# Cathodic Protection of Aboveground Petroleum Storage Tanks

API RECOMMENDED PRACTICE 651 THIRD EDITION, JANUARY 2007

# Cathodic Protection of Aboveground Petroleum Storage Tanks

Downstream Segment

API RECOMMENDED PRACTICE 651 THIRD EDITION, JANUARY 2007

### SPECIAL NOTES
API publications necessarily address problems of a general nature. With respect to particular circumstances, local, state, and federal laws and regulations should be reviewed. Neither API nor any of API's employees, subcon tractors, consultants, committees, or other assignees make any warranty or representation, ei ther express or implied, with respect to the accuracy, completeness, or usefulness of the information contained he rein, or assume any liability or responsibility for any use, or the results of such use, of any information or process disclosed in this publication. Neither API no r any of API's employees, subcontractors, consultants, or other assignees represent that use of this publication would not infringe upon privately owned rights. API publications may be used by anyone desiring to do so. Every effort has been made by the Institute to assure the accuracy and reliabilit y of the data contained in them; however, the Institute makes no representation, warranty, or guarantee in connection with this publication and hereby expressly disclaims any liability or responsibility for loss or damage resultingfrom its use or for the violation of any authorities having jurisdiction with which this publication may conflict.API publications are published to facilitate the broad availability of proven, sound engineering and operating practices. These publications are not intended to obviate the need for applying sound engineering judgment regarding when and where these publications should be utilized. The formulation and publication of API publications is not intended in any way to inhibit anyone from using any other practices. Any manufacturer marking equipment or materials in conformance with the marking requirements of an API standard is solely re sponsible for complying with all the applicable requirements of that standard. API does not re present, warrant, or guarantee that such products do in fact conform to the applicable API standard.

All rights reserved. No part of this work may be reproduced, stored in a retrieval system, or transmitted by any means, electronic, mechanica l, photocopying, recording, or otherwise, without prior written permission from the publisher. Contact the Publisher,

API Publishing Services, 1220 L St reet, N.W., Washington, D.C. 20005. Copyright © 2007 American Petroleum Institute

### FOREWORD
Nothing contained in any API publication is to be construed as granting any right, by impli

cation or otherwise, for the manufacture, sale, or use of any method, apparatus, or product covered by letters patent. Neither should anythi ng contained in the publication be construed as insuring anyone against liability for infringement of letters patent. This document was produced under API standard ization procedures that ensure appropriate notification and participation in the developmen tal process and is designated as an API stan

dard. Questions concerning the interpretation of the content of this publication or comments and questions concerning the procedures under which this publication was developed should be directed in writing to the Director of Standards, American Petroleum Institute, 1220 L Street, N.W., Washington, D.C. 20005. Requests for permission to reproduce or translate all or any part of the material published herein should also be addressed to the director. Generally, API standards are reviewed and revised, reaffirmed, or withdrawn at least every five years. A one-time extension of up to two ye ars may be added to this review cycle. Status of the publication can be ascertained from the API Standards Department, telephone (202) 682-8000. A catalog of API publications and materials is published annually and updated quarterly by API, 1220 L Street, N.W., Washington, D.C. 20005. Suggested revisions are invited and should be submitted to the Standards and Publications Department, API, 1220 L Street, NW, Washington, D.C. 20005, standards@api.org. Persons planning to construct an aboveground st orage facility, replace existing aboveground storage tanks and associ ated piping systems, or cathodically protect existing aboveground storage tanks and associated pipi ng should refer to applicable local, state, and federal fire, safety, and environmental regulations as well as the most recent edition of the following pub

lications: a. API Std 650; b. API RP 652; c. API Std 653; d. API Spec 12B; e. API Spec 12D; f. API Spec 12F; g. API Std 2610; h. NACE RP0193; i. NACE RP0285; j. NFPA 30; k. NFPA 70; and, l. PEI RP200. The appropriate government authority having jurisdiction should be consulted for regula

tions that apply to the area of installation prior to taking any action suggested in this recom

mended practice.

iii

### CONTENTS

Page 1 SCOPE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1
2 REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 2.1 Standards, Codes, Publications, and Specifications . . . . . . . . . . . . . . . . . . . . . . . .1 2.2 Other References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2
3 DEFINITIONS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
4 CORROSION OF ABOVEGROUN D STEEL STORAGE TANKS . . . . . . . . . . . . . . .5 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5 4.2 Corrosion Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6 4.3 Internal Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7
5 DETERMINATION OF NEED FOR CATHODIC PR OTECTION . . . . . . . . . . . . . . .8 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8 5.2 Tank History. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9 5.3 Tank Pad and Soil Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10 5.4 Other Factors Affecting Cathodic Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
6 METHODS OF CATHODIC PROTECTION FOR CORROSION CONTROL . . . . .14 6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14 6.2 Galvanic Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14 6.3 Impressed Current Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15
7 DESIGN OF CATHODIC PROTECTION SYSTEMS. . . . . . . . . . . . . . . . . . . . . . . . .16 7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16 7.2 Influence of Replacement Bottoms, External Liners (Release Prevention Barriers), and Secondary Containment on Cathodic Protection System Design .17 7.3 External Cathodic Protection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19 7.4 Internal Cathodic Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23
8 CRITERIA FOR CATHODIC PROTECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23 8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23 8.2 Protection Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23 8.3 Measurement Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .24 8.4 Alternative Reference Electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .24
9 INSTALLATION OF CATHODIC PROTECTION SY STEMS . . . . . . . . . . . . . . . . .25 9.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25 9.2 Galvanic Anode Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25 9.3 Impressed Current Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .26 9.4 Corrosion Control Test Stations, Undertank Monitoring Methods, and Bonds . .29
10 INTERFERENCE CURRENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .30 10.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .30 10.2 Sources of Interference Currents. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31 10.3 Detection of Interference Currents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31 10.4 Control of Interference Currents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31

vPage

vi 11 OPERATION AND MAINTENANCE OF CATHODIC PROTECTION SYSTEMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 11.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31 11.2 Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .32 11.3 Cathodic Protection Surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .32 11.4 Cathodic Protection Reco rds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .33 Figures 1 Electrochemical Corrosion Cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5 2 Oxygen Concentration Cell Caused by Rocks or Clay in Tank Pad . . . . . . . . . . . . .6 3 Example of Stray Current Corrosion of an U nprotected Tank Bottom . . . . . . . . . . .7 4 Galvanic Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7 5 Cathodic Protection with Galvanic Anodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 6 Impressed Current Cathodic Protectio n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 7 Impervious External Line r Beneath Storage Tank . . . . . . . . . . . . . . . . . . . . . . . . . 17 8 New Steel Bottom on Top of Old Bottom . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 9 Current Requirement Test Se tup. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 10 Potential Measurement Schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 11 Typical Galvanic Anode Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 12 Typical Shallow Anode Bed Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 13 Commonly Installed Deep Anode Bed. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 14 Permanently Installed Reference Electrode and Test Station. . . . . . . . . . . . . . . . . 29 15 Perforated Pipe Installed for Reference Electrode . . . . . . . . . . . . . . . . . . . . . . . . . 30
Tables 1 General Classification of Resistiv ity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 2 Partial Galvanic Series . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 3 Commonly Used Reference Electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251 

## Cathodic Protection of Aboveground Petr oleum Storage Tanks

1 Scope 1.1 The purpose of this recommended practice is to present procedures and practices for achieving effect ive corrosion control on aboveground storage tank bottoms through the use of cathodic protection. It contains provisions for the application of catho dic protection to existing and new storage tanks. Corrosion control meth ods based on chemical control of the environment or the use of protective coatings ar e not covered in detail. 1.2 When cathodic protection is used for aboveground storage tank a pplications, it is the intent of this recommended practice to provide information and guidance specific to aboveground steel storage tanks in hydrocarbon service. Certain practices recom

mended herein may also be applicable to ta nks in other services. It is intended to serve only as a guide to persons interested in cathodic protection. Specific cathodic protec tion designs are not provided. Such desi gns should be developed by a person thor

oughly familiar with cathodic protection practices. 1.3 This recommended practice does not designat e specific practices for every situation because the varied conditions in which tank bottoms are installed preclude standard ization of cathodic protection practices. 

### 2 References 2.1 STANDARDS, CODES, PUBLICATIONS, AND SPECIFICATIONS Unless otherwise specified, the most recent editions or revisions of the following standards, codes, and specifications shall, to the extent specified herein, form a part of this standard. API Spec 12B Bolted Tanks for Storage of Production Liquids Spec 12D Field Welded Tanks for Storage of Production Liquids Spec 12F Shop Welded Tanks for Storage of Production Liquids RP 500 Recommended Practice for Classificati on of Locations for Electrical Installations at Petroleum Facilities RP 575 Inspection of Atmospheric and Low Pressure Storage Tanks Std 620 Design and Construction of Large, Welded, Low-pressure Storage Tanks Std 650 Welded Steel Tanks for Oil Storage RP 652 Lining of Aboveground Petroleum Storage Tank Bottoms Std 653 Tank Inspection, Repair, Alteration, and Reconstruction RP 1615 Installation of Underground Petroleum Product Storage Systems RP 1621 Bulk Liquid Stock Control at Retail Outlets RP 1632 Cathodic Protection of Underground Petroleum Storage Tanks and Piping Systems RP 2003 Protection Against Ignitions Arising Out of Static, Lightning, and Stray Currents ASTM 1 C 144 Standard Specification for Aggregate for Masonry Mortar C 778 Standard Specification for Standard Sand D 512 Standard Test Methods for Chloride Ion in Water D 516 Standard Test Method for Sulfate Ion in Water D 1557 Standard Test Methods for Laboratory Compaction Charac teristics of Soil Using Modified Effort (56,000 ft-lbf/ ft

3 (2,700 kN-m/m 3 )) G 51 Standard Test Method for Measuring pH of Soil for Use in Corrosion Testing G 57 Method for Field Measurement of Soil Resist ivity Using the Wenner Four Electrode Method EPA 2 0376.1 Test Method for Sulfide—Titrimetric Iodine

1 ASTM International, 100 Bar Harbor Drive, West Conshohocken, Pennsylvania 19428, www.astm.org

. 2 U.S. Environmental Protection Agency, Ariel Rios Building, 1200 Pennsylvania Avenue, N.W., Washington, D.C. 20460, www.epa.gov
.2 API R ECOMMENDED P RACTICE 651

NACE 3

Peabody’s Control of Pipeline Corrosion , ISBN 1-57590-114-5 TM0497 Measurement Techniques Related to Criteria for Cathod ic Protection on Underground or Submerged Metallic Piping Systems RP0177 Mitigation of Alternating Current and Lightning Effects on Metallic Structures and Corrosion Control Systems RP0193 External Cathodic Protection of On-Grade Metallic Storage Tank Bottoms RP0285 Corrosion Control of Underground Storage Tank Systems by Cathodic Protection RP0388 Impressed Current Cathodic Protection of Internal Submerged Surfaces of Steel Water Storage Tanks RP0572 Design, Installation, Operation, and Maintenance of Impressed Current Deep Groundbeds RP0575 Internal Cathodic Protection Systems in Oil Treating Vessels TPC 11 A Guide to the Organization of Underground Corrosion Control Coordinating Committees NFPA 4 30 Flammable and Combustible Liquids Code 70 National Electrical Code PEI 5

