Hot-corrosion behaviour of silica and silica-formers:: External versus internal control

Hot corrosion of silica and silica-formers was investigated in powder experiments and using Na(2)SO(4)-pellets on polished samples in air or oxygen. While silica reacts only mildly, silica-formers show strong gas-producing reactions. Worm-like textures of Na-silicate glasses appear on the surface. The texture is seen as due to the interaction of two immiscible liquids, where the bottom one (Na-silicate) has a higher surface energy then the top one (Na-sulfate). The dynamic nature of this interaction does not only account for the distribution of bubbles and provides sites for pit formation, it also promotes the enhancement of the main difference between silica and silica-formers. the latter act as reducing agents, either directly or via ait oxygen gradient in the oxide layer, decomposing the sulfate with pressures of SO(2) > 1 bar. Only after a critical thickness is reached a closed Na-silicate layer exists and external parameters such as the p(SO(3)) of the external atmosphere play, a key role. Hot corrosion of silica-formers consists thus of a rapid, internally controlled initial reaction, followed by external control. The strong initial reaction,vith ifs interplay of chemical and physical process is a major factor pre-conditioning the mode and strength of attack. (C) 1999 Elsevier Science Limited. All rights reserved.