To conclude,the corrosion evolution of low carbon steel in simulated geological disposal environmrnts is strongly influenced by the dissolved oxygen contents,the chemical compositions of the groundwater and the temperature.When the contents of dissolved oxygen is high,the groundwater contains a high concentration of chloride or the temperature is high,the corrosion rate of low carbon steel would be high,which could not satisfy the requirements for service life of the metallic canister in the geological disposal of HLW.