FAILURE ANALYSIS OF BOTTOM PLATE OF POTABLE WATER TANK, PH-II, MEDINA-YANBU PLANTS 1 Mohammad Mobin, Anees U. Malik, Saleh Al-Fozan and Fahd Al-Muaili Saline Water Desalination Research Institute Saline Water Conversion Corporation (SWCC) P.O.Box 8328, Al-Jubail 31951, Saudi Arabia. E-mail: rdc@swcc.gov.sa INTRODUCTION The Plant Manager, SWCC Medina-Yanbu Plants in his letter No. SWCC/MY/2220/0182 dated June 15, 2005 addressed to the Manager, R&D Center, Al-Jubail informed about the holes and deep pitting in the bottom plate of Phase-II potable water tank No. 2 [01-TK-002]. The former requested the R&D Center for a detailed failure analysis and corrosion study and to provide suitable recommendations. A sample of the bottom plate was also received along with the letter. BACKGROUND The potable water tank No. 2 at PS-I in SWCC Medina-Yanbu plants, is in service since July 1998. There are four other identical potable water tanks at PS-I. Potable water from Phase-II and RO plant is stored in these tanks and pumped to Al-Madina Al-Munawarah. The inner side of the bottom plate which is in direct contact with water has a coating of Hempadur multi-strength 3553 whereas the external surface is in contact with a layer of asphaltic material. The tank is protected cathodically (impressed current) from internal as well as external side. Each tank is designed with separate cathodic protection system. Recently, the tank was opened for the first time for internal inspection. Some paint blisters and rust spots were observed on the bottom plate and 1 st to 3 rd course shell plate from the bottom. As a remedial measure it was decided to sand blast and repaint the bottom plate and 1 st course shell plates completely. However, during the sand blasting holes and deep pitting were observed on the bottom plate. 1 Issued as Troubleshooting Technical Report No. TSR 3804/04014 in February 2005.