Fireside corrosion in coal-fired power plants is a major obstacle to increase the overall efficiency for power producers because the increased use of cheap fuels or the requirements for clean combustion modes have aggravated corrosion on furnace walls. Corrosion-induced equipment failure can lead to catastrophic damage and inflict significant loss of production and cost for repair. Present corrosion monitoring technologies, however, were limited in effectively measuring high temperature corrosion,as these measurement techniques typically require planned periodic shutdowns, usually after the damage occurs. This book developed a novel sensor based on a new principle and thin-film technologies. Experiments using a measurement system were conducted at the simulated combustion environments to determine corrosion rates. The results indicated that an accurate measure of corrosion rate can be made in as short as a few hours. The advantage of obtaining the corrosion information quickly, without relying on extensive electronic peripherals and statistical data processing, provides a basis for further development of this corrosion monitoring system for use in power plant combustion furnaces.