Effect of Temperature Gradient in Ash on High-Temperature Corrosion of Super-Heater Material in a Waste Power Generation Boiler

The effect of a temperature gradient associated with ash deposition on the high-temperature corrosion behavior of heat-resistant steels used for super-heater tubes in a waste-to-energy (WTE) boiler plant was investigated by conducting ash embedded corrosion tests using combustion ash from the actual boiler. The ash-embedded corrosion test was carried out under two conditions: (1) an isothermal corrosion test at 460 degrees C, and (2) a temperature-gradient corrosion test with a specimen temperature of 460 degrees C; and an atmospheric temperature of 685 degrees C in air. The corrosion mass loss was found to be much greater under higher atmospheric temperatures where the same specimen temperature was maintained than under the same atmospheric temperatures where the specimen temperature was kept at a constant 460 degrees C. Cross-sectional observations of the ash after the temperature gradient corrosion test revealed that the alkali salt mixture was melted in the region above the melting point of the ash, however, no penetration of the melt reaching to the specimen surface was confirmed, suggesting that the corrosion was produced by a gas-solid reaction. Thus, the faster corrosion kinetics of the heat-resistant steels under an ash deposit with a temperature gradient can be attributed to breakdown of an initially formed protective Cr-rich oxide scale by a reaction with a vapor of alkali salts to form less protective alkaline chromates. The difference in the potentials of the alkali salt across the ash deposit due to the temperature gradient could be the driving force that increases the flux of the alkali salts toward the specimen surface, which accelerates the formation of chromates and the rate of breakdown of the protective Cr-rich oxide scale