ABSTRACT

Corrosion control of above ground storage tank (AST) tank bottoms remains critical over the life of a tank. A primary form of corrosion control has been the application of cathodic protection (CP). Over time, these systems can fail or reach the end of their design lives. In recent years, vapor corrosion inhibitors (VCI) have been recommended to ensure continued protection of tank bottoms. VCIs have been added as an aqueous slurry mixture under tank bottoms as part of a retrofit of corrosion control systems.

Regulations require monitoring of the efficacy of the corrosion control method. For VCI applications under ASTs, the most common monitoring form has been the use of electrical resistance (ER) probes. ER probes indicate local corrosion action via self-corrosion that decreases the cross-sectional area of exposed metal. In AST applications, concerns have been raised as to the long-term accuracy and relevance of ER probes.

The present research indicates that VCIs can indeed control corrosion on steel materials in contact with AST tank bottom backfill materials; however, the efficiency of the VCIs is affected by the presence of salt contamination or impurities (e.g., clay) in the backfill. The research also showed that corrosion of small areas of steel, be it an ER probe or a steel coupon, buried in the sand backfill are not indicative of the range of corrosion that may occur on an AST tank bottom. The research suggests a path forward.

BACKGROUND

Cathodic protection (CP) is the most common soil-side corrosion control method for aboveground storage tank (AST) steel bottoms. The present industry standard for corrosion control on AST tank bottoms is a CP system installed in the tank-floor base in conjunction with sub-base plastic liners to provide for long-term hydraulic integrity. This system has functioned well when maintained. However, the systems have a finite life and retrofitting new CP systems can be costly. Over the past several years, corrosion control via installation of vapor corrosion inhibitors (VCI) in the sand-base has been suggested as an alternative to CP or as a replacement for non-functioning CP systems. 1,2,3,4,5,6

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