Abstract
Electrochemical probes originally designed for use in aqueous environments were shown to measure corrosion rates in a high-temperature gaseous environment where electrolyte consisted of conductive ashes and liquid slag. The field trial of the probes was conducted over a five-month period to monitor fireside corrosion in a waste-to-energy (WTE) plant. The three-electrode air-cooled corrosion sensors, each including a thermocouple to monitor sensor temperature, were installed in four different ports at approximately the same level of the WTE boiler. A total of 12 sensors were tested: six with electrodes using the carbon steel boiler tube material and six using the nickel-chromium alloy used for weld overlays for the electrodes. Corrosion rates and temperatures of the sensors were monitored continuously throughout the trial. Metallographic measurements of sensor thickness loss were used to calibrate the electrochemical corrosion rates. Air cooling of the sensors was found to be necessary to bring the sensors to the temperature of the boiler tubes, to better match the corrosion rate of the tubes, and to increase survivability of the sensors and thermocouples. Varying the temperature of the sensors simulated corrosion rates of boiler tubes with steam temperatures above and below that in the actual WTE plant. Temperatures of two of the sensors were successfully held at controlled temperatures close to the steam temperature for a three-hour test period. Trends in the corrosion rates of the two materials tested were similar though of different magnitude. An expression relating the corrosion rate of the boiler tube material to the corrosion rate of weld overlay was determined for a seven-day period in the middle of the field trial. Results from the field trial suggested that corrosion rate sensors controlled to the outer waterwall temperature can monitor successfully fireside corrosion in WTE plants and can be used as a process control tool by plant operators.
