THE INFLUENCE OF ELECTROCHEMICAL PROCESSES ON HIGH-TEMPERATURE CORROSION REACTIONS IN COMBUSTION SYSTEMS

Electrochemical investigation techniques (a.c. and d.c.) have been used to assess laboratory-simulated high-temperature fireside corrosion reactions. Tests have involved typical iron-base super-heater and reheater alloys subjected to molten salt corrosion in a combustion rig and a furnace-wall alloy subjected to dry oxidation/sulphidation conditions. The techniques have also been applied to a ceramic material, silicon carbide, exposed in a dry oxidation environment. Conclusions regarding corrosion rates estimated from the electrochemical data have been compared with those from metallographic examination of the test elements at the completion of the exposure tests. The results have shown that the ability of the electrochemical measurement techniques to evaluate the reaction processes is dependent on the ionic transport in the scale or in the deposits formed on the high-temperature surfaces during the combustion processes. Thus, dry oxidation/sulphidation of the alloys produces electronically conductive scales, while fluxing of the scales formed in thin-film molten salts results in a change from ionic to electronic conduction in the films; in both cases, electrochemical assessment of the corrosion reactions by using simple test cell configurations is prevented. However, the high ionic transport number of silicon oxide scales on silicon carbide allows measurement of charge-transfer resistances, which may be correlated with the reaction kinetics.