There are 4 main reasons that cause corrosion in heat exchangers to need to be concerned during the inspection: Water impingement, Temperature, Vibration, Velocity.

Refer to: Typical location of Corrosion on Heat Exchanger

Cooling Water Corrosion in Heat Exchanger Tubes at High Water Temperature Zone;

Another important factor affecting Corrosion in Heat Exchanger Tube Service at Cooling Water is the temperature.

The area where the Cooling Water has a high temperature ( on the Outlet side ), there will be more fouling and corrosion than the low-temperature area. In the example, you can see from the corrosion that occurred in front of the Tube-sheet Channel, and the results of the IRIS as below:

Will find that the Cooling Water Outlet or the Cooling Water with the highest temperature will cause the most severe Cooling Water Corrosion.

Cooling Water Corrosion in Heat Exchanger Tubes due to Low Flow Velocity; 

For Heat Exchanger that uses Cooling Water to exchange heat with Process Fluid, the Cooling Water runs on the Tubeside side, where the Tubes have a low-velocity flow of cooling water (<1 m/s), often sediment (sedimentation) and scale (fouling), and cause the area to be more corrosive.