Development of SiO₂/TiO₂/Al₂O₃-based/TiO₂ coating for preventing sulfide corrosion in thermal power plant boilers

In thermal power generation equipment, wall thinning on boiler tubes due to sulfide corrosion has been a problem. The methods for resolving or preventing the problem are coating of a nickel-chromium film by plasma spraying, weld cladding, and replacement of water wall tubing. These methods are expensive and time consuming. Therefore, an economical and straightforward coating technique was developed for preventing sulfide corrosion on boiler tubes. The developed coating has a four-layered structure comprising, in order from the boiler tube substrate, thin films of (1) a SiO₂ layer, (2) a TiO₂ layer, (3) an Al₂O₃-based layer, and (4) a TiO₂ layer. The coating reduced corrosion to 25% or less compared with an uncoated part under a simulated condition (Gas: CO₂/H₂/N₂/CO/H₂O = 12%/4%/67%/8%/9% H₂S 330 ppm, Temperature: 500 °C). It was found that the coating on boiler tubes is exceptionally durable and continues to be effective for more than 2 years in actual power plants. This coating process can be applied to a large area, and the work period is short. When the coating area is 100 m², the working time is 3 days and total spraying time is only 7 h.