The Corrosion Behavior Of Low Carbon Steel In Simulated Deep Geological Disposal Environment

Deep geological disposal is one approach being considered in China for the long term management of the high-level radioactive nuclear waste (HLRNW). In this method, the metal container is the critical factor that separates the HLRNW from the surroundings. As one of the most important structural materials, low carbon steel (LCS) is currently under assessment to make the container. The service life of the container is directly determined by the active/passive behavior of LCS. If the steel was active and corroded in a general manner with low rate, the container would be safe by using thick-walled overpacks to make allowance for corrosion penetration. If the steel became passive, localized corrosion might occur, which would lead to mechanical failure. Reliable long-term data for the anaerobic corrosion of LCS under repository conditions are of fundamental importance for developing a model to predict the life of the container. The aim of this work is to investigate the effect of HCO3-, Cl-and SO42-on the active/passive behavior of LCS in simulated underground water.The presence of HCO3-enhances both the anodic Fe dissolution and cathodic hydrogen evolution reaction. The situ-measurement of open circuit potential (OCP) reveals that the higher concentration of HCO3-and longer immersion time leads to the higher OCP. When [HCO3-]=0.01mol/L, the OCP is in active region. When [HCO3-]>0.02mol/L, the OCP is in passive region. EIS results shows that the electrochemical reaction is kinetically controlled when [HCO3-]=0.01mol/L and diffusion controlled when [HCO3-]≥0.02mol/L. Results of XRD analysis illustrates that the key corrosion products are mainly composed of Fe3O4and a-FeOOH.After long term immersion in0.01mol/L HCO3-+x mol/L Cl-(x is between0and0.5),0.01mol/L HCO3"+x mol/L SO42-and0.01mol/L HCO3-+y mol/L Cl-+z mol/L SO42-(y+z=0.1) solutions, the LCS is in active state and undergoes uniform corrosion. However the LCS is in transpassive state in0.05mol/L HCO3-+x mol/L CP(x≤0.2),0.05mol/L HCO3-+x mol/L SO42-(x≤0.1) and0.05mol/L HCO3-+y mol/L Cl-+z mol/L SO42-(y+z=0.1) solutions. Cl-and SO42-ions breakdown the passive film, which leads to localized corrosion. Results of XRD analysis illustrates that the key corrosion products are mainly composed of Fe3O4, a-FeOOH and Fe2(OH)2CO3. When Cl-or SO42-ions are present in deaerated bicarbonate solutions. Besides, y-FeOOH is found in the rust formed in0.01mol/L HCO3-+x mol/L Cl-solutions.