Shutdown protection and shutdown corrosion inhibitors for boilers

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Abstract: With the development of power generation and changes in operating conditions, boiler units are shut down for longer periods of time more frequently. This leads to an increased degree of corrosion while the units are in a shut-down state, which severely affects their lifespan as well as their safety and economic efficiency. To this end, common methods for protecting boilers during standby periods are introduced, the scope of application and existing problems of the currently used methods are discussed, and the research progress on standby corrosion inhibitors is elaborated in detail. Keywords: shutdown protection; corrosion inhibitors; boiler equipment; corrosion. Due to changes in electrical load and the operating conditions of thermal equipment, boilers will inevitably be in a state of short-term, medium-term, or long-term standby. During shutdown, if no protective measures are taken, the inner surfaces of the boiler’s steam system are exposed to air with 21% oxygen content; such an environment is highly detrimental to steam systems made of carbon steel and low-alloy steel, which account for over 95% of such systems. During the period when the boiler is out of service, various factors such as the relative humidity of the air and the degree of air pollution collectively cause varying degrees of corrosion to the equipment, and this corrosion is much more severe than that that occurs during operation when strict deoxygenation measures are in place. Currently, the problem of corrosion in shut-down boilers is becoming increasingly prominent. This is due to: (1) the growing maturity of water treatment technologies, which ensures stable water quality and gradually resolves issues related to scaling and salt deposition; as a result, corrosion becomes more apparent, directly affecting the lifespan of the units as well as their safe and economical operation; (2) an increasing number of units being taken out of service, with longer periods of inactivity. 　　In recent years, some power plants have been operating in peak-shaving mode, forcing certain units to undergo frequent start-up and shutdown cycles; this has led to increased corrosion of those units when they are not in use. As a result, measures for protecting units during shutdowns are receiving increasing attention. 1　During the period when thermal equipment such as corrosion-prone boilers, steam turbines, condensers, and heaters are out of use, if no protective measures are taken, severe corrosion will occur on the metal surfaces on the water and steam side (i.e., standstill corrosion). The factors affecting corrosion during shutdown are similar to those in atmospheric corrosion; the main ones include: (1) Humidity – A relative humidity of less than 20% inside the shut-down equipment can prevent corrosion. (2) Salt content: As the salt concentration in the water or on the liquid film on the metal surface increases, the corrosion rate rises. (3) Metal material: Carbon steel and low-alloy steel are prone to shutdown corrosion, whereas stainless steel or alloy steel is less susceptible to it. (4) Degree of metal surface cleanliness: When there are deposits or water droplets on the metal surface, the corrosion rate increases, and local corrosion occurs due to differences in oxygen concentration on the metal surface. (5)pH: As pH increases, corrosion decreases; when pH reaches above 10, corrosion is well suppressed. During the shutdown period, the equipment is in contact with the atmosphere, resulting in high oxygen concentrations and a large area of corrosion; therefore, corrosion on equipment that is shut down is more severe than that on equipment that is in operation. When the unit is restarted, corrosion products enter the boiler and turbine; in the loose coatings on the water wall tubes in areas under high heat loads, the boiler water becomes locally concentrated 102 to 105 times, leading to alkaline or acidic corrosion – the latter being the main cause of failure in large-capacity boilers. Secondary scaling during unit operation is often caused by corrosion due to shutdown. It can be seen that shutting down corrosion control has a severe impact on the safe and economic operation of the unit, posing a great threat. 2 Disabling protection methods: Corrosion of equipment occurs due to the simultaneous presence of oxygen and water. Therefore, to prevent corrosion due to shutdown, it is necessary to reliably eliminate these two factors or at least one of them. Based on their mechanism of action, the methods for disabling protection are classified as follows: (1) Preventing air from entering the vapor system of thermal equipment, including nitrogen filling, maintaining steam pressure, and boiler full-water protection methods; (2) Reducing the humidity inside the vapor system of thermal equipment, via drying methods, desiccant methods, vacuum methods, etc.; (3) Adding corrosion inhibitors to form a protective film on the metal surface, or removing dissolved oxygen from water. The corrosion inhibitors used include hydrazine, ammonia solutions, and gaseous corrosion inhibitors. 