Most older MAN diesel marine engines eventually suffer from corrosion of the aluminum heat exchanger tank. MAN reccomends removing the tube bundle and changing the o rings every 2 years. As aluminum and salt water do not play well together the surface where the o-ring seals becomes pitted very quickly.
The aluminum tank contains a copper alloy tube bundle. Once this corrosion starts the aluminum in the presence of salt water expands as it corrodes and eventually crushes the bundle.
The most common repair is to machine the tank and install an aluminum sleeve. While this is good in the short term eventually even the new sleeve will corrode and the process will start again.
After installing a few aluminum sleeves we decided to machine and install sleeves made out of bronze. The bronze is many times more corrosion resistant than the aluminum and will not expand and crush the bundle.