ABSTRACT
Aboveground storage tanks (ASTs) soil-side corrosion protection is implemented using the cathodic protection (CP) technology. The U.S. Department of Transportation mandates use of CP systems for ASTs falling under federal regulations purview. For an AST with double bottom, CP is applied to the new bottom by installing anodes in the sand bed, laid in-between old and new bottoms. Large diameter tanks usually have impressed current cathodic protection (ICCP) systems with an MMO anode due to high CP current demand, and small diameter tanks have galvanic anode CP systems. Current output of a galvanic CP system is primarily dependent on the circuit resistance and the potential differences between anode and cathode, i.e., driving voltage. Vapor Corrosion Inhibitors (VCIs) are being used in combination with CP systems for the tank bottom soil-side applications. When VCIs are exposed to metal surfaces, they tend to change native potential of the metal (anode and cathode); this change in potential difference between the two metals changes the driving voltage. VCIs are highly conductive due to their ionic strength and tend to lower the sand bed resistivity when injected underneath ASTs, consequently changing the circuit resistance of CP system. Current output of the galvanic CP system varies greatly in presence of VCI depending on the compatibility of anodes with VCIs. This paper investigates effects of VCIs on galvanic anodes, their ability to function in a VCI saturated electrolyte, and long-term performance of the galvanic CP systems in the presence of VCIs.
INTRODUCTION
Aboveground Storage Tanks (ASTs) soil-side bottoms are usually protected with a cathodic protection (CP) system. For ASTs where bottoms are not serviceable, a new bottom is installed 100 mm to 300 mm (4 to 12 inches) above the old bottom with a dielectric membrane in between acting as a secondary containment. Figure 1 shows schematic of a double bottom tank with secondary containment and CP system anodes. Sand as tank pad is also placed in between the secondary containment and the new bottom. Secondary containment made of materials such as HDPE liners potentially act as a dielectric shield, hence anodes are installed in the sand base in between the membrane and the new bottom. A similar approach is also used for new tank construction, as secondary containment is a regulatory requirement. For larger diameter tanks, an impressed current cathodic protection (ICCP) system using mixed metal oxide (MMO) anode is preferred because of higher current demand from the anodes and for longevity of CP system. For smaller diameter tanks with lower CP current demands, galvanic anode CP systems have been used. MMO anodes are in either wire or strip form whereas galvanic anodes are usually in ribbon form; both are arranged either concentrically or in a grid pattern underneath a tank bottom to deliver CP current throughout the tank bottom surface.
