Abstract: Water seepage and corrosion occurred in a heat exchanger after long-term use. Macroscopic examination, scanning electron microscope and energy spectrum analysis were performed on the leakage site. The results show that the copper solder in some areas was not fully fused with the stainless steel substrate during welding. During the use of heat exchanger, multiple thermal cycles and periodic impact of water flow separated the copper solder from the stainless steel substrate in the unmelted area of welding, and eventually led to local water seepage.