[en] Geological disposal is thought as the best ultimate disposal way of High-Level-Radioactive -Wastes (HLW) to separate them from biosphere permanently with a multiple-barrier system. The man-made barriers are composed of bentonite and canisters, which play an important role in avoiding HLW leaking. However, once bentonite is soaked by underground water, corrosion of may happen causing the leakage of HLW. Therefore, it's necessary to learn about the corrosion behavior of carbon steel simulated geological disposal conditions. Former studies focus more on the corrosion behavior of carbon steel in underground water without bentonite. The states of the carbon steel surfaces are determined by corrosion potential and the corrosion rates could be calculated by electrochemical impedance spectroscopy. This passage compared the corrosion behavior of carbon steel with bentonite and without bentonite using in-situ electrochemical measurements. Experiments showed that carbon steel would rather passivate when in simulated underground water with high concentrations of HCO₃^- and without bentonite while it is more prone to activate with low concentrations of HCO₃^-. But carbon steel would activate with bentonite and the concentrations of HCO₃^- had little influence on the corrosion modes. What's more, the in-situ EIS results showed that corrosion rates of carbon steel were smaller in bentonite/underground-water than in according underground-water. (authors)