Abstract: The air cooler of a ship has been found that the heat exchange tube was leaked three months after delivered,a systematic research was made to analysis the reason of the leakage of heat exchange tube, including macro examination, chemical component analysis, metallographic examination, SEM and energy spectrum analysis. The results showed that the reason of leakage was pitting corrosion, because the corrosion products of cooling system and marine organism along with the sea water into the heat exchange tube of air cooler core, attached to the inner wall of heat exchange tube and formed sediments, the distribution of sediments was uneven, the different parts of it had oxygen differences and medium concentration difference, thereby forming a micro cell effect, and the pitting corrosion resulted in leakage.