| Thermal power is China’s main source of electricity now and in the coming decades.70%of unplanned downtime incidents in thermal power plants were caused by boiler tubes failure, half of these failures were water wall tube failure. Hydrogen corrosion failure of water wall tube was the most complicated and difficult to monitor, also the most frequently took place among all kinds of water wall tube failure. In this paper, a study about the hydrogen corrosion failure caused by steam corrosion, acid corrosion and alkali corrosion was made. A comprehensive analysis of failure characteristics and failure reasons of hydrogen corrosion in water wall tube was conducted.The main conclusions are summarized as follows:1. Hydrogen corrosion failure had the general characteristics that along cross section of the thickness direction from inner wall to outer wall which distribution of micro cracks along the grain boundaries and has decarburization. The number and size of micro cracks with decarburization thickness were related to hydrogen corrosion degree.2. Hydrogen corrosion failure caused by steam corrosion, acid corrosion and alkali corrosion also had its own uniqueness. Hydrogen corrosion cracks caused by steam corrosion were generally free from corrosion and oxidation products. Macroscopic characteristics were typical "windowed" explosion, no wall thinning, and the inner surface of the fireside tube was typical iron oxide scale. Hydrogen corrosion at the place of fireside tube caused by acid corrosion and alkali corrosion generally had corrosion pits, and the micro cracks along the inner wall of the corrosion pits had corrosion products. Alkali corrosion was more obvious along the grain boundary. Macroscopic characteristics of hydrogen corrosion caused by acid corrosion and alkali corrosion characterized by loose corrosion deposition in the inner surface of fireside tube, and also the presence of scales, obviously wall thinning. Three reasons caused hydrogen corrosion failure were:The tube wall temperature fluctuations or over-temperature in some places of fireside wall, when the temperature reached a certain level that the hydrogen corrosion failure caused by steam corrosion would occur. When there was residual acid or low pH in boiler water, or hard water and Cl element coexist, the hydrogen corrosion failure caused by acid corrosion would occur. If the boiler water had a high concentration alkali that can’t go out with recycled water or the water pH is high, the hydrogen corrosion failure caused by alkali corrosion would occur.3. When heated at850癈:10-20um cracks could not be healed with lh insulation;30-40um cracks could not be healed, but some smaller cracks could be healed with3h insulation;30-40um cracks could be healed with8h insulation, but there are still spherical holes exist. When heated at900癈:20-30um cracks could not be healed completely with lh insulation; few40-50um shallow cracks could be healed, but most40-50um deep cracks could not be healed with3h insulation; most40-50um shallow cracks could be healed, but deep cracks could not be healed completely with8h insulation. When heated at950癈:Few30-40um shallow cracks could be healed, but most cracks could not be healed with lh insulation; most20-30um cracks could be healed with3h insulation; some30-40um cracks could be healed with6h insulation;40-50um shallow cracks could be healed, but few deep cracks could not be healed with8h insulation; most40-50n.m cracks could be healed with lOh insulation, but there are still spherical holes exist;30-40[xm cracks could be healed with12h insulation, but there are still spherical holes exist.4. Improving the heating temperature and insulation time could increase the degree of crack healing. There was a lowest critical temperature of crack healing, and cracks could not be healed below this temperature. Insulation when above the critical temperature, there was also an optimal holding time. The extent of crack healing degree could not be improved greatly when extension of the holding time indefinitely. |
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