Corrosion Behaviour of Ferritic and Martensitic Power Plant Steels Under Conditions of Dual Atmospheres

The most efficient construction materials for steam power plant boiler water walls and superheater are ferritic and martensitic steels. In practical operation the tubes are simultaneously exposed to combustion gas and steam or additional gas in case of water walls on their opposite surfaces. The corrosion behaviour of ferritic-martensitic steels under such dual atmospheres is nondistinctive and was therefore investigated in a special designed test equipment between 500 and 620°C. The power plant conditions were simulated as 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 compared 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.