Inspection revealed patterned severe corrosion of the tubes. An investigation was thus conducted to understand the corrosion mechanism and propose potential mitigation measures.Corrosion was found dominantly on the external surface of the tubes i.e. cooling water side and the severe wall loss was mostly located just beneath the top tube sheet.Corrosion in the cooling water side was in the form of pits and was the result of tuberculation under deposits. The severe wall loss of the tubes just beneath the top tube sheet was due to concentrated corrosive compounds on the tube surface within the empty space.Polymer fines deposited on the top tube sheet at one single event some of the tubes were totally or partially plugged. Deposit of polymer fines on the top tube sheet was the major contributor to cause the patterned severe corrosion and the severe corrosion occurred on the clear tubes.To mitigate this severe corrosion it is recommended to eliminate the empty space close to the top tube sheet by modifying the water exit and upgrade the metallurgy to duplex stainless steels.