Monitoring a degradation of steam generator boiler tubes

An on-line degradation monitoring is disclosed for boiler tubes in large combustion facilities, in particular waste incineration plants. The degradation of the tubes, for example corrosion or caking, is determined in a more or less continuous manner while the combustion facility is operating, and thus not restricted to optical inspection during downtimes. A rate or accumulated degradation can be related to an operation mode of the facility. Appropriate degradation sensors are arranged outside of a the flue gas tract where the boiler tubes are exposed to noxious flue gases from the combustion process, and receive ultrasonic signals from a space or volume within the flue gas tract. A test tube is provided exclusively for the monitoring of the degradation progress and introduced into the flue gas tract at a particularly corrosion-exposed location.