Research On The Corrosion Characteristics Of Water-wall Tubes In 1000MW Fossil Power Plant

With the increasing demands of electricity and power generation technologies continuous development in our country, 1000MW and other degrees large-capacity and high parameter of supercritical fossil power units become an important force of guarantee for China’s electricity supply. However, the units’parameter be higher, the cycle’s tolerance for impurities be lower. Especially for the (ultra-)supercritical fossil generating units, with the structural characteristics of no drum, once the erosive anions into the water-steam cycle that will be a serious threaten of safe and economic operation. Water-wall is an important evaporation surface of fossil power plant, the failure of water-wall tubes occurred that will be cause an serious economic losses and even adverse social impact.Characteristics of forming passive film and corrosion behavior of Cl-/SO2-for the 1000MW ultra-supercritical boiler water-wall tubes specimens in simulated reducing all-volatile treatment[AVT(R)] and oxidizing conditions oxygenated treatment(OT) were studied by the autoclave static method, X-ray diffraction analysis, flame atomic absorption, scanning electron microscope and other technologies. Characteristics of formed passive film and corrosion behavior for the water-wall tubes in different conditions were compared. The results were as follows:(1) The surface of the specimens mainly occurred electrochemical reaction at simulated AVT(R) and OT condition without Cl- and SO42- at· 300℃/8.5MPa,Specimens were.formed black film and the oxide particles on the surfaces, pitting was not occurred. The effects of heterogeneity of specimen chemical composition and structure etc. on the reaction were weak at the reducing and without oxygen condition at simulated AVT(R) condition, so the oxide particles on the surfaces were relatively small and uniform. The effects of heterogeneity of specimen chemical composition and structure etc. on the reaction were strong at the reducing and with oxygen condition at simulated OT condition, so the oxide particles on the surfaces were relatively big and uneven.(2) Black films were formed on the specimens’surfaces at simulated AVT(R) condition with Cl- and SO42- at 300℃/8.5MPa,and occurred different degrees of pitting. At simulated AVT(R) condition, effects of residual oxygen can be ignored, a few pitting was caused by the existence of erosive ions。At simulated OT condition, Pitting corrosion was promoted by the oxygen and erosive anions, the integrity of passive film was severely damaged.(3) The surface of the specimens mainly occurred chemical reaction at simulated OT condition without Cl- and SO42- at 450℃/25MPa, Specimens were formed black film and the oxide particles on the surfaces, pitting was not occurred. The formation and repair of the oxide film on the specimen was promoted by the presented oxygen and high temperature, the effects of heterogeneity of specimen chemical composition and structure etc were attenuated. In the supercritical simulation condition without erosive anions, specimens were formed uniform and relatively large size. In the supercritical simulation condition with erosive anions, because of the lower solubility and dissociation of inorganic salt under supercritical conditions, corrosion of Cl- and SO42- became weaker, black uniform film with dark spots formed on the surfaces of specimens. Pitting was not occurred. Due to different solubility between Cl and SO4 under the test conditions, the number of dark spots in the condition with Cl was more than the condition with SO4.(4) Whether or not simulated solutions contained S, S is existed in the pitting areas at 300℃/8.5MPa simulated AVT(R) or OT condition with Cl- or/and SO42-No Cl was detected in pitting area of specimens in simulated solution with Cl- or Cl-/SO42-.The main components of the oxide film on specimens surfaces is Fe3O4 after 6 hours at simulated AVT(R)(300℃/8.5MPa) and OT(300℃/8.5MPa) with or without Cl- or/and SO42- and supercritical OT condition(450℃/25MPa) without Cl-and SO42-.(5) Dissolved oxygen has double impacts on metal corrosion in aqueous solution:dissolved oxygen promoted oxide film forming on the water-wall specimens in simulated OT solution without erosive anions; oxygen is cathode depolarizer in the simulated OT solution with Cl- or/and SO42- which will promote initiation and progression of pitting as well deposition of Fe2+ of the anode product. Fe in the simulated OT is less than simulated AVT(R) at same concentration of Cl- or/and SO42-. Fe content is not significantly increased when erosive anions introduced into the simulated AVT(R).(6) The presence of Cl- and SO42- not only promote the initiation and progression of pitting, but also the dissolution of the passive film. Dissolution increased of Fe with the increasing concentrations of erosive anions in simulated OT solution, and it is followed:Cl-/SO42-> SO42-> Cl-. In the OT conditions with same ion concentration, the same ion for promote dissolution on different materials is slightly different. With increasing test time, the presence of Cl- and SO42- accelerant the increase the content of Fe in simulated OT conditions.(7) In three test conditions, the oxide particle size and the film formation is: OT (450℃/25MPa)> OT(300℃/8.5MPa)> AVT(R)(300℃/8.5MPa); uniformity of particles:OT (450℃/25MPa)> OT (300℃/8.5MPa), AVT (R) (300℃ /8.5MPa)> OT (300℃/8.5MPa); damage degree of water-wall tube specimens in simulated condition with erosive anions:OT (450℃/25MPa)<AVT(R) (300℃ /8.5MPa)<OT(300℃/8.5MPa).