A DuPont plant has experienced severe localized corrosion from a complex corrosion phenomenon in its cooling water system. A large semi-closed cooling water system was created ten years ago and since has had aggressive corrosion attack particularly on carbon steel surfaces. A new treatment program has been applied and is currently providing improvement. The paper will present performance data on the treatment program and other steps that have been taken to upgrade system performance. This program is in progress and additional data will be includes as it is developed.