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, their application remains limited in subcritical steam drum boilers. The low phosphate-low NaOH process has emerged as a promising alternative, effectively eliminating the "phosphate hiding" phenomenon and significantly reducing boiler corrosion. This method offers excellent protection for boiler systems and has led to increased research and application of carbon steel as a primary material in water-vapor systems. Under high-temperature and high-pressure conditions, a protective magnetic Fe layer can form on carbon steel surfaces, preventing further corrosion. However, factors such as uneven surface conditions, unstable feedwater quality, and imperfect furnace water treatment can damage this protective film, leading to corrosion. Therefore, optimizing the boiler water treatment process and controlling water quality are crucial for reducing corrosion and scaling, extending chemical cleaning cycles, and improving overall operational safety.
This study refers to the equilibrium process parameters used in American subcritical steam drum furnaces and employs an autoclave coupon test to simulate the corrosion of 20A carbon steel under the low phosphate-low NaOH process. The effects of dissolved oxygen concentration, pH, aggressive anions like Clâ», and temperature on carbon steel corrosion were investigated. The results provide valuable insights for the practical implementation of this process.
The experimental setup involved using 20A carbon steel test pieces. The samples were polished sequentially with 200, 400, and 600 mesh quartz sandpaper, ensuring that each polishing direction was perpendicular to the previous one until all scratches were removed. After polishing, the samples were cleaned with pure cotton and acetone, then placed in a desiccator for a period before weighing. A simulated low phosphate-low NaOH furnace water solution was prepared according to literature guidelines, and five pre-treated 20A carbon steel coupons were suspended in a 2L autoclave. Dissolved oxygen was removed by purging with nitrogen, and the temperature was gradually increased to the desired level. After maintaining the temperature for 9 hours, the samples were cooled, rinsed with deionized water, scrubbed with alcohol and acetone, and then weighed again. Surface morphology and composition analysis were conducted, along with measuring total iron ion concentration in the solution.
The corrosion rate of carbon steel was calculated using the gravimetric method, based on the weight change of the test pieces before and after corrosion. Oxygen entering the surface increases the weight, while Fe dissolving into the solution causes weight loss. The difference between these two rates reflects the net corrosion behavior. The total iron ion concentration in the solution was used to approximate the dissolution rate of Fe. Surface analysis was performed using electron probe secondary electron imaging or backscattered electron methods, and X-ray diffraction was used to analyze the morphology of corrosion products.
Results showed that the corrosion rate of carbon steel increased with temperature and pressure. Below 250°C, the oxidation rate was lower than the dissolution rate, leading to weight loss and poor corrosion resistance. Above 250°C, the oxidation rate exceeded the dissolution rate, resulting in weight gain and the formation of a thicker, more uniform protective oxide layer. Dissolved oxygen had a significant impact on corrosion, with higher concentrations increasing both oxidation and dissolution rates. At high pH levels (around 9.6), the corrosion rate was minimized, but excessive alkalinity could dissolve the oxide layer, accelerating corrosion. Aggressive anions, particularly Clâ», promoted pitting corrosion by adsorbing at surface defects and accelerating local dissolution. Controlling Clâ» concentration below 0.4 mg/L was found to be optimal for minimizing corrosion.
In conclusion, temperature, dissolved oxygen, pH, and aggressive anions all play critical roles in carbon steel corrosion under the low phosphate-low NaOH process. Maintaining pH between 9.2 and 9.6, controlling dissolved oxygen, and limiting Clâ» concentration are essential for effective corrosion protection in boiler systems.
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