RP100 Recommended Practices for the Installation of Underground Liquid Storage Systems
2.2 OTHER REFERENCES Although not cited in the text of this recommended practice, th e following publications provide additional information pertaini ng to cathodic protection of aboveground storage tanks in petroleum service. API Std 2610 Design, Construction, Operation, Maintenance & Inspection of Terminal and Tank Facilities UL 6 142 Steel Aboveground Tanks for Flammable and Combustible Liquids 

### 3 Definitions Definitions in this section reflect the common usage among prac ticing corrosion control personnel. In many cases, in the intere sts of brevity and practicality, the strict scientific definitions have been a bbreviated or paraphrased. 3.1 aboveground storage tank: An on-grade, stationary, uniformly supported container, usually cylindrical in shape, con

sisting of a metallic roof, shell, bottom, and support structure where more than 90% of the tank volume is above surface grade. 3.2 anode: The electrode of an electrochemical cell at which oxidation (corrosion) occurs. Antonym: cathode

. 3.3 anode bed: Consists of one or more anodes installed below th e earth’s surface for the purp ose of supplying cathodic protection. 3.4 backfill: Commercial, uniformly conductive material placed in a hole to fill the space around anodes, vent pipe, and buried components of a cathodic protection system. Anodes can be pr epackaged with backfill mate rial for ease of installation. 3.5 breakout piping/tanks: Piping/tanks used to relieve surges in a hazardous liquid pipeline system or piping/tanks used to receive and store hazardous liquid transp orted by a pipeline for reinjection an d continued tr ansportation by pipeline. 3.6 cathode: The electrode of an electrochemical cell at which a reduction reaction occurs. Antonym: anode

. 3.7 cathodic protection: A technique to reduce corro sion of a metal surface by making th e entire surface the cathode of an electrochemical cell.

3 NACE International (formerly the National Asso ciation of Corrosion Engineers), 1440 South Creek Drive, P.O. Box 218340, Houston , Texas, 77218-8340, www.nace.org
. 4 National Fire Protection Association, 1 Batterymarch Park, PO Box 9101, Quincy, Massachusetts 02269-9101, www.nfpa.org

. 5 Petroleum Equipment Institute, P.O. Box 2380, Tulsa, Oklahoma 74101-2380. www.pei.org. 6 Underwriters Laboratories , 333 Pfingsten Road, Nort h brook, Illinois 60062, www.ul.com

.C ATHODIC P ROTECTION OF A BOVEGROUND P ETROLEUM S TORAGE T ANKS 3

3.8 chime (or chine): The portion of the tank bottom steel floor plate that extends horizontally past the outside vertical sur

face of the shell (i.e., the external lip formed at the base of the ta nk where the bottom steel floor plate protrudes and is welded to the bottom of the shell, around the entire tank peri meter). Also referred to as bottom extension. 3.9 coke breeze: A commercial carbonaceou s backfill material. 3.10 concentration corrosion cell: A form of localized corrosion initiated by the difference in metal ion or oxygen con

centration due to crev ices or deposits. (NACE definition: An electrochemi cal cell, the electromotive force of which is caused by a difference in concentration of some component in the electrolyte. [This diff erence leads to the format ion of discrete cathodic and anodic regions.]) 3.11 continuity bond: A metallic connection that pr ovides electrical continuity. 3.12 corrosion: The deterioration of a material, usually a metal, th at results from a reaction with its environment. 3.13 current density: The current per unit area flowing to or from a metallic surface. 3.14 current requirement test: A test where the use of direct current flow from a temporary anode bed to the structure to be protected is used to determine the amount of current necessary to protect the structure. 3.15 deep anode bed: One or more anodes installed vertically at a nominal depth of 15 m (50 ft) or more below the earth’s surface in a single drilled hole for the purpose of supplying cathodic protection. 3.16 differential aeration cell: An electrochemical cell the electromotive force of which is due to a difference in air (oxy

gen) concentration at one electrode as compared with that at another electrode of the same material. 3.17 electrical isolation: The condition of being electrically sepa rated from other metallic structures. 3.18 electrochemical cell: An electrochemical system consisting of an anode and a cathode immersed in an electrolyte so as to create an electrical circuit. The anode and cathode may be se parate metals or dissimilar areas on the same metal. The cell includes the external circuit, which permits the fl ow of electrons from the anode toward the cathode. 3.19 electrode potential: The potential of an electrode as measured agains t a standard reference electrode. The electrode potential does not include any voltage measur ement errors due to the pass age of current through the re sistance of the metallic or electrolytic paths. 3.20 electrolyte: A chemical substance containing ions that migrate in an electric field. For the purposes of this recommended practice, electrolyte refers to the soil or water adjacent to and in contact with the bottom of an aboveground petroleum storag e tank, including the contaminants and chemicals contained therein. 3.21 environmental cracking: The brittle fracture of a normally ductile materi al in which the corrosive effect of the envi

ronment is a causative factor. 3.22 external circuit: Consists of the wires, metallic connectors, measurin g devices, current sources, etc., that are used to bring about or measure the desire d electrical conditions within an electrochemical cell. It is this portion of the cell through which electrons travel. 3.23 external liner: A system or device, such as a non-conductive memb rane, installed beneath a storage tank, in or on the tank dike, to contain any accidentally escaped product. 3.24 foreign structure: Any metallic structure that is not intended as a part of a system under cathodic protection. 3.25 galvanic anode: A metal that, because of its relative position in th e galvanic series, provides sacrificial protection to another metal that is more noble, when electrically coupled in an electrolyte. These anodes are the source of current in galvan ic or sacrificial cathodic protection. 3.26 galvanic cathodic protection: The reduction or prevention of corrosion of a metal in an electrolyte by electrically connecting it to a more anodic metal. 3.27 galvanic series: A list of metals and alloys arranged according to their relative potentials in a given environment. 3.28 holiday: A discontinuity in a protective coating that exposes the underlying substrate to the environment.4 API R ECOMMENDED P RACTICE 651

3.29 impressed current: An electric current supplied by a device employin g a power source that is external to the electrode system. (An example is the direct current output of a cathodic protection rectifier.) 3.30 interference bond: A metallic connection designed to control stray elec trical current discharge from a metallic structure. 3.31 IR drop: The voltage generated across a resistance by an elec trical current in accord ance with Ohm’s Law: E = I × R . For the purpose of this recommended practice, the most significant IR drop is the portion of a structure-to-soil potential caused b y current flow through a re sistive electrolyte from th e anode to the structure. 3.32 isolation: See electrical isolation

. 3.33 membrane: A thin, continuous sheet of nonconductive synthetic ma terial used to contain and/ or separate two different environments. 3.34 oxidation: The loss of electrons by a constituent of a chemical reaction. 3.35 polarization: The change from the open-circuit po tential of an electrode as a result of current across the electrode/elec

trolyte interface. 3.36 rectifier: A device for converting alternating current to direct cu rrent. Usually includes a step-down AC transformer, a silicon or selenium stack (rect ifying elements), meters, and other accessories when used for cathodic protection purposes. 3.37 reduction: The gain of electrons by a constituent of a chemical reaction. 3.38 reference electrode: An electrode whose open-circuit potential is co nstant under similar conditions of measurement. 3.39 release prevention barrier (RPB): Includes steel bottoms (when used in a double bottom or secondary containment system), synthetic materials, clay liners, and all other barriers or combination of barriers placed in the bottom of, or under an aboveground storage tank, which have the following functions: (a) preventing the escape of stored product, and (b) containing o r channeling released mate rial for leak detection. 3.40 resistor: A device used within an electrical circuit to control current flow. 3.41 secondary containment: A device or system used to co ntrol the accidental escape of a stored product so it may be properly recovered or removed from the environment. 3.42 shallow anode bed: A group of cathodic protection anodes installed individually, spaced uniformly, and typically bur

ied less than 20 ft (6 m) below grade. 3.43 shunt: A conductor of a known electrical resi stance through which current flow may be determined by measurement of the voltage across the conductor and calculation using Ohm’s Law. 3.44 stray current: Current flowing through paths other than the intended circuit. 3.45 stray current corrosion: Corrosion resulting from direct current flow through paths other than the intended circuit. 3.46 stress corrosion cracking: The fracture of a metal by the combined action of corrosion and tensile stress. The fracture occurs under a tensile stress that may be well below the tensile strength or even the yield strength of the material. 3.47 structure-to-electrolyte voltage (also structure-to-soil potential or pipe-to-soil potential): The voltage dif

ference between a metallic structure and the electrolyte which is measured with a refe rence electrode in contact with the elect rolyte. 3.48 structure-to-structure voltage (also structure-to structure potential difference): The difference in voltage between metallic structures in a common electrolyte. 3.49 tank pad: The material immediately adjacent to the exterior steel bottom of an aboveground storage tank. 3.50 test lead: An electrically conductive wire or cable attached to a structure and terminat ed in a test station. It is used for the measurement of structure- to-electrolyte potentials and other measurements. 3.51 test station: A small enclosure or housing that is the termination point of one or more test leads. 3.52 voltage: An electromotive force, or a differ ence in electrode potentials expressed in volts. Also known as a potential. 3.53 water bottom: A water layer in the bottom of a ta nk caused by separation of water and pr oduct due to differences in sol

ubility and specific gravity.C ATHODIC P ROTECTION OF A BOVEGROUND P ETROLEUM S TORAGE T ANKS 5


### 4 Corrosion of Aboveground Steel Storage Tanks 4.1 INTRODUCTION 4.1.1 Corrosion may be defined as the deteriora tion of a metal resulting from a reaction with its environment. Corrosion of steel structures is an electrochemical process. For the corrosion proce ss to occur, areas with differen t electrical potentials must e xist on the metal surface. These areas must be electrically connected and in contact with an electrolyte. There are four components required for a co rrosion cell: an anode, a catho de, a metallic path connec ting the anode and cathode, and an electrolyte (see Figure 1). The role of eac h component in th e corrosion cell is as follows: a. At the anode, the base metal goes into solution (corrodes) by releasing electrons and forming positive metal ions. For steel, the anodic reaction is:

b. At the cathode, chemical reactions take place using electrons released at the anode. No corro sion takes place at the cathode. One common cathodic reaction is:

c. The metallic path provides a way for electrons released at the anode to flow to the cathode. d. The electrolyte cont ains ions and conducts current from the anode to the cathode by ionic movement. The electrolyte contains both negatively charged ions called anions and positively charged ions called cations that are attracted to the anode and catho de, respectively. Moist soil is the most common electrolyte for extern al surfaces of the tank bottom, while water and sludge genera lly are the electrolytes for the internal surfaces. 4.1.2 There are many forms of corrosion. The two most common types relative to tank bottoms are general and localized (pit

ting) corrosion. In general corrosion, thous ands of microscopic corrosion cells occur on an area of the metal surface resulting in relatively uniform metal loss. In localized (pitting) corrosion, the individual corrosion cells are larger and distinct anodic and cathodic areas can be identified. Metal loss in this case may be concentrated within relatively small areas with substantial ar eas of the surface unaffect ed by corrosion.

