In order to meet the growing demand for electrical energy in the 21st century, China’s thermal power generation is shifting toward high-parameter and high-capacity units. As these units operate at higher temperatures and pressures, the heat transfer efficiency increases, but so does the risk of corrosion within the boiler. Corrosion of the water wall tubes is a leading cause of leaks and ruptures in boiler systems. Since furnace water and steam are directly in contact with the water wall tubes, their chemical properties significantly influence the corrosion process. While numerous studies have examined the causes of water wall failures, there remains limited research on the impact of trace components in boiler water and how temperature affects corrosion. To address this gap, this study simulates phosphate-based water treatment conditions. Carbon steel No. 20 was used as the test material, and corrosion tests were conducted in an autoclave. The corrosion rate was measured by weight loss, while surface analysis was performed using electron probe microanalysis (EPMA) and X-ray diffraction (XRD). The results aim to establish the optimal conditions for controlling boiler water quality.
The theoretical values presented in Table 1 represent equilibrium data between pure water and vapor. Experimental values, however, come from slow heating processes in the autoclave. As temperature rises, the discrepancy between theoretical and experimental values grows exponentially. Near the critical point, vapor pressure increases linearly with temperature, and the deviation becomes more pronounced. This is primarily due to the unbalanced heating process and the presence of dissolved salts in the water. As temperature increases, other properties of water, such as density, surface tension, and dielectric constant, also change. For instance, water’s dielectric constant decreases significantly near the critical point, which affects the solubility of ions and compounds like trisodium phosphate. This leads to increased partitioning of salts into the gas phase or deposition on surfaces, contributing to scaling and corrosion. Therefore, strict control of boiler water quality is essential to prevent damage to the water wall and turbine blades.
To further understand the effects of dissolved oxygen on carbon steel corrosion, a series of tests were conducted under simulated high-temperature conditions. Carbon steel coupons were exposed to different levels of dissolved oxygen, and their weight changes and surface characteristics were analyzed. Results showed that the corrosion rate increases with rising oxygen concentration. At low oxygen levels (below 0.104 mg/L), the oxide film formed is dense and protective, limiting iron dissolution. However, as oxygen increases, the oxide film becomes porous, allowing faster corrosion. The optimal pH range for minimizing corrosion was found to be around 9–10. At higher pH levels, the formation of insoluble iron oxides can be hindered, leading to increased corrosion rates.
Chloride ion content also plays a significant role in corrosion. Higher Clâ» concentrations increase the likelihood of pitting and localized corrosion on carbon steel surfaces. Tests revealed that when chloride levels exceed 0.14 mg/L, the corrosion rate rises sharply, and surface etching becomes more pronounced. Sulfate ions exhibit similar behavior, though their impact is slightly less severe. Controlling chloride and sulfate levels is crucial to maintaining the integrity of boiler systems.
Temperature has a direct effect on corrosion rates as well. At lower temperatures, the dissolution rate of iron is higher, while near the critical point, it decreases significantly. This is attributed to the reduced dielectric constant of water, which lowers the solubility of iron hydroxides and promotes the formation of protective oxide films. These findings highlight the importance of monitoring and controlling both temperature and water chemistry to prevent long-term damage to boiler components.
In addition to these factors, the new method of BW deactivation protection offers several advantages. It forms a uniform, protective film on boiler and turbine surfaces, reduces start-up time, and provides long-term atmospheric protection. The process is simple, cost-effective, and compatible with existing water treatment systems. However, during maintenance, care must be taken to avoid issues such as pipe blockage due to interactions with ammonia or hydrazine. Proper flushing and careful handling of chemical solutions are essential to ensure the effectiveness of the protection system.
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