Corrosion and protection of shut-down boilers

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Corrosion and protection of shut-down boilers: If appropriate protective measures are not taken during the shutdown period, the metal surfaces of the boiler itself and the entire steam-water system will be corroded by dissolved oxygen, and the rate of corrosion is much faster than when the boiler is in operation. It has been found that boilers with severe corrosion are mostly due to improper shutdown protection, and if these corrosion products are not removed, they will accelerate electrochemical corrosion once the boiler is restarted. Therefore, shutdown corrosion not only shortens the service life of the boiler but also creates potential hazards during operation. After the boiler is shut down, large amounts of air from the outside environment enter the boiler’s steam and water system. Especially when the boiler is cooled and drained without being dried, a layer of water film forms on the metal surfaces; oxygen in the air can easily dissolve in this water film and reach saturation. When oxygen reacts with iron, it is quickly replenished by more oxygen from the air, which makes oxygen corrosion of the metal very likely to occur. Corrosion becomes more severe if the furnace water remaining on the metal surface contains Cl- or SO42-. I. Causes of corrosion during shutdown The extent of corrosion that occurs during shutdown is related to various factors. The main points are as follows: 1. Humidity – For boilers that have stopped producing steam, the moisture on the metal surface has a significant impact on the rate of corrosion. When air humidity is high, dew tends to form on metal surfaces, creating a water film that accelerates corrosion. When the metal surface is dry and the relative humidity inside the boiler is less than 30%, iron corrosion can be completely prevented. 2. Salt content in water: An increase in the salt content in the water film inside boilers or on metal surfaces accelerates corrosion; in particular, an increase in the levels of oxides and sulfates leads to a significant rise in the corrosion rate. 3. Cleanliness of the metal surface: When there are deposits or scale on the metal surface, residual moisture tends to accumulate in those areas, keeping the metal surface beneath the scale in a moist state. At the same time, the scale prevents oxygen from penetrating, while the metal surface surrounding the scale has sufficient oxygen. In this way, due to differences in concentration, a concentration cell is formed; the area around the scale where the oxygen concentration is high acts as the cathode, while the area beneath the scale where the oxygen concentration is low acts as the anode, causing corrosion of the metal beneath the scale. As can be seen from the above, to prevent corrosion when a boiler is taken out of service, it is necessary to keep the metal surface of the boiler dry and clean, and to remove scale or accumulated sediment in a timely manner ； For wet protection, the salt content in the water should be reduced as much as possible to prevent accelerated oxygen corrosion. Common methods for boiler shutdown protection I. Basic methods for boiler shutdown maintenance 1. Preventing external air from entering the steam and water system of the idle boiler ； 2. Keep the metal surfaces within the soda system dry ； 3. Form a film with anti-corrosive properties on the inner surface of the metal to isolate it from air ； 4. Immerse the metal surface in a medium containing a deoxidizer or other protective agent. II. Two main categories: dry protection and wet protection. (I) Dry protection 1. Desiccant method The desiccant method is suitable for protecting boilers that will be out of service for a long period of time (more than three months). This method is more convenient for boilers in which the volume of the drum accounts for a larger proportion of the total water volume in the boiler. The specific methods are as follows: (1) After the waste heat boiler is taken out of service, it should be subjected to shutdown maintenance. When the boiler inlet temperature drops below 300°C and the boiler water temperature falls to around 100°C, open all blowdown valves to completely and rapidly drain the boiler water from the heat recovery steam generator. Open the inspection ports to remove the scale and sludge accumulated inside the boiler, as well as external soot, etc. Use the residual heat from the furnace to dry all the heating surfaces and equipment in the boiler, and then close the valves of the boiler. (2) Place desiccants in the steam drum and various header tanks (the location and quantity of the desiccants placed must be recorded to prevent forgetting). Immediately after insertion, seal the drum manway and all steam-water valves to ensure a tight isolation of the steam-water system from external air. Amount of desiccant used and criteria for determining its effectiveness (3) When maintaining the desiccant, the following points should be taken into account: The desiccant should not come into direct contact with the metal surface of the boiler; it can be placed in containers such as iron trays ； After placing the desiccant, seal the handholes and other coverings tightly; the valves on the steam and water pipes must be closed ； It is necessary to check the condition of the desiccant regularly. Generally, it should be checked about every half month at first, and then once a month thereafter; if it loses its effectiveness, it must be replaced promptly. 2. Drying method: The drying method is commonly used for corrosion protection during boiler maintenance. When the temperature of the boiler water drops to around 100°C, all the water in the boiler is drained, and the metal surface of the boiler is dried using the residual heat from the furnace. 3. Ammonia filling method: The ammonia filling method is generally used for the protection of power plant boilers when they are out of service for extended periods. Since ammonia is corrosive to copper, when using the ammonia filling method, all copper components in the boiler’s steam and water systems must be removed or sealed off. The ammonia filling method involves filling the boiler with ammonia gas after draining it when the boiler is not in use, maintaining the ammonia pressure inside the boiler at 13 kPa in order to remove air and reduce the oxygen content in the water film on the metal surface. Since ammonia, when dissolved in the water film, confers good corrosion resistance to metal surfaces, the ammonia filling method does not impose strict requirements on the dryness of the metal surface; however, it is necessary that the entire system be leak-free. Use a moist red litmus paper to check the areas where ammonia leakage may occur; if the paper changes from red to blue, it indicates an ammonia leak, and sealing the leak should be carried out immediately. Open-flame operations should also be strictly prohibited to prevent explosions. 