High Temperature Corrosion

A large number of important engineering systems operating at high temperatures (480-1100°C) involve contact of metallic or ceramic materials with combustion product gases or other oxidizing gases containing inorganic impurities, e.g. gas turbines, steam generators, incinerators, petrochemical process vessels. As the gases are cooled, fused salt films may condense on the hardware to generate highly corrosive conditions analogous in some aspects to aqueous atmospheric corrosion.
Some other engineering systems, such as the molten carbonate fuel cell, heat treatment baths, high-temperature homogeneous catalysis etc. use molten salts to accelerate the electrochemical and chemical reactions. When materials come into contact with corrosive molten salt films one can also expect high rates of the corrosion reactions but in this case the acceleration is undesirable and makes corrosion more severe than in aqueous solutions.
However the availability of high quality fossil fuels becomes limited, and technologies of solid waste utilization and other efficient energy conversion processes are under development. The damage of hot corrosion is likely to increase.
The most important features of hot corrosion are:
1. Generally molten salts are ionic conductors so that the corrosion attack must be electrochemical in nature.
2. Oxyanion molten salts (silicates, sulphates, carbonates, phosphates etc.) exhibit an acid-base behavior. Acidity of the electrolyte often defines the mechanism of the corrosion process.
3. The main products of the reaction between metal and molten salt layer are oxides. Solubility of these oxides in the melt defines the corrosion resistance.