2.1 Recent Advances in Research on Deactivation Protection In recent years, some new progress has been made in research on deactivation protection in China, mainly focusing on the study of deactivation protection corrosion inhibitors. The following is an introduction and review of several deactivated protective corrosion inhibitors. 2.1.1 Octadecylamine appears as white crystals; its molecular formula is CH3(CH2)17NH2, with a molecular weight of 269.57. Its boiling point is 348°C and its freezing point is 53.1°C. It is insoluble in water but soluble in ethanol and ether. Its protection principle is as follows: Octadecylamine passes through the corrosion products in water or steam to come into contact with the clean metal surface, causing some of the corrosion products to be displaced. Octadecylamine is adsorbed onto metal surfaces to form a dense hydrophobic film that prevents water vapor, oxygen, and hydrogen ions from reaching the surface, thereby acting as a barrier against corrosion. Implementation method: The timing of chemical addition is determined by the decomposition temperature of octadecylamine (400–500°C); therefore, chemical addition begins only when the pressure in the steam drum drops to 9–10 MPa during the gradual shutdown of the unit. The location for chemical addition is at the outlet of the deaerator. The lowest concentration of octadecylamine throughout the water and steam system should be maintained at 2–4 mg/L, though the actual amount of chemical added can be increased by 3–5 times. After all the chemical has been added, the unit is shut down within 30–60 minutes, the boiler is drained while still under pressure, and it is allowed to dry naturally. Generally, this method can be applied when the standby time exceeds 10 days. Studies have shown that the factors affecting the film formation of octadecylamine include: (1) temperature. When the temperature is between 120°C and 170°C, the electrode Rf value increases rapidly; it reaches its maximum at 220°C. Above 220°C, the Rf value shows a downward trend. Therefore, the best film formation occurs at 220°C. (2) Concentration: The higher the concentration of octadecylamine, the better the film-forming effect. (3) Metal surface roughness: The greater the roughness, the worse the film-forming effect; however, even thermal equipment with very rough metal surfaces achieves good protective effects after treatment with octadecylamine. (4) Holding time: A film-forming effect is better at 1 hour or more. Octadecylamine can form films in both vapor and liquid phases; the films formed in the liquid phase have lower density and protective properties compared to those formed in the vapor phase. According to relevant information, as determined by Auger electron spectroscopy, the structure of the iron surface after treatment with octadecylamine is as follows: iron | iron oxide layer | octadecylamine layer containing iron oxide | octadecylamine layer. When using the octadecylamine method for shutdown protection, the following points should be noted: (1) The film formation of octadecylamine on metal surfaces is closely related to pH, with an ideal pH range of 7–8; (2) The corrosion products removed after adding octadecylamine must be discharged promptly; (3) When using octadecylamine for shutdown protection in units equipped with condensate treatment systems, it is necessary to avoid any contact between the ion exchange resin and octadecylamine. 2.1.2 Acetaldoxime The molecular formula of acetaldoxime is CH3CHNOH. The chemical reaction that takes place during its use for corrosion prevention is as follows: 4CH3CHNOH + 5O2 → 4CH3CHO + 4NO2 + 2H2O; 2CH3CHNOH + 3Fe2O3 → 2CH3CHO + 2Fe3O4 + N2 + H2O. In wet corrosion prevention methods, it is first necessary to remove the O2 from water, after which the metal surface is passivated to form a protective layer of magnetic iron oxide, which serves to prevent corrosion. Implementation method: The pH of the solution is adjusted to 10.5–10.8 using ammonia water, and the concentration of acetaldoxime is set at 300–400 mg/L. The solution is then poured into the boiler until the economizer, superheater, and the boiler itself are completely filled with the protective liquid. Under conditions of long anti-corrosion protection duration and incomplete sealing, an acetaldoxime concentration of greater than 400 mg/L should be selected. For cases with strict conditions, it is better to choose a higher concentration. Industrial research has shown that the acetaldoxime-ammonia solution filling protection method provides excellent corrosion prevention effects in industrial applications; from a technical and economic perspective, it is superior to the hydrazine-ammonia solution method. 