4. Corrosion inhibitor protection method: The corrosion inhibitors used for protecting the boiler when it is shut down generally have good volatility; hence, this method is also known as the vapor-phase corrosion inhibitor method. The main corrosion inhibitors include inorganic ammonium salts such as ammonium carbonate, ammonium bicarbonate, and diammonium hydrogen phosphate, organic amines such as cyclohexylamine carbonate and benzoic acid, as well as the composite TH901 corrosion inhibitor. The characteristic of such corrosion inhibitors is that they are volatile and can generate a certain vapor pressure ； It can form a protective film on metal surfaces, offering strong corrosion resistance ； The degree of dryness on the metal surface is not a major requirement; in particular, the TH901 corrosion inhibitor also provides excellent corrosion protection even on wet metal surfaces. Usage: After draining the water from the boiler, remove any scale or debris. Add the chemical to the boiler in the specified proportion in a uniform manner, close all valves, and ensure that the system is tightly sealed. It should be noted that most of these corrosion inhibitors emit ammonia gas, which can erode copper components. Therefore, all copper components in the boiler’s steam and water system should be removed or sealed off ； Alternatively, use a mixed corrosion inhibitor with the ratio of **benzotriazole:cyclohexylamine carbonate = 3:2:7 (by mass)** to prevent corrosion of copper components. (II) Wet protection: Wet protection refers to a type of protection method in which the boiler is filled with a protective aqueous solution during shutdown, in order to prevent air from entering the system. It can be divided into the following types: 1. Alkali solution method: This involves using an alkali solution to fill the boiler with an aqueous solution having a pH value of over 10, thereby passivating the metal surface and preventing corrosion of the metal by dissolved oxygen. A mixture of sodium hydroxide and trisodium phosphate is commonly used; this method can be applied when the shutdown period is within three months. (1) After shutting down the boiler, first remove all dirt, sludge, and other debris from inside and outside the boiler, and seal off the pipelines connected to the outside world ； Remove all copper components from the soda system or seal and isolate them. (2) Sodium hydroxide (amount: 4 kg/m3) and trisodium phosphate (amount: 1.5 kg/m3) are dissolved in softened water or condensate water and then added to the boiler; a pump is used to circulate the alkaline solution evenly within the boiler. (3) It is best to heat the boiler water to around 105°C in order to remove gases such as oxygen from the water. Then close all door openings and valves to ensure no leaks. 2. Pressure retention method: If the boiler is out of service for a short period (within a week), it can be maintained by keeping pressure in it. Small-capacity boilers can utilize the residual pressure of the boiler (0.05–0.1 MPa) to maintain the boiler water temperature slightly above 100°C℃ ； Medium and high-pressure boilers can be filled with qualified feed water, and a feed water pump is used to maintain pressure within the range of 0.5–1.0 MPa. Close all valves in the soda water system to prevent air from entering the boiler. To maintain the pressure and temperature of the boiler, a low flame can be kept burning regularly under the furnace evaporator, or it can be heated periodically using steam from the boiler of an adjacent furnace. If a drop in boiler pressure is detected, the cause should be identified promptly, and the pressure should be restored to the specified level using a feed water pump. 3. Sulfite protection method: Sodium sulfite is a reducing agent that can react well with oxygen in alkaline solutions, thereby preventing oxygen from causing corrosion of metals. The method involves adding sodium sulfite to the alkali solution approach. It is generally required to maintain an excess amount of sodium sulfite in the water at 10–30 mg/L. 4. Ammonia water method and ammonia-hydrazine method: This method is used when the boiler is out of service for an extended period of time. After the boiler is taken out of service, drain all the water from it, remove or isolate the copper components in the system, and fill it with an ammonia solution prepared from condensate or feedwater at a concentration of 700–800 mg/L ； Or, after the boiler is taken out of service, water is not drained, and ammonia and hydrazine are directly introduced. Maintain the pH value of the water in the boiler between 10 and 10.5, with an excess amount of hydrazine of 200 mg/L. The liquid medicine is pumped into the system and circulated to ensure uniform distribution; once the boiler is fully filled with the protective liquid, all valves are closed. During the shutdown period, the entire system must be airtight to prevent air from entering. Using this method, when the boiler is to be put back into operation, all the protective fluid must be drained completely and the boiler must be thoroughly flushed. After ignition, steam should first be vented through the exhaust valve to the atmosphere, and steam can only be supplied once the ammonia content in the steam is below 2 mg/kg, in order to prevent excessive ammonia concentrations from corroding the copper tubes of the condenser. 5. Nitrogen filling method: The boiler can be filled with nitrogen using a wet method without draining water, or it can be done in a dry method with the boiler empty. Since nitrogen is very inert and non-corrosive, filling the boiler with nitrogen at a certain pressure can prevent oxygen from entering. For boilers with more complex structures, water can accumulate in certain areas and is difficult to remove completely; therefore, dry maintenance is not suitable, and nitrogen filling can be used instead. The method is as follows: Before shutting down the boiler, connect the nitrogen-filled pipelines; when the pressure in the boiler drops to around 0.15 Mpa, use nitrogen cylinders to supply nitrogen to areas such as the boiler drum and economizer through temporary pipelines. The specific requirements are as follows: (1) The purity of nitrogen must be over 99% ； (2) When filling the empty boiler with nitrogen, the nitrogen pressure inside the boiler should be above 0.15 Mpa ； Before filling the boiler with nitrogen using a wet method, it is advisable to add an appropriate amount of hydrazine to the boiler, and adjust the pH of the boiler water to above 10 using ammonia water ； (3) When filling with nitrogen, all valves in the boiler’s steam and water system must be closed to ensure that the boiler remains airtight ； If there is a leak, it must be eliminated immediately to prevent excessive consumption of nitrogen due to the leak and to ensure that the nitrogen pressure can be maintained.