[en] In order to clarify the corrosion behavior of carbon steel in buffer material and improve the long-term integrity of carbon steel overpack, immersion tests of carbon steel were performed in compacted bentonite simulating the buffer material for 10 years duration. The behavior of the corrosion propagation has already been published in previous report. This report focuses on the behavior of corrosion products formation on the coupon surface. The results of observation and analysis of corrosion products are summarized as follows; As the results of the observation of surface appearance, black or dark gray corrosion products were formed on the coupons and were tightly adhered to the surface in the case of the test in 80degC. By contrast, in lower temperature of 50degC, no tight corrosion products were observed on the coupon surface. In case of seawater type test solution, ferrous carbonate corrosion product such as siderite, FeCO₃ or Fe₂(OH)₂CO₃ were identified by XRD as the crystallized corrosion products through the test periods. In case of dilute solution, although only FeCO₃ had been identified in short term tests up to 3 years, magnetite (Fe₃O₄) was identified and no carbonate compounds were found for 10 years test. As the results of the analysis by EPMA, Ca, S, Si and Al were detected in corrosion products layer besides Fe, and their distribution patterns were dependent on the experimental conditions. The presence of Al and Si indicates that bentonite was mixed to the corrosion product film, or dissolved species from bentonite were involved in the film. These results were discussed form the view point of thermodynamics. The regions where ferrous carbonates are stable were well agreed with environmental conditions expected in the experiments. The results of calculations of FeCO₃ solubility indicated that the degree of adhesion of corrosion product to metal surface or corrosion rate under film formation seemed to be correlated to the behavior of FeCO₃ dissolution/precipitation. Since the long-term corrosion behavior of carbon steel seems to be associated with properties and conditions of corrosion products, the processes which affects to corrosion rate, such as mass transport, chemical reactions and electrochemical reactions were summarized and an approach for modeling of the corrosion in compacted bentonite in the presence of corrosion product film was suggested with reference to the existing corrosion models. (author)