Corrosion of austenitic steels in molten sulphate deposits

Abstract

Thermal analysis has been used to determine the melting behaviour of alkali metal sulphate deposits typical of those found on superheater tubes in coal-fired power station boilers. Additions of 5–30 m/o Fe 2(SO 4) 2 to Na 2SO 4K 2SO 4 mixtures under a simulated flue gas (N 2 + 15 v/o CO 2 + 1 v/o O 2 + 0.3 v/o SO 2) are shown to reduce the melting point from 820°C to below 550°C. Alkali-iron trisulphates are formed which re-solidify on heating above 720°C by decomposition of Fe 2(SO 4) 3 at low thermodynamic activity. It is suggested that the effect of heat flux and SO 3 potential gradient on the melting behaviour of a superheater deposit could account for the observed ‘bell-shaped’ temperature dependence of corrosion rate in the range 550–750°C. Results are presented of laboratory corrosion tests on three commercial austenitic stainless steels using a melt of composition 74 m/o Na 2SO 4 + 20 m/o K 2SO 4 + 6 m/o Fe 2 (SO 4) 3. A model is proposed for corrosion by an acid fluxing mechanism involving refractory metal alloy elements in the steels. The model offers a quantitative basis for prediction of fireside corrosion resistance in coal-fired boilers, and a comparison of relative corrosion rates of a range of steels in laboratory tests, probe trials and service performance confirms the validity of the present model.