ABSTRACT

In this paper a general description of aboveground storage tank (AST) foundations and corrosion mitigation technques to provide long term service is presented. Case studies involving earth foundation, soil corrosion, and double bottom tank are provided. The case studies apply standard electrochemical and failure analysis techniques to determine the primary causes and modes of failures. Soil chemistry, Microbiologically Induced Corrosion (MIC), pH and presence of chlorides in the soil will provide evidence for accelerated corrosion if there is deficiency in cathodic protection. Soil chemistry can be used to predict the pentration due to corrosion attack. If air traps or shielding is present, localized corrosion attack will take place in corrosive soil. Concrete foundations and corrosion inhibitors may be considered in corrosive conditions.

INTRODUCTION

Unless protective measures are taken, ungrade steel storage tanks, piping, and other metallic components of fuel storage systems corrode and leak product into the environment. Corrosion can attack the metal either over the entire surface of the metal (general corrosion) or in a small, localized area, creating a hole. Localized corrosion can perforate an unprotected tank in little as a few years and is the most common form of corrosion.

Tank bottom corrosion from the soil could be prevented by using a concrete foundation but corrosion could still occur due to moisture accumulation between the tank bottom and the concrete pad due to condensation, blowing rain or snow, or flooding due to inadequate drainage and moisture entrapment. Proper measures should be taken for concrete foundation construction to eliminate the ingress of water and other corrosive contaminants between the tank bottom and the concrete pad.

A typical system for Monitoring and Mitigation of Corrosion in the Interstitial Space includes a) sealing any gaps between the tank floor and dead shell on double-bottom tanks, or gaps between the tank floor and concrete ring wall on single bottom tanks to prevent intrusion of fresh water and air into the interstitial spaces of these tank systems, b) engineered application of the Vapor Phase Corrosion Inhibitors (VCI) into the interstitial space in such a way that effective distribution of the chemistry is ensured and c) a corrosion rate monitoring system utilizing electrical resistance probe technology to measure the real-time rate of corrosion shoud be placed within the interstitial space and near the tank floor.

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