Experimental study on corrosion behavior of carbon steel in buffer material, 2; Analysis of corrosion products on coupons immersed for 10 years duration

Taniguchi, Naoki ; Kawasaki, Manabu; Naito, Morimasa 

Immersion tests of carbon steel were performed in compacted bentonite simulating the buffer material for 10 years duration, and corrosion products formed on the coupon surface were observed and analyzed by various methods. And then, the stability and dissolution/precipitation behavior of corrosion products were discussed from the view point of thermodynamics. The results of this study are summarized as follows; (1) In the case of the test in 80 C, black or dark gray corrosion product was formed on the coupons, and it was tightly adhered to the surface. By contrast, in lower temperature of 50 C, no tight corrosion product was observed on the coupon surface. (2) Ferrous carbonate corrosion products such as siderite, FeCO or Fe(OH)CO were identified as the crystallized corrosion products in seawater type test solutions. In the test result in dilute test solution, magnetite, (FeO) was identified and no carbonate compounds were found. The presence of these compounds was agreed with thermo dynamical aspects assuming the test conditions. (3) Ca, S, Si and Al were detected in corrosion products layer besides Fe, and their distribution patterns were dependent on the experimental conditions. (4) It was indicated that the corrosion rate of carbon steel in compacted bentonite was correlated to the behavior of dissolution/precipitation of the corrosion products. (5) A conceptual model of the corrosion propagation behavior of carbon steel in compacted bentonite was proposed with reference to the existing corrosion models.