4.1.3 The composition of the metal is a factor in determining which areas on a metal surface become anodes or cathodes. Differ

ences in electrochemical potential between adjacent areas can resu lt from uneven distribution of a lloying elements or contamina nts within the metal structure. Corrosion can also be caused by diff erences between weld metal, heat affected zone, and parent meta l. 4.1.4 Physical and chemical properties of the electrolyte also influence the location of cathodic and anodic areas on the metal surface. For example, differing oxygen con centrations between areas on a steel surface may generate differences in potential. Areas with lower oxygen concentrations beco me anodic and areas with higher oxygen c oncentrations become cathodic. This phe

nomenon can cause corrosion of steel tank bottoms on a sand pad c ontaminated with clay or other debris or natural soil that doe s not have a uniform consistency (see Figure 2). 4.1.5 Soil characteristics substantially affect the type and rate of corrosion on a structure in contact with soil. For exam

ple, dissolved salts influence the current carrying capacity of the soil electrolyte and help determine reaction rates at the anodic and cathodic areas. Moisture conten t, pH, oxygen concentration, and other fact ors interact in a complex fashion to influence corrosion. Figure 1—Electrochemical Corrosion Cell Fe Fe +2 2e – + →

O 2 2H 2 O 4 e– + + 4OH – →6 API R ECOMMENDED P RACTICE 651

4.2 CORROSION MECHANISMS 4.2.1 Stray Current Corrosion Stray currents (also known as interference currents) travel through the soil electrolyte and on to structures for which they ar e not intended. Usually, the affected structure collects the interference currents from the el ectrolyte; the source of these currents is not electrically connected to the affected structure. As shown in Figure 3, stray current may ente r an unprotected tank bottom and travel through the low resistance path of the metal to an area on the tank closer to the pr otected structure (pipeline). At this locat ion, the current discharges back into the electrolyt e (soil) at point B with resultant metal loss. The most common, and potentially the most damaging, stray currents are direct curren ts. These currents are genera ted from grounded DC electric power systems including el ec tric railroads, subways, welding machines , impressed current cathodic protection sy stems, and thermoelectric generators. The severity of corrosion (metal loss) resulting from interference cu rrents depends on several factors: a. separation and routing of the interferi ng and affected structures and location of the interfering current source; b. magnitude and density of the current; c. quality or absence of a coating on the affected structures; d. the presence and location of mechanical joints having high electrical resistance; e. temperature. Further information related to proper design to avoid stray cu rrents, and detection and control of stray current corrosion, can be found in Sections 7 and 10.
4.2.2 Galvanic Corrosion Galvanic corrosion occurs when two metals with different compos itions (thus, different electroly tic potentials) are connected i n an electrolyte (usually soil). Current will flow from the more active metal (an ode) to the less active metal (cathode) with acc eler

ated attack at the anodic metal. For exam ple, galvanic corrosion can occur when a bron ze check valve is joined to carbon steel piping or where stainless steel, copper pipe or a copper ground rod is connected to a carbon steel tank. In the pipe/steel tank exam ple, the stainless steel or copper pipe be comes the cathode and the steel tank is the anode. Since current takes the path of le ast resistance, the most severe corrosion attack will occur in the area on the steel tank immediately adjacent to the stainless ste el or copper pipe as shown in Figure 4. The extent of such a prob lem is dependent on several factor s. The most significant factors ar e: a. the relative surface areas of the cathode and anode, b. the relative potential difference between the two materials as determined by their position in the galvanic series, and c. temperature. Corrosion tends to be more severe when th e anodic area is small with respect to the cathode surface and the two metals are far apart in the galvanic series. Figure 2—Oxygen Concentration Cell Caused by Rocks or Clay in Tank PadC ATHODIC P ROTECTION OF A BOVEGROUND P ETROLEUM S TORAGE T ANKS 7

4.3 INTERNAL CORROSION Experience has revealed that corro sion may occur on the inside surface of a tank bottom. The extent or nature of corrosion depends on many factors associated with the composition of the fluid in contact with the steel bottom. Major factors which influence the severity of corrosion include: a. conductivity (a function of dissolved solids); b. suspended solids; c. pH level; d. dissolved gases such as CO 2 , H 2 S, or O 2 ; e. sulfate reducing bacteria; f. temperature. Figure 3—Example of Stray Current Corrosion of an Unprotected Tank Bottom

Figure 4—Galvanic Corrosion Stainless steel or copper pipe

Copper ground rod Tank bottom

Current Corrosion occurs on tank bottom8 API R ECOMMENDED P RACTICE 651
Three major types of corrosion to be considered are general corr osion, localized (pitting) corrosion, and to a lesser extent in tanks, environmental cracking. For furthe r discussion of inte rnal corrosion mechanisms, see API RP 652. 

### 5 Determination of Need for Cathodic Protection 5.1 INTRODUCTION The need for cathodic protection shall be determined for all aboveground storage tanks. This section discusses parameters for consideration when determining whether a steel aboveground stor age tank bottom requires cathodic protection. If it is determine d that corrosion may occur, adequate corrosion control procedures should be adopted to ensure metal integrity for safe and econom

ical operation over the service life of the tank. The location of a facility or the presence of a leak detection system alone s hould not be used to determine the need for cathodic protection. The decisi ons governing the need for cathodic protection should be based on data from inspections performed in accordance with API Std 653 , corrosion surveys; operating re cords; prior test results wit h similar tank systems in similar environmen ts; national, state, and local code require ments; and the recommendations made within this document. Due to the wide variety of tank pad types and conditions, and their variability in preventing corrosion with or without cathodi c protection, the practices of this standard should be used in co njunction with the inspection and maintenance practices of API S td 653 to determine that external corrosion of tank bottoms is adequately controlled.
5.1.1 New Aboveground Storage Tanks Corrosion control by cathodic protection for new aboveground stor age tanks should be evaluated in the initial design, and if applied, should be maintained during the service life of the tank.
5.1.2 Existing Aboveground Storage Tanks Studies should be made within a suitable time frame in accorda nce with API Std 653 concerning the possible need for cathodic protection. When these studies in dicate that corrosion will affect the safe or ec onomic operation of the system, adequate corro sion control measures should be used.
5.1.3 Internal Cathodic Protection Pure hydrocarbon fluids are usually not corrosive and do not requ ire corrosion control for intern al surfaces. However, based up on experience, internal corrosion ma y occur in aboveground storage ta nks that have internal surfaces exposed to water, sediments, or other contaminants. Generally, co atings are used to reduce or el iminate corrosion on internal su rfaces. For tanks in petroleum ser vice, internal cathodic protection in conjunction with coatings has not gained wi despread use, but under certain conditions it can be effective in protecting against corrosion at holidays in the coating. For more detailed information on internal cathodic pro tec

tion, see NACE RP0575 and RP0388.
5.1.4 Limitations of External Cathodic Protection Cathodic protection is an effe ctive means of corrosion control on ly if it is possible to pass electrical current between the an ode and cathode (tank bottom). Many factors can either reduce or e liminate the flow of electrical current and, therefore, may limit the effectiveness of cathodic protection in some cases or preclude its use in others. Such factors include: a. tank pads such as concrete, asphalt, or oiled sand; b. an impervious external liner between the tank bottom and anodes; c. high resistance soil or rock aggregate pads; d. old storage tank bottoms left in place when a new bottom is installed. These and other related factors are discussed in more detail in 5.3 and 5.4. It should be recognized that external cathodic pro tec

tion has no effect on internal corrosion.C ATHODIC P ROTECTION OF A BOVEGROUND P ETROLEUM S TORAGE T ANKS 9

5.2 TANK HISTORY 5.2.1 General Before determining the need for cathodic protection, a full eval uation of tank history is advised. If this evaluation indicates that external corrosion is a known or potentia l concern, then cathodic protection or other corrosion control measures should be used . If internal corrosion is known to be a problem, use of a lining shoul d be considered (refer to API RP 652); in certain cases, inte rnal cathodic protection in conjunction with a lining may be appropriate. The following items should be investigated and determined.
5.2.2 Tank Design/Construction History The following items should be investig ated and determined in the evaluatio n of tank design/construction history: a. ringwall foundation and tank pad design; b. site plan, including tank farm layout; c. construction dates; d. soil properties and resistivity; e. water table; f. presence and type(s) of coatings or linings; g. previous repairs; h. change in soil conditions; i. impervious external liner; j. secondary bottom; k. existing cathodic protection on nearby structures; l. maintenance history; m. expected life; n. rectifier and anode bed location.
5.2.3 Type of Service The following items should be investigated and determined in the evaluation of types of service: a. type of product stored; b. product temperature; c. presence and depth of water bottoms; d. frequency of fill and discharge.
5.2.4 Inspection/Corrosion History The following items should be investigated and determined in the evaluation of inspection/corrosion history: a. tank inspection per API Std 653; b. corrosion rate records; c. corrosion problems on nearby tanks; d. corrosion problems on tanks of similar construction; e. stray current problems; f. design and performance of previous corrosion protection systems; g. structure-to-soil potential surveys.
5.2.5 Other Factors The following items should be investigated and determined in the evaluation of other factors: a. foreign buried metallic structures; b. foreign cathodic protection systems.10 API R ECOMMENDED P RACTICE 651

5.3 TANK PAD AND SOIL CONDITIONS 5.3.1 Introduction 5.3.1.1 Different types of pads are constructed for aboveground stor age tanks due to a wide variety of surface, subsurface, and climatic conditions. The pad material unde r the tank has a significant effect on ex ternal corrosion of th e tank bottom and can influence the effectiveness and applicability of external cathodic protection. It is very important to ensure that there is no debris such as, rocks, lumps of clay, welding elect rodes, paper, plastic, wood, etc. in the pad material. The pad material particles s hould be fine and uniform. Tanks should be built on an elevated berm to allow adequate drainage aw ay from the tank bottom. The use of fine particles will provide a denser pad to help reduce the influx and outflow of ox ygen from the perimeter of the tank as it is emptied and filled . If large particle sizes are used, differential aeration corrosion may result at points wh ere the large particles or debris contact the steel tank bottom. In this case, cathodic protection will not be effective in eliminating the pitting. There are a wide variety of pad materials available, some of which may actually prevent the beneficial effects of cathodic prot ec tion. Conversely, there are situations where some of these materials, when properly selected and installed, can be beneficial i n reducing corrosion to the extent that cathodic protection may not be needed, as described later in this section. 5.3.1.2 Soil resistivity provides valuable information about the corrosivity of the material used under and around a tank. A gen

eral resistivity classification is given in Table 1. There are several techniques fo r measuring soil resistivity. A common meth od is described in ASTM G 57.

