Destructive and Non-destructive Assessment of the Thickness of Oxide Layers of Super-heater Tubes

Tubes of super-heaters and heaters used in conventional power plants are exposed to coolant and high temperatures. The growing oxide layer on the inner surface reacts over time as a heat insulator on the water side and reduces heat transfer through the wall of the tube. A relatively thin oxide layer already contributes to the boiler efficiency and causes a permanent overheating of the tube wall. As a result of overheating at the site, the intercrystalline cracks leading to the bursting of the tube are developing. The secondary problem of the growth of oxide layer thickness is so-called exfoliation. For non-destructive evaluation of the thickness of the oxide layer directly at the power station, ultrasonic method (UT) can be used with a high frequency probe. In order to verify the accuracy of the measurement and the qualification of the ultrasonic testing methodology, light and Scanning Electron Microscopy (SEM) was used on specimens that were removed from the super-heater after the UT measurement. The standard longitudinal cut surface imaging in BackScatter Electrons (BSE) and Energy Dispersive Spectroscopy (EDS) analysis for accurate thickness determination with the chemical composition of the layer confirmed the accuracy of the UT measurement.