The performance of thermal-sprayed coatings applied to uncooled components installed in the high-temperature zone of a 300-MW boiler was investigated. The environment in the boiler is highly corrosive because of the burning of heavy fuel oil with high contents of vanadium, sodium, and sulfur. The additive feeder components coated by thermal spraying were exposed to a gas temperature of 1,1oooe during approximately 3,240 hours. A nickel-chromiumfsilicon-iron two-Iayer based coating demonstrated the best performance. Oetails of the microstructure in the interfaces between the coating and the corrosive environment and of the coating and the base metal are discussed. U sually, heavy fuel oils con-tain ash in quantities less than 0.2% of their weight. However, when the ash has a high content of vanadium, sulfur, and so-dium, catastrophic corrosion and , fouling problems can occur in the steam boiler components. In general, the ash content is related to the amount of asphaltenes in the fue!. High concentrations of deleterious el-ements in the slag in contact with metals are the cause for such low con-tents of ash to cause severe corrosion problems. In the boiler, the atomized oil fuel is burned in two steps. First, the volatile compounds are evapo-rated and burned to form combus-tion gases and cenospheres, porous carbon-rich materials. The secónd step of combustion consists of the burn-ing of the cenospheres, which have a much higher content of the deleteri-ous elements. In this step, the cenospheres are the fuel and produce a higher percentage of ash than the original fuel oilY At the end of com-bustion, the concentration of vana-dium, which in the fuel oil was about 250 ppm, riges to 40% in the slag deposits that are in contact with me-tallic components of the boiler.