5.3.1.3 The resistivity of the pad material may be higher than th e existing surrounding soil. Corrosive soil beneath the higher resistivity pad material may contaminate th e pad fill by capillary action. Thus, resistivity of surrounding soil may be used to deter

mine the probability of corrosion on the tank bottom. The results of soil resistivity surveys can be used to determine the need for cathodic protection. However, other properties of the soil (see 5.3.2.1g, h, i, and j) should also be considered. 5.3.1.4 In coastal areas, salt spray on tank surfaces will be washed do wn the sides of the tank by rain and may flow beneath the tank to contaminate the tank pad. This also can occur in areas where fertilizers or chemicals may be in the atmosphere either f rom spraying or industrial operations. The tank pad also can become contaminated by wicki ng action that can draw contaminates such as chlorides up from the water table. Cat hodic protection is usually necessary for corrosion prevention in these situations. 5.3.1.5 If a leak occurs in a tank bottom, the leaking material can also influence corrosion on th e external side. If water leaks from the tank, the environment under the tank may become more corrosive. If product leaks from the tank, it could create corro

sion cells that did not previously exist or adversely affect the effectiveness of catho dic protection. A leak may wash away par t of the pad material and eliminate the contact of the tank bottom w ith the ground in some areas. Cathodic protection will not be ef fec

tive in such areas. Additionally, the drainage properties of the pad material may be deteriorated by a leak and allow water and con taminates to remain in contact with the tank bottom.
5.3.2 Sand Pad Clean sand is the most common material used as a pad beneath st orage tank bottoms. The use of clean sand alone normally does not eliminate the need for cathodic protection since corrosion ma y occur due to intrusion of water from rain, snow, or a shallo w water table. Guidelines for the chemical an alysis of the sand for corrosive contaminants are discussed in 5.3.2.1g, h, i, and j

. These are suggested guidelines or considerat ions for the construction of aboveground storage tank bottom sand pad (non oiled) that were developed from practices represented by industry experience. Table 1—General Classifica tion of Resistivity

Resistivity Range (ohm-cm) Potential Corrosion Activity

<500 Very corrosive 500 – 1,000 Corrosive 1,000 – 2,000 Moderately corrosive 2,000 – 10,000 Mildly corrosive >10,000 Progressively less corrosiveC ATHODIC P ROTECTION OF A BOVEGROUND P ETROLEUM S TORAGE T ANKS 11

5.3.2.1 Sand Pad Material The following are issues to sand pad material. a. The sand should be clean, screened, and debris free (i.e., no wo od, sticks, vegetation, paper, rocks; clay, silt, or other so il; welding rods or other metallic objects; etc.). b. Cleaning should be done at the supply source and is accomplis hed by mechanical washing with water that will not alter the chemical composition or electrical resistance of the sand material. CAUTION: A municipal potable water supp ly may not be acceptable because of chlori nation which may prod uce high chloride levels. c. Masonry mortar quality sand is sometimes specified. d. Sand should conform to specifications such as one of the following: 1) ASTM C 778 type “20-30 sand” or equivalent, 2) ASTM C 778 “graded sand” or equivalent, or 3) ASTM C 144 or equivalent. e. Portland cement, at an approximate 33:1 sand to cement ratio, or lime, at an approximate 95:5 sand to lime ratio, is sometime s added at the supply source to elevate pH le vels and/or to facilitate good compaction. CAUTION: Do not use an excessive amount of portland cement or lime as the pad material should be somewhat yielding and not hard like concrete in order to allow the tank bottom to uniforml y bear on the sand pad material when loaded and make intimate contact with it. f. Electrical resistivity of the sand material is a commonly used method for determining its corrosivity because it is relativel y easy to measure. The resistivity of a soil depends on its chemical properties, moisture content, and temperature. Th e resistivity of the sand material may be determined in acco rdance with ASTM G 57, or equivalent. The results of the testing shall be forwarded to the cathodic protection designer. g. Measuring pH indicates the hydrogen ion content of a soil. Corrosion of steel is fairly independent of pH when it is in the range of 5.0 to 8.0. The rate of corrosion increases appr eciably when pH is < 5.0 and decreases when pH is > 8.0. pH may be determined in accordance with ASTM G 51 or equivalent. h. Chlorides will affect the resistivity of soil, and act as a depolarizing agent which will increase the current requirement fo r cathodic protection of steel. Pitting corrosion on steel can begin at chloride levels of 10 ppm. Chloride content may be deter

mined in accordance with ASTM D 512 or equivalent. There is currently no industry consensus on an acceptable range for chloride levels, therefore the tank owner/operator should speci fy the acceptable chloride level. There are practical and possib le economic limitations in achieving minimum levels of chloride content. i. Sulfate levels >200 ppm frequently indicate high concentrati ons of organic matter. Sulfate content may be determined in accordance with ASTM D 516 or equivalent. There is currently no industry consensus on an acceptable range for sulfate levels, therefore the tank owner/operator should specify the acceptable sulfat e level. There are practical and possible economic limita

tions in achieving minimum levels of sulfate content. j. Sulfide levels > 0.10 ppm, may indicate that sulfates have be en reduced by bacteria. Sulfide content may be determined in accordance with EPA 0376.1 or equivalent. There is currently no industry consensus on an accep table range for sulfide levels, therefore the tank owner/operator should sp ecify the acceptable sulfide level. There are practical and possible economic limita

tions in achieving minimum levels of sulfide content. k. Random testing of the sand material should be conducted at the supply source to determine if the electrical resistivity and chemical properties are at accep table levels. Sand samples used to determine th e properties of the material should be taken fro m the actual material that is to be used du ring construction. If test results do not meet owner/operator specified levels then ad ditional steps such as rewashing and/or adding portland cement or lime , or secure another source of sand, may need to be taken.
5.3.2.2 Tank Pad Construction The following are considerations when constructing tank pads. a. Care should be taken to use clean mixing, handling, and constr uction equipment to ensure that the sand pad material remains free from foreign matter and debris. b. Sand is often installed by loose lifts in maximum layering of 6 in. – 8 in. (15 cm – 20 cm) thickness. The total depth or thi ck

ness of the sand pad is determined by engineering design and ta nk loading requirements and may be from one to several layers thick dependent upon owner/operator specifications. c. Mechanical vibratory compaction and rolling is suggested to be done for each layer to achieve compaction to 95% of the max

imum dry density per ASTM D 1557 or equivalent.12 API R ECOMMENDED P RACTICE 651

CAUTION: Compaction by water flooding is not recommended because the water used to flood the sand pad may cause contamina

tion and deterioration of the original chemical properties of the sand material. Additionally, flooding can cause bulking of th e pad and subsequent adverse settlement when loaded, possibly causing damage to the tank bottom, attached piping or other appurtenances. d. The bottom side of each steel plate to be used for tank bot tom construction should be insp ected immediately before placement onto the pad to ensure that any contaminating debris that is ad hered to it (e.g., mud) is removed and that the plate surface is clean. e. Cathodic protection and leak detection materials/components ma y be placed in, or pass through, the sand pad only in accor

dance with owner/operator approved designs. Otherwise, the pad shall be completely free of any material other than the specified sand material.
5.3.3 Continuous Concrete Pad 5.3.3.1 A properly designed concrete tank pad constructed on stable , properly prepared subsoil may be effective in eliminating intrusion of groundwater, soil-side corrosion, and the need for cathodic protection. Preparation of a stable soil to support th e con crete slab is very important to ensure the continued integrity of the pad. Unstable soil may i nduce cracks in the slab through which water and contaminants can permeate to the steel tank bottom and provide a corrosive environment. 5.3.3.2 Although corrosion from the soil may be prevented by a conc rete pad, there may still be a collection of moisture between the tank bottom and the pad due to condensation, blowing rain or snow, or flooding due to inadequate drainage. Corro

sion may occur due to this moisture accumulation. Cathodic protec tion is generally not considered an effective way to combat th is corrosion. A free-draining concrete pad or ringwall and a seal around th e periphery of the tank may be effective in eliminating the accumulation of moisture between the pad and the tank bottom where flooding in the dike area above the tank bottom does not occur. In situations where water may condense on the tank bottom or water is retained above the concrete pad, accelerated corro

sion may occur. 5.3.3.3 Due to numerous complex factors that ca n affect the corrosion of a tank bottom underside in the presence of concrete, prediction of the propensity of corrosion in this case is extremely difficult. Thus, care should be observed with tanks on conc rete pads since cathodic protection most likely will not help reduce any corrosion that might occur.
5.3.4 Crushed Limestone or Clam Shell Pad In certain locations, the tank pad could consist of a layer of crushed limestone or clam shells. Such tank pads without the use of cathodic protection have produced mixed result s. The tank pad should be fine and unif orm, since differential aeration corrosion cells will cause pitting at contact areas be tween the large particles and the metal. The intrusion of water from rain or ground water makes the environment under the tank alkali ne, which may reduce corrosion. If contaminants are present in the pad, or with time infiltrate the pad, corrosion may accelerate. Thus, the use of cr ushed limestone or clam shells does not clearly eliminate the need for cathodic protection.
5.3.5 Oiled Sand Pad Historically, in some cases oil has been added to the sand fo r various reasons, including compaction and corrosion control. How

ever, if cathodic protection is applied, the higher resistivity of oiled sand may prevent it from being effective.
5.3.6 Continuous Asphalt Pad A pad of new asphalt may provide many of the same advantages and disadvantages as a concrete pad for reducing corrosion and eliminating the need for ca thodic protection. Proper support to prevent cracks and to prevent accumulation of water between the pad and the tank bottom is an important consideration. Asphalt de grades with time and can provide a path for water and dissolve d contaminants to come into contact with th e steel tank bottom, allowing corrosion to o ccur. Cathodic protection, if applied, may or may not aid in stopping corrosion when the asphalt becomes deteri orated. In fact, deteriorated asphalt may shield cathodic prot ec tion current in a manner similar to a disbonded coating on a pipe line. The condition of the external surface of the tank bottom as well as the asphalt can be determined if coupons are cut from the tank bottom.
5.3.7 Native Soil Pad 5.3.7.1 Soil analysis is often a useful test for helping to determine whether the potential corrosion activity will be high enough to make cathodic protection necessary and whether cathodic protection will be a practical application to prevent corrosion. Det er mination of aggressive ions such as chlori des and sulfates along with measurement of pH and resistivity are helpful for furtherC ATHODIC P ROTECTION OF A BOVEGROUND P ETROLEUM S TORAGE T ANKS 13
corrosion analysis. The variety of particle sizes and chemical di fferences as discussed in 5.3.1. 1 should also be considered in the effectiveness of a cathodic protection system. 5.3.7.2 Cathodic protection or other corrosion control measures should be considered when soil analysis data indicate that soil is corrosive.
5.4 OTHER FACTORS AFFECTIN G CATHODIC PROTECTION 5.4.1 Contents of Tank Storage tank temperature can influence corros ion on tank bottoms. Accelerated corrosion can occur on the exte rnal surface of th e bottom of heated tanks due to elevated temperatures if the area is wet. Note: The corrosion rate of steel may double with every 18°F (1 0°C) increase in temperat ure above 77°F (25°C). Storage tanks containing a hot product may require an increase in current density to achieve adeq uate protection on the externa l surface of the bottom. Conversely, sufficient heat could dry out a well drained tank pad, thus, increasing its resistivity and reduc

