Abstract: Active amine and ammonia water were used as alkalizing agent to simulate the air cooling island environment of the direct air cooler group. Electrochemical tests, immersion tests, and morphology observation were used to study the corrosion resistance of No. 20 steel in different alkalizing agent environments. The results showed that when active amine as an alkalizing agent, No.20 steel had revelatively high corrosion resistance in both vapor and liquid phase environments. The practical application results showed that when active amine was used as the alkalizing agent for unit shutdown and maintenance in direct air cooling units, the water vapor system had low iron content, small particle size, and short start-up time during the process of restarting the unit after maintenance. The protective effect of using active amines was better than that of ammonia water as alkalizing agents for shutdown maintenance.