Accelerated Corrosion of Stainless Steels in the Presence of Molten Carbonate below 923 K
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Ken-ichiro OTA, Katsuya TODA, Naobumi MOTOHIRA, Nobuyuki KAMIYA Department of Energy Engineering, Yokohama National University 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240, Japan ABSTRACT The high temperature corrosion of stainless steels (SUS316L and SUS310S)
in
the presence of molten carbonate [(Lio.62Ko.3 8)2 C03 and (Lio. 52Nao.48)2CO 3] has been
studied in a C0 2-0 2 atmosphere by measuring the weight gain of the specimens. The corrosion of SUS316L significantly depended on the reaction conditions. With the carbonate coating (both Li/Na and Li/K carbonates), severe corrosion occurred during the initial period of the corrosion test below 923 K, especially around 823 K. The initial severe corrosion was a local corrosion which produced through holes in the metal specimens and occurred more clearly at low Pco 2 with the Li/Na coating than with the Li/K coating. The corrosion became more severe at higher CO 2 pressures and lower 02 pressures. In a pure CO 2 atmosphere (without 02), the corrosion rate significantly increased at 823 K. The steel was corroded uniformly at that time. INTRODUCTION
Generally the energy conversion processes utilizing a molten salt have many advantages, such as a high reaction rate and high conversion efficiency. However, the The molten processes always have material problems for commercialization. carbonate fuel cell (MCFC) is expected to be one of the most promising power generation systems for the coming century due to its high efficiency, excellent environmental characteristics and the ability to utilize a wide variety of fuels. Besides the MCFC, molten carbonate could be used as the electrolyte for other electrochemical systems, such as the high temperature carbon dioxide separator or the high temperature electrochemical heat pump using the water gas shift reaction[I]. In these systems, material durability is quite important for long term operation[2]. Although the long term operation of 40,000 h was proved by using a 100 cm2 cell[3], several materials should be improved before commercialization. In present MCFCs, the stainless steels such as SUS316L or SUS310S, which have been used for the separator alloy or the current corrector, have some corrosion in an oxygen-containing atmosphere (cathode side) the presence of molten carbonate. The corrosion separator or the current collector causes an increase in the ohmic resistance due to the formation of oxide scale and the loss of the electrolyte due to the corrosion reaction. The decrease in the metallic corrosion in the presence of molten carbonate is thought to be very important in order to decrease the decay rate of the cell. Besides this, SUS316L has been reported to have severe corrosion with the presence of the Li/Na carbonate melt below 923 K[4] that causes trouble during the start of MCFCs. In this paper, the corrosion of stainless steels (SUS310S and SUS316L) has been studied in the presence of molten carbonates [(Li 0.62K0.38)2CO 3 and 231
Mat. Res. Soc. Symp. Proc. Vol. 496 01998 Materials Research Society
(Lio. 52Nao.48)2CO
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