ing the need for cathodic protection. However, tank operators should be aware that if water penetrates the previously dried out tank pad (such as could be caused by above average rainfall), the resistivity of the tank pad can decrease, developing a more corrosive condition. For this situatio n, the installation of a cathodic protection system should be considered.
5.4.2 Bottom Replacement Replacement of tank bottoms is an accepted practice. Cathodic pr otection systems can be used to ensure long-term integrity of existing and replacement bottoms. The methods of installing repl acement bottoms, release preventi on barriers, and internal lin

ings shall be considered when determining the need for and the method of installation of a cathodic protection system. These fa c tors relate to both existing and new cathodic protection system s and have a significant impact on the feasibility and effective ness of cathodic protection. The effect of replacement bottoms and release prevention barriers on cathodic protection system design is discussed in 7.2.
5.4.3 Release Prevention Barriers 5.4.3.1 There are a variety of methods available for releas e prevention. These include, but are not limited to: a. use of impervious clay pad in tank dike; b. dual bottom tank design; c. impervious nonmetallic membrane. 5.4.3.2 The use of release prevention barriers (RPB ) will reduce the environm ental risk in the event of a leak. However, the use of certain RPBs may preclude the use of cathodic protection a nd in some cases may cause accelerated corrosion of the tank bot

tom. An example of a double bottom release prevention system is the installation of a new steel bo ttom over an existing steel b ot

tom which has been repaired. If water or other electrolyte intrude s into the interstitial space, a galvanic cell may be formed which will cause the new steel tank bottom to co rrode at an accelerated rate. In order to apply cathodic protection to a new tank bot tom, anodes shall be installed between the old and new bottom. 5.4.3.3 If a release prevention system utilizing a nonconductive, imperviou s external liner is in place or is installed under a new tank, the cathodic protection anodes shall be placed between the ex ternal liner and tank bottom in a conductive material such a s sand. Cathodic protection systems are rendered ineffective when an external liner is installed between the anodes and the tank bot

tom to be protected, because the external liner acts as a barr ier to the flow of electrical cu rrent necessary for protection. A nother consequence of release prevention involves the use of an impervious external lin er which may trap corrosive liquid, resulting i n more severe corrosion of the tank bottom. There are advantages and disadvantages to using release prevention barriers and these should be evaluated to determine the appropriate corrosion prot ection methodology. Refer to 7.2 for a further discussion of the effect of RPBs on cathodic protection design.
5.4.4 Thick-film Internal Linings The use of a thick-film laminate or other types of internal lini ng should not be considered as sufficient justification to elim inate the need for external cathodic protection of tank bottoms.14 API R ECOMMENDED P RACTICE 651


### 6 Methods of Cathodic Protect ion for Corrosion Control 6.1 INTRODUCTION Cathodic protection is a widely accepted method of corrosion control. Corrosion of aboveground steel storage tank bottoms may be reduced or eliminated with proper application of cathodic protection. Cathodic protection is a technique for preventing corr o sion by making the entire surface of the metal to be protected act as the cathode of an electrochemical ce ll. There are two sys tems of cathodic protection—galvanic and impressed current. 6.2 GALVANIC SYSTEMS 6.2.1 General Galvanic systems use a metal more active than the structure to be protected to supply the current required to mitigate corrosio n (see Table 2 for a partial galvanic series). The more active metal is called an anode, commonly referred to as a galvanic anode

. The anode is electrically connected to the st ructure to be protected and buried in the soil. A galvanic corrosion cell develops and the active metal anode corrodes (is sacrificed) while the metal structure (cathode) is protected (see Figure 5). Metals commonl y used as galvanic anodes in soil are magnes ium and zinc in either cast or ribbon fo rm. The anodes are usually distributed around the perimeter of the tank or buried beneath the tank bottom. Ga lvanic systems are normally applie d only to small diameter tanks (e.g., less than 60 ft [18 m]) or for tanks with external ly coated bottoms.
6.2.2 Advantages of Galvanic Systems There are several advantages of galvanic systems: a. no external power supply is required; b. installation is relatively easy; c. capital investment is low for small diameter tanks; d. maintenance costs are minimal; e. interference problems (stray currents) are rare; f. less frequent monitoring is required.
6.2.3 Disadvantages of Galvanic Systems There are several disadvanta ges of galvanic systems: a. driving potential is limited; b. current output is low; c. use is limited to low-resistivity soils; d. not practical for protection of large bare structures. Table 2—Partial Galvanic Series

Metal Potential V (CSE) Active or anodic Commercially pure magnesium –1.75 Magnesium alloy (6% Al, 3% Zn, 0. Zn, 0.15% Mn) –1.6 Zinc –1.1 Aluminum alloy (5% Zinc) –1.0 Commercially pure aluminum –0.8 Mild steel (clean and shiny) –0.5 to –0.8 Mild steel (rusted) –0.2 to –0.5 Cast iron –0.5 Lead –0.5 Mild steel in concrete –0.2 Copper, brass, bronze –0.2 High silicon cast iron –0.2 Mill scale on steel –0.2 Noble or cathodic Carbon, graphite coke +0.3C ATHODIC P ROTECTION OF A BOVEGROUND P ETROLEUM S TORAGE T ANKS 15

6.3 IMPRESSED CURRENT SYSTEMS 6.3.1 General The second method of applying cathodic protection to an aboveg round storage tank bottom is to use impressed current from an external source. Impressed current systems use direct current usua lly provided by a rectifier attached to an AC power source. T he rectifier converts alternating current to direct current. Direct current from the re ctifier flows to the buried impressed curre nt anode, from the anode through the soil electrolyte, and onto the tank bottom as shown in Figure 6. Figure 5—Cathodic Protection with Galvanic Anodes

Figure 6—Impressed Current Cathodic Protection Galvanic anode with backfill Tank bottom

Current

Tank bottom Rectifier

Current

Impressed current anode with backfill16 API R ECOMMENDED P RACTICE 651

6.3.2 Advantages of Impressed Current Systems The advantages of impresse d current systems include: a. availability of large driving potential; b. high current output capable of protecting large structures; c. capability of variab le current output; d. applicability to almost any soil resistivity.
6.3.3 Disadvantages of Impressed Current Systems The disadvantages of impressed current systems include: a. possible interference problems (stray currents) on foreign structures; b. loss of ac power causes loss of protection; c. higher maintenance and operating costs; d. higher capital cost for small installations; e. safety aspects of rectifier location; f. safety aspects of negative lead connection; g. more frequent monitoring.
6.3.4 Cathodic Protection Rectifiers A typical cathodic protection rectifier has two major components: (a) a step-down transf ormer to reduce the AC supply voltage, and (b) rectifying elements to convert AC input to DC output. Un its may be obtained with either selenium or silicon rectifier e le

ments. Silicon rectifiers are generally more efficient; however, they are more suscep tible to damage from power surges. There

fore, protective devices should be consider ed for these units to prevent lightning damage. Because of decreased life at higher temperatures, selenium rectifiers are not recommended if ambient temperatures are expected to exceed 130°F (55°C).
6.3.5 Impressed Current Anodes Impressed current anodes used in soil are made of materials such as graphite, steel, high silicon cast iron, or mixed metal oxi des on titanium. Anodes are usually buried in a coke breeze backfill to ex tend their life and reduce ci rcuit resistance. They may b e located in remote anode beds, distributed around the tank, in stalled underneath the tank, or installed in deep anode beds. 

### 7 Design of Cathodic Protection Systems 7.1 INTRODUCTION 7.1.1 Cathodic protection systems are designed and installed to prevent corrosion of a tank bottom by satisfying the require

ments of one or more of the criteria listed in Section 8. In order to achieve the desired results, a cathodic protection system shall be properly designed. The cathodic protection system should be designed after a study of the following items: a. design and engineering specifications and practices (see 5.2); b. operating procedures; c. safety, environmental, and hazardous area requirements; d. field testing. 7.1.2 In general, the design should provide adequate corrosion protection while minimizing installation, maintenance, and operation costs. The major objectives of cathod ic protection designs for tank bottoms are to: a. Deliver and distribute sufficient current to the external ta nk bottom to ensure that the criterion for protection is met. b. Provide a design life of the anode system and other equipment commensurate with the design life of the tank or provide for periodic replacement of anodes and maintenance of equipment. c. Provide adequate allowance for anticipated changes in current requirements with time. d. Place anodes, cables, rectifiers, and test stations where the possibility of physical damage is minimal. e. Minimize interference currents on neighboring structures. f. Provide sufficient monitoring points so measurements can be taken to determine that the protection criterion is met on the entire surface of the tank bottom.C ATHODIC P ROTECTION OF A BOVEGROUND P ETROLEUM S TORAGE T ANKS 17

7.1.3 There are many factors for consideration in the design of both internal and extern al cathodic protection systems. Cathodic protection systems should be designed only by a person thoroughly familiar with cathodic protection practices. 7.1.4 Whenever possible, the design should be based on standard components provided by manufa cturers regularly engaged in the production of cathodic protection system components.
7.2 INFLUENCE OF REPLACEMENT BO TTOMS, EXTERNAL LINERS (RELEA SE PREVENTION BARRIERS), AND SECONDARY CONTAINMENT ON CATHODIC PROTECTION SYSTEM DESIGN 7.2.1 Barriers to Cathodic Protection Cathodic protection is achieved by directing the flow of curren t from an anode to a cathode, re sulting in protection of the cat hode. Anything that acts as a barrier to the flow of current will pr event the application of cathodic protection. RPBs and replacemen t bottoms can have such an effect if not properly considered.
7.2.2 Tank Bottom Replacement Replacement of tank bottoms is an accepted practice. Whether the old bottom is left in place or removed has a significant impac t on the types of cathodic protection systems that are feas ible for corrosion control of the new bottom (see 7.2.4.3).
7.2.3 Considerations when External Liners are Used in a Diked Area 7.2.3.1 Impervious External Liner One method used to provide secondary containment is to line the entire diked area with an impervio us external liner. An existin g external liner under a tank or one proposed for a new tank can have a significant impact on the choices and design of a cathodi c protection system. In either case, if a nonconductive liner is us ed, anodes shall be placed betw een the external liner and the tank bottom in order for cathodic protection to work (see Figure 7). Note: If a clay liner is installed instead of a non conductive external liner, experience has shown that it will not significan tly affect the operation of a conventional cathodic protection system.

