Seawater is a frequently used cooling media for seaside plants. Hence copper and copper alloys with excellent corrosion resistance to seawater are one of the major construction materials for heat exchangers and condensers for such applications. This paper investigated several possible failure mechanisms and intended to determine the dominating cause for Naval brass condenser tubesheet to suffer from a severe wall thinning and even perforation. And its service life was as short as 5 years or so. The influence of hypochlorous ions, which was introduced by electrolysis of seawater, on corrosion rate was also studied.