PURPOSE: To prevent a multitubular heat exchanger from being corroded during standstill by a method wherein titanium tubes, onto the surface of each of which copper alloy is interposed, are connected to a copper alloy tube plate.

CONSTITUTION: Copper alloy, which is same material as that of a tube plate, is installed onto the surface of a titanium tube 2 closely contacting with a tube plate made of copper alloy 1 in electrically connected state as the sacrificial anode 3. No electrical corrosion due to the contact between different metals occurs on the sacrificial anode 3 during the operation even though the anode 3 contacts with the titanium 2, because electrical corrosion protection is in action during the operation with sea water circulation. However under the state that no sea water exists in a water chamber due to the stoppage of water circulation or the state that a sea water film 6 exists onto the surface of a combinational structure of different materials, the electrical corrosion occurs between the titanium 2 and the sacrificial anode 3 and is prevented from occurring between the titanium 2 and the copper alloy (tube plate) 1. The sacrificial anode 3 is dealt as expendables and renewed when damaged, resulting in permanently preventing the copper alloy (tube plate) 1 from being electrically corroded.