Vapor Corrosion Inhibitors (VCIs) are increasingly being used as a corrosion control measure for soil-side corrosion of Aboveground Storage Tanks (ASTs). VCIs are primarily applied to the tank bottoms using one of the following two delivery methods: (i) through-the-sandpad, and (ii) through-the-floor. Through-the-sandpad delivery is employed for tanks that are in-service, while through-the-floor delivery is used for out-of-service tanks. Access to the tank bottom is relatively uninhibited for the out-of-service tanks, and therefore, VCIs applied using the through-the-floor delivery method are expected to provide coverage to the entire bottom plate. However, for in-service tanks using the through-the-sandpad delivery method, VCIs are discharged through the multiple injection ports that are at discrete locations along the ring-wall (single bottom) or dead-shell (double bottom). After VCIs are injected, it is expected that VCIs would migrate via various mechanisms such as molecular diffusion, volatilization, convection, etc., and provide corrosion mitigation coverage throughout the bottom plate. A Pipeline Research Council International (PRCI) sponsored study was conducted to quantify VCI migration for the through-the-sandpad delivery method. Laboratory-scale experiments were conducted to determine VCI migration; two tubular-shaped sand-filled glass vessels were used. VCIs were injected at one end, and electrical-resistance data pre- and post-VCI injection were collected and analyzed to determine the VCI migration rates. The experimental data and associated analysis showed that VCIs migrated quickly from the injection port.
Soil-side corrosion of the bottom plates of aboveground storage tanks is one of the main reasons for the tank bottom failures. Literature data1,2,3 and experimental work have shown that soil-side corrosion rates could reach as high as 50-100 mpy, indicating that soil-side corrosion could cause the tank-bottom failure in relative short periods, i.e., less than five years after initial installation or repairs.
In addition, a recent study4 has shown that cathodic protection (CP) systems’ effectiveness could be questionable in up to 40 percent of the tanks. Tank operators have been increasingly using VCIs to address these issues; VCIs are being used either as stand alone or in combination with CP to mitigate tank bottom corrosion. An industry consortium sponsored a study5 to measure the migration characteristics of the VCIs; the other objectives of the study were to evaluate CPs’ effectiveness for the tank bottom application, monitor tank pad corrosivity to predict tank bottom corrosion, and assess VCIs performance under extreme conditions, such as high levels of chloride or high levels of bacteria.
