Research On Characteristics Of Oxide Scale Of Boiler Tube Under Different Water Treatment Conditions

At present,supercritical coal fired power generation is still one of the main electric power industry in China,due to the supercritical unit greatly improves the main steam temperature and pressure,that have brought great challenges to the furnace tube.The oxide scale of boiler superheater tube especially in the final stage superheater peel off severely,it may lead to pipeline jam and even the pipe burst,which brings huge potential safety hazard and economic losses to Power Plant.Therefore,it is necessary to study the growth and peeling mechanism of scale cinder to provide theoretical basis for prevention of boiler pipe burst.In this study,final stage superheater of 600MW supercritical unit tube was oxidized with different time in three working conditions,namely,AVT?R?,AVT?O?and OT.Surface morphology,cross section and thickness of oxide skin layers and chemical composition of the corrosion product film was analyzed by EDS,SEM,XRD and metallographic analysis.The main research results are as follows:?1?Through the SEM analysis,granular oxide was firstly formed on the surface of samples,which has a loose structure.With the extension of oxidation time,granular oxide growths into crystal,the crystal structure is not very closely and has small holes.The crystal oxide growths into a thin but dense oxide film transversely,then growths into oxide clusters lengthways together with holes.Holes disappear with the transverse crystal growth,the Oxide layers thicken constantly and peel off at last.?2?Through the EDS analysis,Fe,O are found to be the main composition of the outer oxide layer.The content of Fe in the outer layer and the oxide layer is all lower than the substrate,but the content of Fe in the outer layer is higher than the inner layer.The outside-in successive decreases happen to the content of O.The content change of Cr and Ni contrary to O.Cr is enriched in inner oxidation layer.The content of Cr in three kinds of materials are Super304,TP347 and T91 respectively.?3?In the same running time,the thinnest oxidation layers of three kinds of material appear in the OT condition without obvious stratification,while oxidation layers layered obviously in AVT?R?and AVT?O?conditions.The size of the thickness of the oxide scale is in order AVT?O?>AVT?R?>OT.In the same running material,the oxide scale peel off most easily in OT condition and most hard in AVT?O?condition.?4?Through the XRD analysis under different conditions,it shows that,The oxide layers have a double-layer structure,the inner layer is the FeCr₂O₄ in spinel structure,the outer layer are Fe₃O₄ and Fe₂O₃ and Fe₂O₃ is detected at first on the Oxide film surface in OT condition.?5?Adding oxygen into feedwater promotes the formation of Fe₂O_3,which also promotes the exfoliation of the oxide scale.?6?After the boiler adopts the method of water supply and oxygen treatment,the dissolved oxygen content in the steam will increase,so that the Fe₃O₄ oxide into non dense Fe₂O₃,so as to speed up the exfoliation process of the oxide scale of the superheater tube.Therefore,the staff of power plant water chemistry condition in the choice of adding oxygen to water supply operation mode must carefully control the amount of dissolved oxygen.