The mechanism of sodium sulphate-induced low temperature hot corrosion of pure iron

The Na2SO4-induced corrosion kinetics of pure iron between 600 and 800°C was studied using a SO3-SO2-O2 mixed atmosphere. At any temperature accelerated oxidation of the metal was observed: this phenomenon is attributed to the formation of a liquid sulphate phase from the initially solid Na2SO4 deposit. A corrosion mechanism based on the dissolution of iron and iron oxides on the scale surface is proposed. The attack occurs through different stages: first a liquid FeSO4-Na2SO4 phase is probably formed from the initially pure solid Na2SO4 layer; then accelerated oxidation goes on by SO3 inward diffusion and iron outward diffusion through the molten salt. Transport models and quantitative estimates of the fluxes of the reacting species are presented.