7.2.3.2 Existing Multiple or Single Tanks With a Dike Liner To install a cathodic protection system on an existing tank in a diked area that is lined from dike wall to dike wall and under the tank with an impervious external liner, one option is to bore under the tank at a very shallo w angle and install anodes under t he tank. In order to obtain adequate protection, an impressed curren t system probably would be required. This would be feasible on ly if there was sufficient depth between the tank bottom and the ex ternal liner so that the integrity of the external liner would not be compromised. At this time, there is limited experience with this type of system. Figure 7—Impervious External Liner Beneath Storage Tank Anodes between lining and tank bottom

Cathodic protection current from a remote groundbed shielded by external liner Lead wire to tank or rectifer (See Figure 5 or 6)

Impervious external liner Dike Tank Dike18 API R ECOMMENDED P RACTICE 651

7.2.3.3 New Multiple or Single Tanks with a Dike Liner New tanks in a diked area lined with an impervious external liner can be equipped with a network of shallow anodes or a grid of ribbon anodes which would be placed between the external liner and the tank bottom during construction. Such an arrangement could be an impressed current system or a galvanic system. Alternatively, a geosynthetic clay liner may be used and a more typi

cal cathodic protectio n system installed.
7.2.4 Replacement or Repair of Steel Tank Bottoms 7.2.4.1 Replacement or repair of steel tank bottoms is a common prac tice since the shell of a tank typically will far outlast the bottom. Linings are frequently inst alled in tanks to protect against internal co rrosion. An acceptable and commonly used method of replacing tank bottoms can be found in API RP 575 and API Std 653. 7.2.4.2 The method of repair or bottom replacement is of major importance in determining if cathodic protection should be installed and whether or not an effective cat hodic protection system can be installed. 7.2.4.3 If an existing tank bottom is protected by cathodic protect ion and/or if cathodic protec tion is planned for the new bot

tom (by means of deep or shallow anode beds), the old bottom will have to be completely removed. As shown in Figure 8, if it is not removed, the old bottom forms a shield that collects the cathodic current flowing through the ground and prevents cathodic protection of the new bottom. Unless the anodes are installed be tween the two steel bottoms (see Figure 8), or the old bottom i s removed, electrically isolated, or coated with a nonconductive material, a galvanic cell can develop between the old and new bo t

tom. Industry experience has sh own that if a conductive electrolyte exists between bottoms, the current flow and metal loss wil l be from the new bottom, resulting in premature failure of the new bottom. Industry experience has also shown that the successfu l operation and monitoring of some impressed anode systems installed in the interstitial space between the old and new tank botto m has been mixed when the distance between the two floors is less than 12 in. (30 cm). The closer the distance between the two floors the greater the likelihood that the anode system and/or reference cells will become “shorted” to the new tank bottom at some point in time due to normal loading and working of the tank. Regardless of the distance between tank bottoms, adequate care should be taken to avoid shorting the anodes to the tank bottom.

7.2.5 Effects of Impervious Nonconductive External Liners 7.2.5.1 General The installation of a external liner between the old and new botto m is an alternative practice. There are advantages and disadv an tages to this practice. Figure 8—New Steel Bottom on Top of Old Bottom Anodes installed between the new and old bottoms when cathodic protection is used

New steel floor

Old steel floor

Current

Old tank bottom shields existing cathodic protection current from the new steel floor Cathodic protection current returns to rectifier, offering no protection to the new floorC ATHODIC P ROTECTION OF A BOVEGROUND P ETROLEUM S TORAGE T ANKS 19

7.2.5.2 Advantages There are several advantages to the installation of a external liner: a. provides a means of detecting and containing leaks and preventing ground contamination if leaks occur; b. eliminates the natural current flow between the old bottom and the new bottom, thus, reducing the accelerated failure of the new bottom due to galvanic corrosion; c. may reduce entry of groundwater into the space between the bottoms.
7.2.5.3 Disadvantages Installing an external liner could be disadvantageous for the following reasons: a. an external liner makes the future addition of cathodic protection difficult; b. an external liner acts as a basin to cont ain water or any other electrolyte that might wet the sand between the old and new b ot

toms, thus, increasing corrosion rates.
7.2.5.4 Mitigation of Adverse Effects To keep the advantages and eliminate or reduce the adverse effe cts of a external liner in the old bottom, install a cathodic pr otec

tion system in the space between the old bottom and the new bottom. Such a system could consist of an array of ribbon anodes embedded in the sand between the old bottom and the new bott om. The connecting wires project through the external liner and through a sealed bulkhead fitting (in the old shell portion) to be connected directly to the tank or through a test station. Th is type of system would have to be installed at the time the bottom is replaced. If unexpected anode depletion or failure occurs, remed ial measures to replace anodes cannot be easily accomplished. The advantages of this syst em include the following: a. the need for future cathodic protec tion system installation is decreased; b. the external liner acts as a barrier to cu rrent flow, protecting the old bottom and directing all current flow toward the new bottom; c. as long as the sand stays dry, corrosion rates and current flow would be low as a result of the high resistivity of the sand; d. corrosion would tend to increase if the sand became wet; howev er, the resistivity of the sand would be much lower, thus, more current would flow and cathodic protection would increase (a self-governing system); e. the cost of the anodes is a small fraction of the total cost.
7.3 EXTERNAL CATHODIC PROTECTION 7.3.1 Introduction The purpose of this section is to recomme nd procedures for designing ca thodic protection that will effectively control external cor rosion by satisfying one or more of the criteria listed in Sect ion 8 for the intended service life of the aboveground storage t anks.
7.3.2 Design Considerations In the design of cathodic protection, the following items should be considered: a. Recognition of hazardous conditions existing at the proposed in stallation site and the selection and specification of materia ls and installation practices that will ensure the safe instal lation and operation of the cathodic protection system. b. Specification of materials and installation practices to conform with applicable codes, such as National Electrical Manufactu r ers Association standards, NACE recommended practi ces, and federal, state, and local regulations. c. Selection and design of the cathodic protection system for optimum economy of installation, maintenance, and operation. d. Selection and specification of materials an d installation practices that will ensu re dependable operation throughout the intended service life of the cathodic protection system. e. Selection of a design to minimize excessive protective currents or earth potential gradients that can detrimentally affect th e tanks, piping, coating, or neighboring buried or submerged metallic structures. f. Provisions for monitoring the cathodic protection system operation. g. The depth of any existing tank ringwall foundation.20 API R ECOMMENDED P RACTICE 651

7.3.3 Information Useful for Design Information that is useful for design can be divided into three categories: a. specifications and practices; b. site conditions; c. field survey, corrosion test data, and operating experience.
7.3.3.1 Specifications and Practices Information pertaining to these specifications or practices may prove useful: a. site plan and system layout; b. construction dates; c. tank design information; d. pumps and power supply; e. coatings; f. corrosion control test stations; g. electrical isolation; h. electrical bonds; i. electrical conduit routing; j. electrical area classification boundaries.
7.3.3.2 Site Conditions These factors relating to site conditions are c onsiderations in designing cathodic protection: a. existing and proposed cathodic protection systems; b. possible interference sources (stray current); c. special environmental conditions; d. depth of bedrock; e. depth of frost line; f. neighboring buried metallic structures (including lo cation, ownership, and corrosion control practices); g. structure accessibility; h. power availability; i. feasibility of electrical is olation from foreign structures; j. release prevention barri ers and secondary contai nment systems, if any; k. areas of poor water drainage.
7.3.3.3 Field Survey, Corrosion Test Data, and Operating Experience The following information may also be useful: a. protective current requirements to meet applicable criteria; b. electrical resistivity of the electrolyte (soil); c. electrical continuity of the system; d. electrical isolation of the system; e. coating integrity; f. leak history of similar structures in the area; g. deviation from construction specifications; h. existence of stray current; i. other maintenance and operating data.
7.3.4 Considerations that Influence Selection of the Type of Cathodic Protection System 7.3.4.1 The following factors influence the selection of a cathodic protection system: a. size and number of tanks to be protected; b. current required;C ATHODIC P ROTECTION OF A BOVEGROUND P ETROLEUM S TORAGE T ANKS 21
c. soil conditions such as resistivity, chemical composition, aeration, and pH; d. possibility of cathodic protection interference on adjacent structures; e. future developments and extensions to the storage system; f. cost of cathodic protection equipment, installation, operation, and maintenance; g. existing or proposed release prevention ba rrier and/or secondary containment system. 7.3.4.2 Several options are available for the protection of one or more tanks, including: a. shallow anodes installed around the periphery of the tank; b. anodes installed directly under the tank prior to construction; c. boring at an angle or horizontally directional drilling under the tank so that anodes can be installed beneath it in a patter n that will provide adequate protection; d. use of a deep anode bed.
7.3.5 Types of Cathodic Protection Systems 7.3.5.1 Galvanic Anode System 7.3.5.1.1 Galvanic systems use galvanic anodes whic h can be made of materials such as ma gnesium or zinc in either cast or rib

bon form. These are installed either bare or packaged in a special back fill. The anodes are connected to the system, either sin gly or in groups. Galvanic anodes are limited in current output by the system-t o-anode driving voltage and the circuit resistance. It may be more economical to cathodically protect bare, poorly coated, or large structures with impresse d current-type systems. Galvan ic cathodic protection systems may be more economical on small diameter tanks (less than 60 ft [18 m]; see NACE RP0193). 7.3.5.1.2 Three galvanic anode materials are co mmonly used for soil installations: a. high potential magnesium alloys; b. standard magnesium alloy; c. zinc. 7.3.5.1.3 Magnesium and zinc anodes prepackaged in special backfill are readily available in a nu mber of size and weight con

figurations to meet various current output and anode life design requirements. The use of a specia l backfill with anodes is des ir

able for installation in soil environments. Special backfill, cons isting of a proper mixture of gypsum, bentonite, and sodium sulfate, promotes anode efficien cy, lengthens anode life, and keeps the anode environment moist. 7.3.5.1.4 The number of anodes required to provide cathodic prot ection for aboveground storage tanks depends upon total cur

rent requirements and the expect ed individual anode discharge rate in the soil . In placing the anodes, current distribution fac tors should also be considered. Sometimes it is advantageous to consider the use of different sizes of anodes so that more anodes wi ll be required and give better current distribution. Typically, better current distribution and uniform polarization is obtained b y dis

tributing anodes around the tank or under the tank (for new construction).
7.3.5.2 Impressed Current System 7.3.5.2.1 Anodes 7.3.5.2.1.1 Impressed current anodes can be of materials such as, but not limited to, graphite, high silicon cast iron, steel, plati

nized metals, magnetite, and mixed metal oxides . These anodes are installed either bare or in special backfill material (usuall y coke breeze). They are connected with an in sulated conductor either singly or in grou ps to the positive terminal of a direct cu rrent source. The structure is connected to the nega tive terminal of the direct current source. 7.3.5.2.1.2 Graphite, high silicon cast iron, or mixed metal oxide anod es are generally preferred fo r soil installations. Plati

nized niobium, tantalum, and titanium are best suited for water rather than soil installations. 7.3.5.2.1.3 Each anode material has an optimum current density that provides maximum anode se rvice life. Anodes may be located in remote anode beds, located in deep anode beds, or di stributed closely about or under the structure to be protected. A proper anode bed design should: a. avoid physical interference with existing facilities; b. provide uniform current distribution; and c. avoid stray current interfere nce with offsite structures.22 API R ECOMMENDED P RACTICE 651

