Abstract: The formation of oxidation film on boiler tubes before and after oxygenation treatment (OT) is studied to figure out the major factors contributing to the metal surface oxide film within different temperature ranges. The characteristics of the oxide film on the economizer and water wall tubes are analyzed and compared under the conditions of feedwater all-volatile (reduction/oxidation) treatment-(AVT(R)/AVT(O)) and oxygenation treatment. The causes of metal oxidation and scale exfoliation in the high-temperature section of boilers are also explored with the data from both test bench and field testing. The results show that feeding oxygen to the water makes the metal surface form a redish brown Fe₂O₃ coating layer in the low-temperature section of the thermal system(under 300 ℃), and this layer requires continuous oxygen feeding to maintain its stability. The causes of scale exfoliation of 18Cr austenitic stainless steel in high-temperature sections are complex. Nevertheless, the temperature and the material itself are believed to be the major culprits.

Key words: boiler piping, metal oxide film, feedwater oxygenated treatment, γ-Fe2O3 film, α-Fe2O3 film, scale exfoliation