Research The Characteristics Of Water-Wall Tube Oxide Film In Different Water Chemistry Conditions

The morphology and protective of the oxide film formed in different water chemistry conditions is different, some compact and uniform oxide film can prevent the erosion of water vapor and ion, but some oxide film is failed to protect the furnace tube, and produce damage. This paper researched the oxide film of 600MW furnace tube formed in AVT(R)/(O) water chemistry conditions oxidation for 18h at 300℃, and analysis morphology, structure and forming mechanism of oxide film by electrochemical methods, X-ray diffraction, SEM and EDAX, The results show that:(1)It's the maxiest of metal corrsion current and the weakest of resistance in the CT water chemistry condition, so the corrosion should be the most erious, and the formation of oxide film must be the most integrated and compact. In AVT(R) and AVT(O), ammonia protective metal is more than the phosphate and sodium hydroxide, the anti-corrosion ability of metal is stronger in the AVT(R) than AVT(O),so the oxide film of formed should be the thinnest in theory.(2)The first water chemistry condition of plant boiler taken phosphate as alkalizer, better reduced the hardness of boiler water mainly to Ca2+, Mg2+ ions, reduced the dirt and played a role in protect furnace. However, in these condition, the metal oxide film formed on the metal wall inner surface was metal corrosion products, which presented the features of depth corrosion (the depth of corrosion is 200μm) and rugged, and the metal was strong destructted.(3)Compared with CPT water chemistry condition, CT treatment had more advantages, as there was no hidden phenomenon when adjusting pH, so that the pH will not change greatly with the increased of temperature and pressure. In these condition, the metal oxide particles on the surface was larger, and the middle pores was also larger, as the oxide was easy to peel off with the flow of the solution. Thus the metal surface of potholes was exposed. At the same time, all kinds of corrosive ions penetrated into the metal oxide film and accelerated the corrosion.(4)The reduction of solution was enhanced in AVT(R), adjust pH while reduce the formation of metal oxide, metal surface was covered with a layer of very smooth and uniform film, to prevent direct contact of metal and solution. However, the surface oxide material particles were too small to easily take away with the solution flowed, and Oxide film is become thinner. Eventually, the metal will be thinner. (5)AVT(O) bases on AVT(R), oxygen was allowed to exist, led to the hard Fe₂O₃layer formed in oxide film outermost, to prevent the oxide film was taken away by solution. As the amount of oxygen is extremely limited, so the outermost is thin.