The most efficient construction materials for boiler water walls and superheaters in steam power plants are ferritic and martensitic steels. In practical operation tubes are simultaneously exposed to combustion gas and air/steam on their opposite surfaces. The corrosion behaviour of ferritic-martensitic steels under such dual atmospheres is nondistinctive and has therefore been investigated in a special designed test equipment between 500 and 620°C. The power plant conditions were simulated with a flowing and pressurised (80 bar) combustion gas on the inner side of the tube which mainly consists of H2O and/or CO2. On the outer side, the tube material was exposed to air. It was discovered that under similar temperatures the oxides formed on the air side under dual atmosphere conditions were significantly different to the oxide scales formed when the alloy was exposed to air only. It is assumed that the anomalous corrosion behaviour during the dual atmosphere exposure is due to the hydrogen transport through the bulk alloy from the combustion gas side to the air side.

Subject
Air pressure, Materials, Oxide formation, Tubes, Atmosphere, Carbon monoxide, Oxide scales, Wall thickness, Scale thickness, Power plants, Steel, Carbon, Hydrogen
Keywords:
high temperature corrosion, dual atmosphere, corrosive gases, mild steels, 9-12% Cr-Steels, hydrogen transport
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2009
Association for Materials Protection and Performance (AMPP)
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