Steam-side Oxidation Corrosion Mechanism and Characteristics of Oxide Scales under the Deposit in a Boiler Water-wall

The corrosion oxidation mechanism and characteristics of steam-side oxidation scale formed on the internal surface of water-wall tube made of 20G steel which was removed from a boiler of a power plant were investigated by metallurgic inspection, SEM-EDX and X-ray diffraction analyses. The results show that the original steam-side oxidation scale normally consists of phosphate and iron oxide with two layers, the inner layer consists of compact and loose zone appearing alternatively and being identified as Fe3O4, the outer layer is loose, porous and identified as Fe2O3 with the deposition of phosphate. The deposition corrosion changed the steam-side oxidation mechanism and characteristics, which promoted the diffusion and mass transfer process of corrosion products (hydrogen and oxygen). And the reaction process of hydrogen and cementite in grain boundarier caused decarburization of water-wall tube, void fraction increase and grain boundar cracking initiation and propagation, then accelerated the oxidation of grain boundaries and materials.