Metal corrosion refers to the phenomenon of metal damage or deterioration caused by the chemical or electrochemical action of environmental media. Corrosion can be divided into dry corrosion, wet corrosion, molten salt corrosion and organic medium corrosion according to different environments; metal corrosion can be divided into chemical corrosion, electrochemical corrosion and physical corrosion according to the corrosion mechanism; according to the corrosion form, it can be divided into comprehensive Corrosion and localized corrosion.
When the metal undergoes general corrosion, the corrosion is distributed on the entire surface of the metal in contact with the medium, and it can be uniform corrosion or non-uniform general corrosion. The risk of general corrosion, especially uniform corrosion, is relatively small, because we can appropriately increase the size of structural components according to the corrosion rate of metal materials and the service life of the equipment. Most of this type of corrosion is predictable and preventable.
When the metal is locally corroded, the corrosion is mainly concentrated in some local areas of the metal surface, while the other parts of the surface are almost undamaged. There are many forms of localized corrosion, mainly including galvanic corrosion, pitting corrosion, intergranular corrosion, crevice corrosion, stress corrosion, selective corrosion, abrasion corrosion, etc. Among them, stress corrosion can be divided into stress corrosion cracking, hydrogen damage and corrosion fatigue; wear corrosion includes erosion corrosion and cavitation corrosion. The risk of local corrosion is much greater than that of general corrosion, mainly because of the concealment of the corroded parts. The metal surface is only corroded in a small area, and the corroded parts are often difficult to be found. When it is discovered, the equipment may have major problems and need to be stopped. Overhauled.
Due to the particularity of the environment, the thermal equipment of thermal power plants has some different characteristics in addition to the general characteristics of metal corrosion.
①The heat load plays an important role in the corrosion of thermal equipment. Many thermal equipment are exposed to high temperature and high pressure for a long time, and the corrosion of thermal equipment is closely related to the thermal load. For example, the corrosion of economizer tubes, water wall tubes, and superheater tubes is mostly concentrated on the parts with higher heat load, such as the fire side of the furnace tube.
②The operating conditions of the unit have a great influence on the corrosion of thermal equipment. For example, changes in the quality of raw water and the operating status of water treatment equipment outside the furnace, changes in the operating conditions of thermal equipment, changes in water supply treatment methods, etc. will all cause changes in the quality of water and steam, and ultimately lead to the type and degree of corrosion. Corresponding changes.
③The corrosion rate of thermal equipment increases with the increase of the random group parameters. The improvement of unit parameters means that the temperature and pressure of the water vapor system will increase, and the rate of metal corrosion reaction will speed up. Therefore, under the same water quality conditions, the corrosion of high-temperature and high-pressure units is more serious than that of medium-temperature and medium-pressure units. Corrosion is more serious. The improvement of unit parameters will also bring about changes in equipment materials, and at the same time require higher purity of make-up water. These changes will in turn cause changes in metal corrosion patterns.
④ The degree of corrosion in the operation of thermal equipment has a certain relationship with the corrosion during shutdown. If effective protection measures are not taken during the outage of the thermal equipment, a large amount of air will enter and a higher relative humidity will cause more serious corrosion of the equipment metal. When the unit restarts operation, the corrosion pits during the shutdown period will further develop. The degree of corrosion in operation is aggravated. If it becomes a fatigue source of corrosion fatigue, the equipment is more susceptible to corrosion damage.