Cathodic Protection of Bottom Plates of Aboveground Crude Oil Storage Tanks

ABSTRACT

 

External protection of above ground storage tank bottoms is now at the top of worldwide priorities on account of safety and environmental considerations. The protection of these buried structures from external corrosion requires providing efficient Cathodic Protection systems, combined with external coatings.

Cathodic protection for these tank bottoms presents 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, requires careful corrosion mitigation Cathodic Protection system.

Indian Oil Corporation took the lead in permitting the use of polymeric anodes for Cathodic Protection of Tank bottom plates in India at one of its bulk crude oil storage location, situated at Chaksu, Near Jaipur (Rajasthan). The polymeric anodes were installed in the year 1995 for Cathodic Protection of three numbers of crude oil storage tanks. The installed Cathodic Protection system at Chaksu exhibits the achievement of effective and homogenous cathodic protection of the entire surface of the tank bottom plates. 

The effectiveness of the Cathodic Protection using Polymeric anodes has since been established by periodic inspection and the fact that there is no substantial increase in current requirement for protection of tank bottom plates at Chaksu (Rajasthan). The use of polymeric anodes for Cathodic protection help in better current distribution, minimal hydrogen over voltage, reduced risk of Cathodic disbondment & reduced CP current requirement. The extremely low DC voltage ensures the electrical remoteness of the anode from the tank bottom plate. The operating current densities of polymeric anodes at Chaksu (Rajasthan) indicate the long operating life.

This paper presents the detailed study and benefits of the installed Cathodic Protection system for tank bottom plates for mitigation of external corrosion for last 15 years. A comparative study has been done highlighting the efficacy of the installed Cathodic Protection system using polymeric anodes versus the conventional method of corrosion mitigation for tank bottom plates.

----x---