Abstract: A tubular probe has spaced apart bands or patches of the same material as the probe body attached to the probe body by an electrically insulating, high temperature material. A series of cooling tubes are provided within the probe body to direct cold air to the regions near each band. One or more probes is placed in a furnace or boiler above the ammonia injection zone. When ammonium bisulfate forms on the probe it completes an electrical circuit between the probe body and the bands and will also cause corrosion of the probe. The presence of ammonium bisulfate is detected by a change in resistance between the bands and the probe body. Electrochemical noise is generated during the corrosion process. A corrosion rate can be determined from the level or amount of electrochemical noise that is detected.