The physical and electrolytic contact between the bottom plate of an aboveground storage tank and the underlying soil typically varies over the area of a tank bottom plates. External tank bottom plates are exposed to both electrolytic and vapor-phase corrosive environments. Cathodic Protection (CP) requires direct electrolytic contact between the tank floor and the underlying soil to effectively mitigate corrosion; hence it is ineffective in a vapor-phase environment.
There is a growing trend to supplement cathodic protection with Volatile Corrosion Inhibitors (VCI) beneath tank floors to specifically address vapor phase corrosion and enhance overall protection of tank bottom plates against soil-side corrosion. The objective of this experimental work is to expand on the study done by Pynn & Abed1 and investigate mutual compatibility and interactions of three different volatile corrosion inhibitors and cathodic protection when applied jointly on an oxygen concentration corrosion macro-cell setup.
The test results varied significantly between the three volatile corrosion inhibitors. One showed it had cathodic polarization effect and resulted in reduction of CP current requirement by 48%. Another had an anodic polarization effect and resulted in reduction of CP current requirement by 2%. Third had no polarization effect, and resulted in an increase of CP current requirement by 10%.
Effective control of soil-side corrosion on aboveground storage tank bottoms during the complete life cycle of the tank is critical both operationally and environmentally. Different foundation construction methods and corrosion protection techniques have been implemented over the last several decades in attempts to mitigate and control soil-side corrosion, including the use of asphalt pad, bituminous sand, cathodic protection and coating. Despite these measures, field experience and inspection activities indicate that soil-side tank floor corrosion persists in some cases.2-3
Underside surfaces of tank bottom plates are typically exposed to a combination of electrolytic and vapor-phase corrosive environments. While cathodic protection can be an effective corrosion mitigation technique where there is an electrolytic contact between the tank bottom surface and the underlying soil, it is ineffective in a vapor-phase environment. Air gaps or where there is intermittent moisture in the soil contacting the tank bottom surface are typical examples of vapor-phase environments under a tank floor. Published technical articles have discussed the practical limitations of the different protection methods, including cathodic protection systems, that are ineffective in providing protection in air gap areas or where cathodic current is shielded.2,4