ABSTRACT
A close coupled mixed metal oxide (MMO) anode, grid or concentric ring, cathodic protection (CP) system is a state-of-the-art technique for corrosion prevention of the soil-side bottom of an aboveground storage tank (AST) resting on sand pads. The regulatory requirement of secondary containment for ASTs, especially with the use of polyethylene liners, has eliminated the use of traditional shallow anodes around the periphery or the deep well anode systems. Recent experiences with close coupled CP systems have shown that adequate corrosion protection is not achieved for ASTs and are subject to leaks, before their scheduled out-of-service inspection interval. Literature information on failed tank bottoms with CP systems have identified several issues such as improper current distribution due to anode spacing, shorting of the anode to tank bottom and acid generation at anode resulting in low pH. One of the issues not adequately discussed with close coupled MMO anodes include oxygen generation at the anode surface due to electrolysis of water at polarized potentials above a threshold. For close coupled CP systems such as ASTs, the oxygen evolution from the MMO anodes can depolarize the cathode (tank bottom) polarized potentials. When the tank bottom plate loses contact with the sand pad due to flexing, oxygen can accumulate in the void space and accelerate corrosion. CP cannot provide protection to the areas of bottom plate that loses contact with the electrolyte. The floor scan data of the tank bottom plates indicate patches of corrosion even with good clean sand and an active CP system.
In this paper, experimental investigations are carried out to understand the evolution of oxygen and its concentration in the void spaces. The effect of oxygen concentration in void space versus corrosion growth rate with increased oxygen concentration is studied. This paper provides information on preventive measures that can be adopted to prevent oxygen concentration corrosion in the void space of AST soil-side bottom.
