Chapter 12 - Corrosion in Hydrothermal and Supercritical Water

Abstract

Abstract In this chapter, the interaction of aqueous solutions with the materials of the experimental or production plants is discussed that can lead to major problems due to corrosion. The aqueous solutions contain many reactive compounds, for example, chloride ions, under oxidative or reductive conditions, that can cause corrosion. Corrosion includes all reactions of components of an aqueous reaction mixture with the walls of the equipment of experimental or production facilities, like pipes, valves, fittings, and autoclaves. Corrosion in hydrothermal and supercritical water processes depends on the properties of the reaction mixture, on the properties of the construction materials, and on the construction of the processing equipment. In general, no satisfying solution to corrosion problems can be achieved with addressing only one aspect. On the contrary, only a comprehensive adjustment of all aspects influencing corrosion can lead to a process configuration that can be operated successfully and for a reasonable extended time span. Various types of corrosion occur, such as general corrosion, intergranular corrosion, pitting corrosion, and stress corrosion cracking. General corrosion occurs mainly in the near critical and supercritical temperature range in a region, where the density is high. Corrosion rates below T = 320 °C are very low. At temperatures T > 320 °C and pressures higher than the critical pressure, corrosion rates increase considerably. At a pressure of P = 24 MPa, high corrosion rates drop down to the level observed at lower temperatures just around the critical temperature of water, while at a pressure of P = 38 MPa, corrosion rates remain high up to a temperature of about T = 450 °C, since at that temperature density reaches the limit for corrosion due to ionic processes. In this chapter, first corrosion phenomena are described as occurring in high-temperature aqueous systems. Then, the parameters influencing corrosion are discussed in some detail together with corrosion mechanisms. The effect of corrosion is illustrated by reporting on a number of detailed investigations on corrosion in aqueous systems. A detailed discussion of measures that can be used against corrosion follows and shows the necessity of combining several approaches and ends in a compilation of means against corrosion.