ABSTRACT
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.
INTRODUCTION
Aboveground Storage Tanks (ASTs) are one of the key assets for owners and operators in the oil and gas transmission industry. Protecting these assets is an economic burden and hence the operators are in continuous struggle to protect and extend the service life of these storage tanks. The soil-side corrosion of the bottom plates of ASTs is a major challenge and directly impacts the service life of tanks. API 6531 makes it mandatory to determine the tank bottom integrity to prevent leakage of storage fluids that could cause environmental damage. Inspection frequency to verify tank integrity is mandated once every 10-20 years, provided the in-service periodic inspections assure continued tank integrity. The tank bottom plate corrosion rates normally determine the frequency of inspection, with higher corrosion rates of the bottom plates the frequency of inspection may be as little as 10 years. The tank owners take precautionary measures to keep the tank in-service for the mandated 20-year inspection cycle, as frequent inspection cycles cause downtime and loss of revenue which is significant in vindicating cost to benefit economics.