7.3.5.2.1.4 The number of anodes in a particular cathodic protection design will be determined by total current requirements of the structures to be protected and the optimum current density of the anode material selected. For a distributed anode design, addi

tional anodes may be installed to provide more uniform current distribution and to provide allowa nce in case of isolated anode connection failures or partial anode depletion.
7.3.5.2.2 Current and Voltage Requirements 7.3.5.2.2.1 For optimum design, the current required for cathodic protection should be calculated using the results of current requirement tests. However, in lieu of a current requirement te st, the generally accepted protective current density is between 1 mA/ft 2 and 2 mA/ft 2 at ambient temperatures, and current density between 2 mA/ft 2 and 8 mA/ft 2 for elevated temperature tanks. If a current requirement test is used , it can be performed only on existing tanks. It is conducted using a temporary ano de bed and an appropriate source of direct current (see Figure 9). The temporary anode bed is typically positioned in the soil nea r the perimeter of the tank. Depending on the cu rrent required, the power source can vary from a 12-volt storage battery to a 300-amp welding unit.

7.3.5.2.2.2 Current requirement tests are conduc ted by forcing a known amount of current from the temporary anode bed through the soil and onto the tank to be protected. The degree of protection at various locations around the tank, and under th e tank center if possible, is evaluated usin g potential measurements. This testing allows approximation of the current required t o protect the tank. Current requirement tests should be conducted with an adequate liquid level in th e tank to maximize contact o f the tank bottom with the pad material. 7.3.5.2.2.3 The voltage necessary to drive the required amount of cu rrent depends largely on the number and location of anodes and on the resistivity of the soil. Since the current is generally known from a current requirement test or is estimated , the voltage requirement can be calculated using Ohm’s Law ( E

= I × R) if the resistance of the circu it is known. The resistance can be estimated in several ways. a. From existing impressed current systems similar to the one to be installed. b. From current requirement tests as described above if the test anode bed is similar to the final one. c. From soil resistivity tests, the anode- to-earth resistance can be calculated using a variation of Dwight’s equation (see Pea

body’s Control of Pipeline Corrosion) . The anode-to-earth resistance is generall y the dominant part of the overall circuit resistance in an impressed current system. Figure 9—Current Requirement Test SetupC ATHODIC P ROTECTION OF A BOVEGROUND P ETROLEUM S TORAGE T ANKS 23

7.3.5.2.3 Rectifier Selection The rectifier output capacity selected will depend upon the following factors. a. Estimated or measured current requiremen ts of the structure to be protected. b. The voltage needed to cause current to flow from the anodes to th e buried structure. c. Rectifiers with a moderate excess capacit y (typically 10% – 50%) should be selected to allow for adjustments during the life of the cathodic protection system and to prevent damage due to vo ltage overloads. Care should be observed if the excess rectifier capacity will be used to drive th e anodes. Increasing the current output of the anodes above the manufacturer’s rating will cau se a decrease in anode life.
7.3.6 Electrical Isolation 7.3.6.1 Isolation devices consisting of flange a ssemblies, prefabricated isolating joints, or couplings should be installed where electrical isolation of the system is required to facilitate th e application of corrosion control. These devices should be prop erly rated for temperature, pressure, and dielectr ic strength. Isolating devices should not be installed in encl osed areas where com bus

tible atmospheres are normally present. 7.3.6.2 Electrical grounding of electrical equipmen t is an essential element in personnel sa fety. It may not be practical or desir

able to electrically isolate st orage tanks from each other or adjacent plant equi pment due to the probab le interconnection via a grounding grid. Attachment of auxiliary equipment such as elec tronic gauges, mixer motors, and lighting may also preclude the possibility of effectively maintaining isolation. When installing a cathodic protection system for a tank bottom, it should be recog nized that some of the current may also be co llected on adjacent buried metallic equipment. 7.3.6.3 Lightning arresters and fault current protection devices installe d at electrical isolation locati ons should be of a size suit

able for short term, high current loading. Where isolation devices are installed in areas of known or suspected influence from high voltage alternating current (HVAC) power lines, precautions should be taken to ensure that the AC potentials across such device s do not present a hazard to personnel. Refer to the latest revision of NACE RP0177.
7.4 INTERNAL CATHODIC PROTECTION 7.4.1 The design of an internal cathodic protection system is comp licated because of the variation in the level of the accumu

lated corrosive medium (usually water). In addition, the presence of sludge and other contaminants could have a detrimental effect on the performance of the cathodic protection system. Ther e are many factors which influe nce the design of an internal cathodic protection system. A few of these factors are as follows: a. condition and type of coating (if any); b. minimum and maximum water level in tank; c. compatibility of stored liquid with anodes and cables; d. internal inspection interval of tank, which affects design life. 7.4.2 Because of the many variables associated with design of internal cathodic protection systems for petroleum storage tanks, the use of these types of systems is limited and industry wide pr actices do not exist. It is recommended that Sections 4 and 5 of NACE RP0575 be consulted when designing similar systems fo r aboveground hydrocarbon storage tank bottoms. In addition, NACE RP0388 should be consulted for design information for impr essed current systems. 

### 8 Criteria for Cathodic Protection 8.1 INTRODUCTION The purpose of this section is to list criteria for cathodic prot ection that will in dicate when adequate cathodic protection ha s been achieved. The selection of a particular cr iterion for achieving the objective depends, in part, on prior experience with simila r structures and environments wherein the criterion has been used successfully.
8.2 PROTECTION CRITERIA 8.2.1 There are several criteria used to determine if adequate ca thodic protection has been achieved on aboveground storage tanks. For a more detailed description, re fer to the latest edition of NACE RP0193.24 API R ECOMMENDED P RACTICE 651

8.2.2 The following criteria were developed through laboratory expe riments or were determined empirically by evaluating data obtained from successfully operat ed cathodic protection systems. It is not intended that person s responsible fo r corrosion cont rol be limited to these criteria if it can be demonstrated that the control of corrosion can be achieved by other means. 8.2.2.1 A negative (cathodic) potential of at least 850 mV with th e cathodic protection current appl ied. This potential shall be measured with respect to a saturated copp er/copper sulfate reference electrode (CSE ) contacting the electro lyte. Voltage drops other than those across the tank bottom-to-electrolyte boundary shall be considered for valid interpreta tion of this voltage measurem ent. Note: “Considered” is unde rstood to mean the application of good engineering practice in dete rmining the significance of voltag e drops by methods such as: a. measuring or calculating the voltage drop(s), b. reviewing the historical performan ce of the cathodic protection system, c. evaluating the physical and electrical characteris tics of the tank bottom an d its environment, and d. determining whether or not there is physical evidence of corrosion. 8.2.2.2 A negative polarized potential of at least 850 mV relati ve to a CSE. (One common method of measuring polarized potential is by using the “instant off” technique.) 8.2.2.3 A minimum of 100 mV of cathodic polarization measured be tween the tank bottom metallic surface and a stable refer

ence electrode contacting the elect rolyte. The formation or decay of this polarization can be measured to satisfy this criterio n.
8.3 MEASUREMENT TECHNIQUES 8.3.1 The standard method of determining the effectiveness of ca thodic protection on a tank bottom is the tank-to-soil potential measurement. These measurements are performed using a high-imp edance voltmeter and a stable, reproducible reference electrode contacting the electrolyte. These measurements are commonly made with the reference electrode pla ced in contact with the soil a t several places around the perimeter of the tank as shown in Figure 10 and, if possible, at one or more points under the tank. U nder

tank measurements are made because measurements at the perimete r of the tank may not represent the tank-to-soil potential of th e center of the tank bottom. Methods to monitor tank-to-soil poten tials under the center of the tank are discussed in 9.4. If und ertank potential measurements are not used, a good en gineering practice, in conjunction with the inspection and maintenance practices of API Std 653, should be used to determin e that cathodic protection is adequately controlling corrosion of the tank bottom. 8.3.2 The actual structure-to-soil potential under the tank should be determined by using permanently installed reference elec

trodes, or by temporarily inserting a waterproof, portable re ference electrode under the tank through a perforated nonmetallic tube (see 9.4). With a perforated tube, it may be possible to determine the least protected area of the tank bottom and thereafter u se that one point as the primary undertank monitoring location. Permanen tly installed reference electrodes may vary from their original reference potential. This possible variation should be taken into account when evaluating structure-to-electrolyte potentials. 8.3.3 The tank-to-soil potential measurements ma y be taken with current applied; howeve r, consideration of IR drop(s) in the soil shall be made. Consideration of the IR drop in the soil is necessary for measurements made at the tank perimeter even if t he reference electrode is placed immedi ately adjacent to the tank. This is especially true if distri buted anodes ar e close to the tank since the perimeter of the tank may be within the electric field gradient of the anodes. 8.3.4 The value of the IR drop and the methods of consideration should be determined by using good engineering practices. Interrupting the flow of current at the rectifiers using the “instant-off” technique is a common method. The IR drop, once dete r mined, can be used for future tests at th e same location if conditions remain similar. 8.3.5 Tank bottom surface area contacting the tank pad may vary wi th the tank content level. Since this condition can cause variations in tank-to-soil potentials, the le vel of the tank contents should be considered at the time of the survey. For more detailed information, see NACE TM0497.
8.4 ALTERNATIVE REFERENCE ELECTRODES Other standard reference electrodes may be substituted for the saturated copper/copper sulfate reference electrode. Three com

monly used reference electrodes are listed in Table 3, along with their voltage equivalent to –0.85 volt referred to a saturate d cop

per/copper sulfate reference electrode.C ATHODIC P ROTECTION OF A BOVEGROUND P ETROLEUM S TORAGE T ANKS 25



