The corrosion of steels in power plants (coal combustion, waste incineration) is mainly due to condensed chlorides in the ash deposited on the boiler tubes. These chlorides are stabilized by HCl in the
combustion gas. In the case of coal as a fuel, chlorine is present as chloride minerals in the raw material which is converted to HCl during the combustion process. Corrosion of steels in chlorine containing
environments occurs by the ‘active oxidation’ mechanism, which is a self - sustaining accelerated oxidation process, catalysed by chlorine. This study shows that solid chlorides react with the oxide scale of the steels to form chlorine, which initiates ‘active oxidation’. In order to prevent chlorine induced corrosion, the deposition of chlorides on the tubes within the coal ash must be avoided. This is possible by the presence of SO2, which is present in the combustion gas, converting the chlorides to sulfates in the gas phase. The paper presents an example of a failure case in a coal fired plant in Germany. In this plant, chlorine induced corrosion was observed after effective removal of SO2 by additions of CaO. From thermodynamic calculations it can be shown that a certain amount of SO2 is necessary in order to avoid deposition of chlorides and to prevent corrosion. Keywords: chlorine induced corrosion, active oxidation, fly ash deposits, solid chlorides, chlorine induced failure case