Corrosion by Fused Salts

Project Number 207

Following the granting of funds by the Board of the VGB Research Foundation, a start can be made on a new research project, the purpose of which is to reduce corrosion in steam generators in refuse incineration plants. The work will be done by Max-Planck-Institut für Eisenforschung GmbH, Düsseldorf, Institut für Mineralogie der Universität Münster and Noell KRC Energie- und Umwelttechnik GmbH, Würzburg. The project will run from 1st October, 2000 to 31st December, 2001, and will be organised and monitored by the VGB Working Panel "Thermal Waste Utilisation".

Corrosion on evaporator walls and superheater pipes in refuse incineration boilers is much increased in the presence of ash deposits compared with areas where there are no deposits. One reason for this phenomenon is the presence of a ZnCl2-KCl-NaCl-PbCl2-based chloridic melting phase with melting points in the deposits from 250°C.

The formation and microscopic phase composition of the deposits, the mechanisms of the corrosion of metal materials in the presence of chloridic melts and the choice of practical, thermally sprayed films on the basis of laboratory tests are the subject of this research project. It is expected that by taking into account the eutectic melts, the corrosion processes in the laboratory will reflect the conditions in the real plants more accurately than has been the case with previous investigations.

The investigations into the corrosion mechanism include laboratory tests on 15 Mo 3, for example, under chloridic melting eutectics with different cations and also the role of temperature gradients in the molten film and in the sample. These investigations will provide information on the kinetics of the corrosion process.

The choice of thermally sprayed films that are suitable in practice is particularly important. Parameter windows will be determined by varying temperature, gas atmosphere and chemical composition of the melting phase on coated samples, which will provide information on optimum applications of the coating concerned.

The deposit formation conditions will mainly be studied taking into account flow, temperature and thermal transfer profiles in the vicinity of the pipe surfaces, which have to be calculated. The aim is to minimise deposit formation through design and process measures. The phase composition will be determined from deposit samples from different plants with the aid of structure analytical methods (XRD) and microchemical methods (SEM, TEM, ESMA).