Salt melt induced corrosion of metallic materials in waste incineration plants

Abstract

This paper presents the analysis of corrosion damages of metallic materials after application in waste fired boilers. The first part gives a description of corroded metallic specimens, collected from different grate fired plants in Germany and it will provide the base information for thermodynamic calculations and laboratory experiments, described in the second part of the paper. In deposits from boiler tubes of waste incinerators there are significant amounts of molten phases, consisting of calcium-, potassium- and sodium-sulfates and/or potassium- and sodium-chlorides containing heavy-metals like lead and zinc. These heavy-metals are decreasing the melting point of the deposits and severe corrosion occurs especially by the molten phase. A thermodynamic approach is given which describes the stability of chlorides with respect to sulfates in deposits from waste fired boilers. It is shown that in HCl-containing gases especially heavy-metal sulfates will react to corresponding volatile chlorides. In addition, laboratory experiments are presented on the effect of heavy-metal sulfates and chlorides in the corrosion of several metallic materials. The experiments were carried out beneath a CaSO4- K2SO4- Na2SO4- ZnSO4- PbSO4 salt mixture at 600 °C, beneath the heavy-metal chlorides PbCl2, ZnCl2 and also beneath a ZnCl2- KCl mixture at 300–600 °C. The results of these studies show clearly the strong corrosive effect of heavy-metal rich salt mixtures in the corrosion of steels and nickel-based alloys.