At present, the steam treatment process in steam drum furnaces primarily relies on a phosphate-based system. However, this method is prone to "phosphate hiding" at high temperatures, leading to issues like alkali embrittlement or acidic phosphate corrosion. In recent years, improvements have been made to random group parameters, resulting in the development of coordinated and balanced phosphate processes. Despite these advancements, they are still limited to subcritical steam drum boilers. The low phosphate-low NaOH process has proven effective in eliminating "phosphate hiding" and significantly reducing boiler corrosion. It offers good protection for boiler systems and has gained attention for its use with carbon steel as the main structural material in both domestic and international applications.
In high-temperature and high-pressure water vapor systems, a protective magnetic Fe3O4 layer can form on the surface of carbon steel, preventing further corrosion. However, due to surface irregularities, unstable feedwater quality, and imperfect furnace water treatment, the protective film can be damaged under harsh conditions such as condenser leakage or frequent start-stop cycles. This damage leads to corrosion, highlighting the need for further optimization of the water treatment process to control water quality, reduce corrosion and scaling, extend chemical cleaning intervals, and enhance overall operational safety.
This study references the equilibrium process parameters used in American subcritical steam drum furnaces and employs autoclave coupon tests to simulate the corrosion behavior of 20A carbon steel under a low phosphate-low NaOH process. The effects of dissolved oxygen concentration, pH, aggressive anions like Clâ», and temperature on the corrosion of carbon steel in aqueous solutions were investigated. The results provide valuable insights for the practical application of the process.
The experimental setup involved using 20A carbon steel test pieces, which were polished sequentially with 200-mesh, 400-mesh, and 600-mesh quartz sandpaper. Each polishing step was performed at a 90° angle to ensure even surface preparation. After polishing, the samples were cleaned with pure cotton and acetone, then stored in a desiccator before being weighed.
A simulated furnace water solution was prepared according to literature guidelines, containing low phosphate and low NaOH concentrations. The solution was placed in a 2L autoclave, and five pre-treated 20A carbon steel coupons were suspended inside using Ni-Cr wire. Dissolved oxygen was removed by purging with nitrogen, and the temperature was gradually increased to the desired level. After maintaining the set temperature for 9 hours, the samples were cooled, rinsed with deionized water, and cleaned with alcohol and acetone. Corrosion was visually assessed, and weight changes were recorded. Surface morphology and composition analysis were conducted using electron probe microscopy and X-ray diffraction, while total iron ion concentration in the solution was measured.
The corrosion rate was calculated based on weight loss or gain, with oxidation contributing to weight gain and dissolution causing weight loss. The difference between these two rates indicated the net corrosion effect. X-ray diffraction was used to analyze the corrosion products, while electron probe techniques provided detailed surface imaging.
Results showed that corrosion rates increased with temperature and pressure. Below 250°C, the dissolution rate of Fe exceeded the oxidation rate, leading to weight loss and poor corrosion resistance. Above 250°C, oxidation dominated, resulting in a more uniform and protective oxide layer. Dissolved oxygen had a significant impact on corrosion, with higher concentrations increasing both oxidation and dissolution rates. Oxygen promoted Fe corrosion, so it was essential to remove dissolved oxygen during testing.
pH control was critical in the low phosphate-low NaOH process. At around pH 9.6, the Fe(OH)₃ saturation point was reached, minimizing corrosion. However, excessively high pH could dissolve the oxide layer, accelerating corrosion. Within the pH range of 9.2–9.6, corrosion rates decreased, and the oxide film remained stable. Beyond pH 13, the dissolution rate spiked, indicating poor film formation.
Aggressive anions like Clâ» significantly accelerated corrosion, especially through pitting. As Clâ» concentration increased, the number of etch points rose, and the oxide film became less uniform. At high Clâ» levels, the film failed to form properly, leading to severe localized corrosion.
In conclusion, temperature, dissolved oxygen, pH, and aggressive anions all play critical roles in carbon steel corrosion. Controlling these factors effectively can improve boiler performance and longevity. The low phosphate-low NaOH process, when optimized, offers a promising approach to reducing corrosion in modern steam systems.
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