During the last 10 years of my professional career, I had the chance to get involved in tens of aboveground storage tank inspections, review Testing & Inspection (T&I) reports, foundation drawings, and analyze cathodic protection (CP) data. This experience supported with readings of many published technical articles in this field confirms that soil-side corrosion is a principal cause of storage tank failure and imposes a major environmental and operational challenge worldwide and requires effective control during the complete life cycle of the tank.

Tank bottom plate with large and deep pits due to localized soil-side corrosion.

Several methods have been adopted to mitigate soil-side corrosion of Aboveground Storage Tank (AST) floors, such as bituminous sand, impressed current cathodic protection and coatings. However, the total effectiveness of these techniques, as standalone or combined, have been questionable in providing the required protection, especially against soil-side corrosion. Al-Sulaiman1 discussed the possibility of bituminous layer trapping moisture and corrosive species between the underside of tank floor and construction pad resulting in creating a corrosive environment. The author also highlighted the likelihood of a bituminous layer when combined with CP to shield protection current and render the CP system ineffective, at least partially. Yu2 concluded that inevitable air gaps between the construction pad and tank bottom plates block CP current at that location and consequently prevent its uniform distribution on the underside surface of the tank bottom. Chatterjee3 emphasized that the underside coating of bottom plates alone cannot prevent corrosion due to unavoidable defects during its application and deterioration during tank operation. Another problematic area is the tank chime. The presence of inevitable gaps and spaces between the annular plates and the ring wall is a major source of corrosion around the tank’s base. Such gaps allow for the ingress of moisture, rainwater, fire sprinkler or tank deluge systems creating water pooling and ponding around the base and diffusion airborne chloride, especially in facilities located in coastal areas. 

Such issues can be avoided by following the latest specifications and international standards for the construction phase and can be improved through good operation and maintenance practices. Below is a list of the most commonly used Industry codes, standards and guides:

Design and Construction:

• API 650 Welded Tanks for Oil Storage
• API 620 API 2610 Design, Construction, Operation, Maintenance, and Inspection of Terminal and Tank Facilities
• EN 14015 Design and Manufacture of Above Ground Welded Steel Tanks for Storage of Liquids at Ambient Temp and Above
• API 2610 Design, Construction, Operation, Maintenance, and Inspection of Terminal and Tank Facilities
• EN 14015 Design and Manufacture of Above Ground Welded Steel Tanks for Storage of Liquids at Ambient Temp and Above

Cathodic Protection:

• NACE SP0193 External cathodic protection of on-grade carbon steel storage tank bottoms
• API 651 Cathodic Protection of above-ground storage tanks
• BS 7361 Cathodic Protection Part 1 Code of practice for land and marine applications Section 4.5
• NACE SP0286 Electrical isolation of cathodically protected pipelines
• NACE SP0169 Control of external Corrosion of underground or submerged metallic piping systems

Inspection and Maintenance:

• API 653 Tank inspections, repairs, alterations, and reconstruction
• API RP-581 Risk-Based Inspection Methodology
• EEMUA 159 Above ground flat bottomed storage tanks: A guide to inspection maintenance and repair
• EEMUA 183 Prevention of Tank Bottom Leakage  

References:

1. S. Al-Sulaiman, H. Sabri, R. Rahim, “Evaluation of Cathodic Protection System criteria On Constructed Tanks Over Bituminous Sand Mix Layer,” 14th Middle East Corrosion Conference, paper no. 63-CP-10 (Manama: NACE,2012) 
2. Xiaming (Andy) Yu, “Evaluation of the Tank Bottom Corrosion and CP Effectiveness at a Saudi Aramco Crude Oil Tank Farm,” 13th Middle East Corrosion Conference, paper no. 10043 (Manama: NACE, 2013) 
3. B. Chatterjee, “Prevention of External (Soil Side) Corrosion on Storage tank Bottom Plates by Cathodic Protection System.” CORROSION/08, paper no. 8058 (Houston TX: NACE, 2008)