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%.
Key words: tank bottom, AST, volatile corrosion inhibitor, cathodic protection, polarization, absorption, macro-cell, vapor phase, corrosion current, air gaps, chime area, soil-side corrosion