Sujay Math / Pavan K. Shukla
Compatibility between vapor corrosion inhibitors (VCI) and cathodic protection (CP) for the tank bottom application. Guidelines presented for selecting an effective corrosion mitigation strategy for combined VCI and CP systems.
Prevention of soil-side corrosion of the bottom plates of Aboveground Storage Tanks (ASTs) is a major challenge for the oil and gas storage industry. AST bottom plates are generally 0.25 inch (6.35 mm) thick A36 carbon steel. Literature suggests that the corrosion rate of the soil–side bottoms can be up to 200 mpy (5 mm/year). The soil-side surfaces of the bottom plates are usually protected by an impressed current cathodic protection (CP) system. However, when the bottom plate flexes the tank fill levels are varied, creating air gaps between the bottom plate and the tank bed leading to reduced CP effectiveness and potential corrosive conditions for the tank bottoms. Furthermore, in many situations, CP could become ineffective due to the poor ionic conductivity within tank beds and failure of anodes. Limited research and fieldwork have shown that vapor corrosion inhibitors (VCIs) by themselves or in combination with CP can be used to protect the bottoms of ASTs from external corrosion. VCIs could reduce corrosion incidents by 70–90 percent and decrease the incidence of pitting corrosion. This paper discusses methodologies to evaluate the compatibility between VCI and CP for the tank bottom application. Experimental work is reported to evaluate the CP criteria applicability in presence of VCIs, the work also provides a guideline for selecting an effective corrosion mitigation strategy for combined VCI and CP systems. In summary, VCI and CP, used in combination, are found to have a synergistic interaction to mitigate tank–bottom corrosion to acceptable levels.
Key words: Vapor Corrosion Inhibitors, Cathodic Protection, Above Ground Storage Tanks, CP Criteria