Mechanistic aspects of carburization in oil-fired power plants

摘要

Materials degradation due to surface carburization has been recently reported in connection tofailures or high corrosion rates of superheaters or reheater tubes of oil-fired and coal-fired boilers.Reducing conditions to explain carburization process of materials are hardly to expect because fuel isburned with oxygen excess to attain its complete combustion. The formation of sodium carbonate asa carburization catalyst has been proposed in order to explain carburization phenomenon in boilers,however this carburization catalyst has been rarely found in ash deposits on carburized tubes. In order to study the role of some compounds present in boiler deposits as possible carburizationcatalysts and the corrosion behavior of 304H-type stainless steel in boiler deposits under reducingconditions, pack-carburization experiments were carried out at 650 ℃ by 1000 hours. Mixtures of60 wt.% carbon and 40 wt. % of different typical compounds of boiler tube deposits were used. Theexperimental results show that ash deposits and several deposit compounds act as carburizationenhancers, they increase carburization of the alloy if carbon is present in ash deposits. Furthermore,corrosion were detected in some cases. However, other mechanisms might be considered to explaincarburization of boiler steels. Analysis of these mechanisms is presented. It is concluded that surfacedeposition of partially burned particles and its further burning supplying carbon monoxide to the localatmosphere is considered as the main mechanism contributing to carburization of boiler tubes.