Fundamental aspects of chlorine induced corrosion in power plants

Abstract

Chlorine induced corrosion is the most important corrosion process in power plants, firing waste, coal and/or biomass. Depending on the process, chlorine is present as HCl gas, as solid KCl and NaCl in ashes or as eutectic melts i.e. KCl–ZnCl2 in deposits of waste fired boilers. In the presence of HCl gas and solid chlorides, ‘active oxidation’ is generally accepted as the major corrosion mechanism. Metal chlorides are formed by inward diffusion of chlorine to the metal/oxide interface, evaporating and subsequently oxidized to non-protective oxides and additional chlorine. In this mechanism, the role of alloying elements has not been fully understood until now. Hence, experiments were conducted on the influence of the alloying elements Mo, Ti, Si, Al on Fe–15Cr model alloys in an N2–5 vol.% O2 gas mixture with addition of 500 and 1500 vppm HCl with the use of thermogravimetric experiments at 600°C. The corrosive attack by ‘active oxidation’ strongly depends on the alloying elements. Fe–(15–25) wt% Cr alloys without any further addition of alloying elements show catastrophic ‘active oxidation’, characterized by the formation of metal chlorides at the metal–oxide interface. The addition of molybdenum, silicon and aluminium generally decreases the corrosive attack, whereas Ti has no beneficial effect and the corrosion is enhanced compared to Fe–15Cr.The presence of molten chlorides on heat exchanger tubes leads to catastrophic corrosion rates, even at relatively low temperatures of 250°C. Thick oxides scales of Fe2O3 are formed in contact with the gas phase. At the metal–oxide interface, a mixture of KCl–FeCl2 chloride phases are detected, probably molten at a reaction temperature of 400°C. Short term experiments on 2.25Cr–1Mo steel at 400°C in N2–5 vol.% O2–HCl gas mixtures covered with a eutectic KCl–ZnCl2 melt have shown that the metal is dissolved in the chloride melt at the melt/scale interface and precipitated as a thin layer of oxide at the melt/gas phase boundary. The kinetics of the corrosion strongly depends on gas phase composition i.e. p(HCl) and p(O2).