26. High Temperature Corrosion

A metal in contact with a hot gas, typically at temperatures above 400 °C, in absence of liquid water phase, can suffer corrosion, also called hot corrosion. While aqueous (wet) corrosion processes are of electrochemical nature, hot corrosion is a chemical process, i.e., governed by chemical process kinetics in gas phase. Nevertheless, the oxide layer that forms at the metal surface is influenced by ionic diffusion and electronic conductivity within the oxide, as typical of an electrochemical mechanism. Corrosion attacks include: thinning due to the formation of non-protective scale, corrosion products and metal evaporation, metal degradation by molten salts, erosion-corrosion assisted by entrained solid particles, localized attack at grain boundaries, embrittlement. In this Chapter, the properties of oxides, as morphology, conductivity, protectiveness are described, together with the oxidation behaviour of metals and alloys; other processes (sulphidation, carburisation) and different environments, like steam and combustion gases, are briefly outlined.