2.1.3 Dimethylthioxime (DMKO) Dimethylthioxime is also a commonly used chemical for the wet protection of boilers as an oxygen scavenger and passivator. Its molecular formula is (CH3)2CNOH, with a molecular weight of 73.09, a density of 0.931, a melting point of 60.0°C, and a boiling point of 134.6°C. It possesses strong reducing properties, and its toxicity LD50 value is 5500 MG/KG. Dimethylthioxime appears as white prismatic crystals; it has an aromatic odor, is volatile in air, is neutral in nature, and is soluble in solvents such as water, alcohols, and ethers. The chemical reaction that occurs during the anti-corrosion process is: 2(CH3)2CNOH + 6Fe2O3 —— 2(CH3)2CO + 4Fe3O4 + N2O + H2O. Dimethylcarbinol acts as a strong reducing agent, which enables the passivation of the metal surface. Implementation method: Add the pre-prepared dimethyl oxime solution to the water supply tank; the dosage is calculated based on the boiler’s volume at a concentration of 300–400 mg/L. At the same time, adjust the pH to above 10.5 using NH3, ensure uniform circulation, feed the solution into the superheater, top up water in the water tank, continue to maintain uniform circulation after adding chemicals, and send it to the reheater via the primary bypass. 　　Summary: (1) DMKO provides excellent protection for metal surfaces and is suitable for protecting boilers during long periods of inactivity. (2) It is generally appropriate to keep the DMKO concentration between 300 and 400 mg/L (too low a concentration results in inadequate protective effects; too high a concentration significantly increases the costs associated with protection). (3) The pH should be maintained above 10.5; once a protective film forms on the metal, the pH decreases, but it still provides good protection within a certain range. It should be noted that DMKO has a certain corrosive effect on HSn70-1 brass. Therefore, contact between the protective fluid and copper or copper alloy components should be avoided as much as possible. As a new type of boiler shutdown protector, DMKO boasts advantages such as low toxicity and high efficiency, as well as easy treatment of waste liquid; it can replace the more toxic hydrazine for the protection of boilers during long periods of inactivity. The main problems include: the method for determining the concentration of DMKO needs to be improved. 2.2 Protection Methods for Deactivation Abroad According to the latest American literature, a relatively effective protection method currently in use is the combination of dehumidification and vapor phase corrosion inhibitor methods: at a certain temperature, the vapor phase corrosion inhibitor (Vapor Phase Corrosion Inhibitor, VPI) has a certain vapor pressure within a closed system. VPI molecules adsorb on the metal surface to form a film with a thickness of about 1–2 molecular layers, thereby suppressing corrosion on the metal surface. These vapors can penetrate through pores and other openings to reach surfaces that are difficult to access, and they possess strong anti-corrosion properties even in conditions of high relative humidity, high temperature, and acidic media. The main types of VPIs currently in use include aromatic compounds, fatty acids, and mineral oils. 　　Both the dehumidification method and the vapor-phase corrosion inhibitor method have their respective limitations. The limitation of the dehumidification method is that it requires a well-sealed system and a structure that is not too complex. On the other hand, the vapor-phase corrosion inhibitor method is relatively costly in terms of labor and resources for large, complex systems. Combining the two methods can yield better results. A small thermal power plant in New England employed a combined protection method of dehumidification and vapor-phase corrosion inhibitors when it was shut down. Among them, the boiler body and turbine are protected by dehumidification, while various pipelines are protected by VPI. The VPI powder is drawn into the pipeline from inside the furnace. The installation of dehumidification equipment takes about a week, while for VPI, 1 person is required per day. The results showed that no corrosion was detected during the equipment inspection one year later. The dehumidification method can be used in large-capacity systems that are easy to isolate, but it involves high installation costs and energy consumption; whereas the VPI method can be applied in areas where the dehumidification method is not feasible. The combined use of the two methods enables safe and cost-effective protection for the entire system (the advantages include ease of implementation and maintenance, lower costs, and comprehensive protection). This combined method has been widely applied and yields good results. 3 Prospects for boiler shutdown protection work: (1) Further study the corrosion inhibition mechanism, use surface analysis techniques to determine the thickness of the film and establish a structural model of the film; employ nonlinear analysis methods such as artificial neural networks to analyze, integrate, and evaluate experimental data in order to identify appropriate corrosion inhibition strategies and predict test results, or apply quantum chemical theory to calculate and infer the adsorption model of corrosion inhibitors on the electrode surface, and analyze the predicted results. (2) Develop new, non-toxic, and efficient furnace shutdown protectants. The shutdown protectants that have been developed to date have certain limitations in terms of application; for example, DMKO is corrosive to copper and copper alloys, and some protectants have poor water solubility, which makes them inconvenient to use in practice. Therefore, it is necessary to develop shutdown protectants with superior performance.