Kinetic investigations on salt melt induced high-temperature corrosion of pure metals

This paper discusses kinetic investigations, accomplished by TG experiments of high temperature corrosion of pure Fe, Cr and Ni beneath a molten chloride. TG tests were conducted with a 50 mol.% KCl/50 mol.% ZnCl2-salt mixture at a temperature of 320 degreesC in an atmosphere consisting of argon and oxygen. In the case of pure Fe, the amount of salt, the oxygen potential and also the exposure time have been varied. On Fe, the corrosion starts with an incubation phase, which is characterised by low mass gain. The duration of the incubation time depends strongly on the amount of oxygen and salt deposit. While the incubation time decreases with increasing amount of oxygen, it increases with increasing amount of salt deposit. The subsequent stage is linear and shows a high mass gain within a short period of time. Subsequently, the TG curve evolves into a stage that follows either a logarithmic or a parabolic law. The different stages can be attributed to the different reaction steps iron chloride formation (incubation phase) oxide precipitation (linear stage) and scale formation (parabolic / logarithmic phase). Experiments stopped in the incubation phase have shown that iron chloride is formed at the metal-salt interface. A corrosion model, based on these observations, is presented. Cr and Ni behave in a totally different manner and a high mass loss is observed during reaction of these materials.