High temperature corrosion monitoring by electrochemical noise techniques

Co-firing biomass with coal in pulverized fuel combustion plants can lead to significant corrosion problems due to the release of aggressive species from some types of biomass. The resulting deposits on heat exchangers in the power plant hot gas path can contribute to breakdown of protective oxide films and lead to accelerated fireside corrosion. The electrochemical noise (ECN) technique has been investigated as a method of monitoring such corrosion where molten salts are present. Three identical sensors manufactured from the same material as the heat exchangers (either superheater/reheater or waterwall) were assessed. Sensor designs were developed as uncooled prototypes, representing geometries suitable for use in probes for waterwall or superheater/reheater locations. Simulated deposits were used to determine the sensitivity of the sensors and monitoring technique to changes in deposit composition and temperature. Further tests investigated the effect of different concentrations of SO(X) in the simulated combustion gas. Analysis of ECN data using statistical and frequency domain treatments has been combined with real-time observations of raw data. Results have shown that appropriately calculated values of resistance can be realistic indicators of the corrosion rate of alloys tested where general, uniform corrosion is taking place. Attempts to identify localised corrosion mechanisms by statistical data treatment will also be presented.