Materials Performance

SEP 2018

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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C L A S S I C T he corrosion process as it relates to buried, partially buried, and submerged metallic structures is a naturally occurring phenomenon. The principles of this process and the benefits of cathodic protection (CP) in controlling this type of corrosion have been demonstrated in many different situations. Concern has increased, at an alarming rate, over failures of ground storage tank bottoms resulting from internal and external corrosion. This paper will discuss various options for CP of the exter- nal (groundside) surfaces of tank bottoms that are in contact with corrosive environments. The advantages and disadvantages of these options will be discussed, along with limitations that exist in determining the effectiveness of CP through traditional measure- ment tech niques. Suggestions for an alternative design approach intended to improve protective current distribution will also be discussed. Introduction THERE HAS BEEN A CONSIDERABLE AMOUNT OF CONCERN over the integrity of underground storage tanks as well as that of storage tanks resting on the ground because of the corro- sive reaction between the metal and the electrolyte. Cathodic protection (CP) is a proven method of controlling the corrosion of buried, partially buried, and submerged metallic structures. Cathodic protection of tank bottoms The application of CP for tank bottoms has been accom- plished in a variety of approaches with varying degrees of suc- cess. The use of sacrificial anodes is typically limited to those applications in which the tanks are of small-diameter construc- tion and they are effectively isolated from other underground metallic structures such that the current requirements to achieve protection are minimal. Impressed current system designs have been used where current requirements are more substantial. Several different types of anode installations intended to distribute the protective current to the tank are This article is published as originally appeared in MP, Vol. 27, No. 4 (1988). Cathodic protection of external tank bottoms ✩ Kevin C. Garrity* and Michael Urbas** T A N K B O T T O M P R O T E C T I O N possible. Some of the system configurations used are as follows: 1. Horizontal or vertical anodes installed around the periph- ery of the storage tank (Figure 1), 2. Horizontal or vertical anodes installed under the tank bottom (Figure 2), and 3. Anodes installed in deep well configurations (Figure 3). Each of these systems has advantages and disadvantages, and the selection of each system is dependent on design and economic factors as well as operational requirements. Paramount with the selection of system type is the method of verification that corrosion control has indeed been achieved. Past experience indicates that standard monitoring proce- dures intended to determine satisfaction of the industry-recog- ✩ Presented during CORROSION/87, Paper No. 320, NACE, San Francisco, CA, 1987. * Harco Corporation, 1055 West Smith Rd., Medina, OH 44256. ** Harco Corporation, 7706-A North W. Third St., Oklahoma City, OK 73127. FIGURE 1 — Commonly installed vertical Impressed current anodes. SEPTEMBER 2018 W W W.MATERIALSPERFORMANCE.COM A40

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