Abstract: Leakage occurred in the inlet pipeline of the evaporation system of a urea units during service. The cause of leakage was analyzed by means of macroscopic observation, penetrant testing, chemical composition analysis, metallographic examination,scanning electron microscope and energy spectrum analysis. The results show that the insulation layer of the outer wall of the pipe contained chloride ions. Due to the wetting of rainwater, a humid environment was formed in the gap between the pipe wall and the heat conducting mud. The dry-wet alternating environment led to the enrichment of chloride ions in the pipe wall, caused pitting corrosion and chloride stress corrosion cracking.