|Content Type||Conference Paper|
|Title||Cathodic Protection of Bottom Plates of Above Ground Crude Oil Storage Tanks|
|Authors||Ajit Kumar Thakur, Indian Oil Corporation Limited|
|Source||CORROSION 2011, March 13 - 17, 2011 , Houston, Texas|
|Copyright||2011. NACE International|
|Keywords||Soil Side Bottom plate Cathodic Protection, Long Line Anode, Corrosion Mitigation|
Safety and environmental considerations call for implementation of a reliable Cathodic Protection system to effectively eliminate the soil side corrosion of tank bottom plates. Tank integrity continues to be adversely affected due to existence of soil side corrosion even with Cathodic Protection systems. The protection of these buried structures from external corrosion requires providing efficient Cathodic Protection systems, combined with external coatings. Designing effective Cathodic Protection system for these tank bottoms is a real challenge due to the large and complex geometries involved and the proximity of the anodes to the tank bottom. Special considerations such as unprotected areas, with preferential corrosion, occurring at the heat-affected zones (HAZ) of the base metal near welds, unprotected areas in contact with soil and normal coating breakdown overtime, require careful corrosion mitigation Cathodic Protection system. This paper discusses the role of passive barrier (primary coating) in prevention of soil side corrosion of tank bottoms. The paper presents the details and benefits of the soil side corrosion prevention measures provided at five locations. The installed Cathodic Protection Systems are effectively protecting 26 numbers of tank bottoms at three locations for 5-15 years. Various anode configurations have also been discussed to examine the limitations of the conventional Cathodic Protection systems.
Bottom plates of above ground storage tanks are subjected to soil side corrosion, which may become severe due to formation of corrosion couples between HAZ near the welds and the base metal of the bottom plate. Accordingly it becomes essential to avoid corrosion of the tank bottoms on account of safety and environmental hazards. Further leak repair is very cumbersome and costly process requiring tank outages for substantial period of time.
PASSIVE CORROSION PROTECTION & CATHODIC PROTECTION
Traditional methods of corrosion prevention include providing passive barrier protection systems such as sand bitumen tank foundation pad or soil side paint / coating of the bottom plates. The protective coating should preferably be a self-priming, high build, immersion-grade coating resistant to wet environments. The coating performance depends on good surface preparation to provide the desired performance. Since the coating is required to perform for many years, it is best to use high performance materials such as fusion-bonded epoxy or solvent free, liquid epoxy. However, the passive barrier protection / coating alone cannot be relied upon to prevent long term soil side corrosion of tank bottom plates due to following reasons:
• During tank construction, individual bottom plates are first coated on soil side and then welded together to complete the entire tank bottom. Near the weld joints the heat of welding destroys the coating, which cannot be repaired, being on soil side. Thus the tank bottoms inherently have unprotected area in contact with soil,
• The heat of welding may result in differences in microstructure of the steel plate adjacent to weld joints causing preferential corrosion at heat affected zones (HAZ).
• The coating on the soil side of the bottom plates is also subjected to normal coating breakdown over time.
|File Size||238 KB||9|