Carbon continued high-temperature corrosion

The hot carbonate system has been promoted with organic materials, which not only increase the rate of CO2 hydrolysis, but also alter the equilibrium to lower the CO2 vapor pressure above the lean solvent. Because this process operates at a continuous high temperature, organic materials may degenerate to produce corrosive by-products therefore, a corrosion inhibitor usually is used in combination with the organic promoter. When H2S is being absorbed, oxidative-type inhibitors must be avoided because of the possible production of solid sulfur in the solution. 

The two types of high temperature fuel cell are quite different from each other (Table 6). The molten carbonate fuel cell, which operates at 650°C, has a metal anode (nickel), a conducting oxide cathode (e.g. lithiated NiO) and a mixed Li2C03/K2C03 fused salt electrolyte. Sulphur attack of the anode, to form liquid nickel sulphide, is a severe problem and it is necessary to remove H2S from the fuel gas to <1 ppm or better. However, CO is not a poison. Other materials science problems include anode sintering and degradation, corrosion of cell components and evaporation of the electrolyte. Work continues on this fuel cell in U.S.A. and there is some optimism that the problem will be solved within 10 years. 

A dense and electronically insulating layer of L1A102 is not suitable for providing corrosion resistance to the cell current collectors because these components must remain electrically conductive. The typical materials used for this application are 316 stainless steel and Ni plated stainless steels. However, materials with better corrosion resistance are required for long-term operation of MCFCs. Research is continuing to understand the corrosion processes of high-temperature alloys in molten carbonate salts under both fuel gas and oxidizing gas environments (29, 28) and to identify improved alloys (30) for MCFCs. Stainless steels such as Type 310 and 446 have demonstrated better corrosion resistance than Type 316 in corrosion tests (30). 

The most critical parts, the contact parts of the rotating assembly, are most frequently made in one of the many stainless steels. Nevertheless some manufacturers continue to supply howls in carbon steel. The 316-type stainless steel is a common material for the smaller decanters, but for the larger machines at higher g-forces, materials with higher strength, such as the duplex stainless steels, must be used. Where high temperatures, or extra high speeds, or corrosive materials are in use, special corrosion-resistant or temperature-resistant materials may be used. 

Dissolution Some waters continuously dissolve appreciable amounts of copper . Factors that favour this action are high free carbon dioxide, chloride and sulphate contents, low hardness, and increase of temperature. The trouble is therefore most prevalent in hot, soft, acid waters. The corrosion is general and the resulting thinning is so slight that the useful life of the pipe or component is virtually unaffected (unless impingement attack... 

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