Research On Behaviors Of Carbon Steel Corrosion Under Alkalescent Water Conditioning

Today the electric power industry is developing rapidly and a lot of high-parameter and high-power fossil power plant is on building. So the working environment of metal materials is near to their limit condition. Carbon steel, as a kind of main constructive material, has been extensively applied in boiler water vapor system. How to minimize the corrosion of carbon steel is very important for improving availability, reliability and economics of the plant. From the practice, the main researching work of this thesis is following:(1) The principle, type and characteristics of water cycle chemistry in fossil power plant were reviewed.(2) The main factors influencing corrosion of carbon steel were summarized, such as temperature, anions, dissolved oxygen, pH value and the solubility of oxide film. And the predominant corrosion mechanisms of carbon steel in water were described in details.(3) The high temperature potential-pH equilibrium was plotted based on the chemistry and electrochemistry equilibrium in the Fe-H₂O system. The practical solution pH values at high temperature were calculated and analyzed. The results indicate that with the increasing in temperature, the stable zone of α-Fe₂O₃ and Fe₃O₄ move to low pH zone and the stable zone of iron move to the negative potential direction. In the practical high temperature water the ability of ammonia to regulate pH is weaker, so the NaOH should be adding to regulate practical pH to avoid acid attack.(4) The main factors influencing corrosion of carbon steel were studied by autoclave test and electrochemical methods. Results showed that the protective film was favorably formed with the increase of temperature. The corrosion rate of carbon steel decreased with raising temperature. The corrosion rate of carbon steel is the lowest while water pH value was near 9.5. The higher the electric conductivity is, the higher pH value was needed for passivation. High concentration of dissolved oxygen accelerated corrosion of carbon steel, the minute dissolved oxygen in water is favorable to form the protective film.The results of electrochemical tests indicated that the oxide film on carbon steel was protective.(5) An outlook is also given for future researches.