### 9 Installation of Cathodic Protection Systems 9.1 INTRODUCTION The purpose of this section is to recommend procedures for the installation of catho dic protection systems that will control co rro sion of the tank bottom if design consideratio ns recommended in Section 7 have been followed. The installation of cathodic protection systems should be under the supervision of trained and qualified personnel to ensure th at the installation is made in stri ct accordance with the drawings and specifications. Exceptions may be made only with the approv al of the owner, operator, or personnel qualified by the owner or operator.
9.2 GALVANIC ANODE SYSTEMS 9.2.1 Packaged anodes should be inspected to ensure integrity of th e container and should be kept dry during storage. If indi

vidually packaged anodes are supplied in waterproof containers , that container shall be remove d before installation. Electrical continuity between the anode and lead wire should be tested wi thout compromising the integrity of the package. Packaged gal

vanic anodes should be backfilled with compacted native soil. Figure 11 shows a typical galvanic anode installation 9.2.2 Where anodes and special backfill are provided separately, anodes should be ce ntered in the special backfill, which should be compacted prior to backfilling with native soil. 9.2.3 When galvanic anodes are used to protect internal surfaces of tank bottoms, they can be either bolted or welded to the tank bottom. The connection should be coated, but care should be taken not to coat or paint the anode. The anodes must be place d in the electrolyte to perform as designed. 9.2.4 Where a ribbon-type anode is used between tank bottoms, it is generally installed in clean dry sand. Ribbons should be carefully straightened and made to lie flat so that the anode wi ll not contact the steel bottom. Figure 10—Potential Measurement Schematic

Table 3—Commonly Used Reference Electrodes
Reference Electrodes –0.850 Voltage Equivalent to CSE Saturated KCI calomel –0.78 Silver/silver chloride (used in sea water) –0.80 Zinc +0.25 As close as possible to tank (consistent placement) Potential measured at several locations aound tank High input resistance voltmeter

Tank Tank Cu/CuSO 4 reference electrodes26 API R ECOMMENDED P RACTICE 651

9.2.5 Care should be taken so that lead wires and connections ar e not damaged during backfill operations. Lead wires should have enough slack to prevent strain. Anodes should not be carried or lowered into the ex cavation by the lead wire.
9.3 IMPRESSED CURRENT SYSTEMS 9.3.1 General 9.3.1.1 Impressed current anodes should be inspected for defects, co nformance to anode material sp ecification, size and length of lead wires, and to ensure that the an ode cap, if used, is secure. Care shall be exercised to avoid cracking or damaging anod es during handling and installation. Cracked anodes should not be used. Lead wires should be carefully inspected for defects in in su lation. Care shall be taken to avoid damage to insulation on wire . Defects in the lead wire sha ll be repaired, or the anode sha ll be rejected. 9.3.1.2 Impressed current anodes can be installe d vertically or horizontally by conventional excavation or augering, angled by boring, in deep vertical holes by drilling, or horizontally by directional drilling. Impressed current anodes are typically ins talled in carbonaceous backfill such as coke breeze. If the backfill is installed properly so that there are no voids around the anode, m uch of the current reaching the anode is conducted to the backfill by electrical contact. This promotes consumption of the backfill in stead of the anode and substantially lengthens the effective anode life . Carbonaceous backfill also tends to reduce total circuit res istance by lowering anode- to-soil resistance. 9.3.1.3 The following are principal points to be observe d in the installation of impressed current anodes. a. The coke breeze shall be correctly installed because loose back fill can result in high resistance and shortened anode life. T he anode should be centered in the coke breeze. Premature anode fa ilure will occur if the anode co mes in contact with the soil. b. Buried anode connections shal l be protected with extreme precautions agains t the entrance of any moisture, because any dis

charge of current to earth from the cab le will destroy it very rapidly. Figure 11—Typical Galvanic Anode Installation To tank

Anode cable Grade

Soil backfill

Galvanic anode

Anode backfillC ATHODIC P ROTECTION OF A BOVEGROUND P ETROLEUM S TORAGE T ANKS 27
c. Care should be taken to protect the cable connection to the anod e; this is the weak point in all anodes, and the joint is pro ne to failure by the entrance of moisture through even the tiniest crack. d. Anodes and cable should be installed at a sufficient depth to protect against accidental damage . The anode lead may be severe d by corrosion if there is the slightest break in its insulation.
9.3.2 Shallow Anode Bed Installation 9.3.2.1 Figure 12 shows an example of a shallow anode bed installati on. For a typical vertical anode installation, the hole is excavated 8 in. – 12 in. (20 cm – 30 cm) in diameter by approximately 10 ft – 20 ft (304 cm – 366 cm) deep. Power auger equip

ment is used where avai lable if both the terrain and right of way will perm it. The anode is centered in the opening and properl y installed anode backfill is carefully tamped when necessary. Many anodes also come prepac kaged in carbonaceous backfill.

9.3.2.2 Sometimes it is necessary to install an anode in a location where rock is encountered at a shallow depth, or where soil resistivity increases markedly with depth. Such sites can be coped with by a horizontal installation of anodes. A ditch is exca vated to whatever depth is practical, and a horiz ontal column of coke breeze is laid therei n, usually square in cross section. The an ode is laid horizontally in the center of this column. 9.3.2.3 In some instances, to improve current distribution to the cente r of the tank, it may be desirable to install anodes in holes which are bored at an angle under the perimeter of the tank bott om or bored horizontally underneath the tank bottom by direc

tional drilling. Prepackaged anodes may be beneficial in such in stallations to ensure that the anode remains centered in the co ke breeze column. Alternatively, centering a wi re- or cable-type anode (e.g., mixed metal oxide, etc.) with centralizers in a perf orated Figure 12—Typical Shallow Anode Bed Installation28 API R ECOMMENDED P RACTICE 651
nonmetallic tube prior to coke breeze placem ent can be beneficial in ho rizontal directionally drilled installations to ensure t hat the anode remains centered in the coke breeze column.
9.3.3 Deep Anode Bed Installation In situations where a deep anode bed similar to that shown in Figure 13 is required, refer to the latest edition of NACE RP0572

. Prior to installation of a deep anode bed, it is important to consider the environmental aspects since the anode bed may be installed through underground aquifers. It is often appropriate to provide an internal and external casing se al to maintain sep ara tion between surface and subsurface environments.

9.3.4 Rectifier Installation 9.3.4.1 The rectifier or other power source should be installed so that the possibility of damage or vandalism is minimized. 9.3.4.2 Rectifiers and associated wiring should comply with local, st ate, and national electrical co des and electrical area classi

fication per API RP 500. An external disconnect switch on AC wiring should be provided. The rectifier case should be properly grounded. 9.3.4.3 Lead wire connections to the rectifier sh all be mechanically secure and electri cally conductive. Befo re the power source is energized, it shall be verified that the negative lead is connected to the structure to be protected and that the positive l ead is con

nected to the anodes. CAUTION: If the leads are reversed, with the positive lead mistakenly attached to the tank, the tank bottom will serve as an anode and rapid corrosion failure can result. 9.3.4.4 An exothermic weld connection is the pr eferred means for connecting the negative rectifier lead wire to the structure to be protected; however, good mechanical connections may be subs tituted if necessary. All positive cable connections and wire Figure 13—Commonly Installed Deep Anode Bed Rectifier Junction

box

Tank

Environmental seals as necessary. Refer to the government authority having jurisdiction for enviromental seal requirements.

Deep ground bedanodes in coke breezefilled column Negative lead connectionC ATHODIC P ROTECTION OF A BOVEGROUND P ETROLEUM S TORAGE T ANKS 29
splices should be carefully waterproofed an d covered with electrical insulating mate rial. If mechanical connections are used, t hey should not be buried.
9.3.5 Cable Installation 9.3.5.1 All underground wire attached to the positive rectifier terminal is at a positive potential with respect to ground. If not completely insulated, the wire may discharge current (act as an anode), which will result in corrosion of the wire and rapid fa ilure of the cathodic protection installation. Ther efore, all anode lead wires, header cables, and any wire splices should be careful ly inspected prior to backfilling. Cable can be installed by plowing if proper precautions are taken. Backfill should be free of s harp stones or other material that could damage wire insulation. Co nsideration should be given to installing cable in rigid conduit in areas subject to frequent excavation or where cable insulation is prone to damage by rodents. 9.3.5.2 Underground splices of the header cable (positive lead wire) to the anode bed should be avoided. Connections between header cable and anode lead wires should be mechanically secure and electrically conductive. Sufficient slack should be left to avoid strain on all wires. All splices and connections shall be sealed to prevent mois ture penetration so that electrical isola tion from the environment is ensured.
9.4 CORROSION CONTROL TEST STATIONS, UNDERTANK MONITORING METHODS, AND BONDS 9.4.1 The structure and test lead wires should be clean, and dry at points of connection. Connections of test lead wires to the structure shall be installe d so that they will remain m echanically secure and electricall y con ductive. A preferred method from the electrical standpoint is th e use of an exothermic weld connection. However, this method is not recommended in areas where a combustible atmo sphere may exist during the attachment process. 9.4.2 Attention shall be given to the manner of installing test l ead wires for corrosion control testing to avoid affecting the physical strength of the structure at the point of attachment. 9.4.3 All test lead wire should be co ated with an electrically insula ting material. Test lead wires should be color coded or other

wise permanently identified. Sufficient slack should be left to avoid strain on all wires. Damage to wire insulation shall be avoided, and proper repairs sha ll be made if damage occurs. 9.4.4 One of the problems associated with monitoring cathodic prot ection systems on tank bottom s is the inability to place a portable reference electrode in close proximity to the underside. For new tank construction, one or more of the following shoul d be installed to permit testing the tank-to-soil potent ial at one or more places under the tank bottom. a. Permanently installed reference electrodes and lead wires underneath the tank pa d to the perimeter of the tank where they can be terminated in a test station for future use in testing (see Figure 14). b. Perforated nonmetallic tubes for use in measuring the tank-to- soil potential at locations along the length of the tube (see Figure 15).

Figure 14—Permanently Installed Reference Electrode and Test Station Test station

Tank

Sand Test lead connection

Permanent reference electrode30 API R ECOMMENDED P RACTICE 651

9.4.5 When a tank bottom is repaired or repl aced, one or more permanently installed reference electrodes and/or perforated nonmetallic tubes should be installed under the tank bottom. For ex isting tanks not scheduled for su ch repairs, one or more per fo

rated nonmetallic tubes should be installe d underneath the tank using a directional boring technique shown in Figure 15. Refer

ence electrodes are inserted in th e perforated pipe, which provides electrical co ntinuity between the soil outside the tube and the reference electrode inside the tu be for the entire length. The re ference electrode should be inse rted into the pipe, using a no nme tallic electrician’s fish tape or small diameter PVC pipe, to obtain a profile of the tank-to-soil potential measurements acros s the tank bottom. If a metallic device is used to insert the refe rence electrode, it should be removed before readings are taken. 9.4.6 Where permanently installed re ference electrodes are specified, installation should be per manufacturer’s recommended procedures.

9.4.7 It is common to contact the tank with a sharp point, such as an awl, when taking a tank-to-soil potential measurement. This repeated action can cause early failure of the tank’s paint system if not recoated. Permanently inst alled test leads, grou nding lugs, or short pieces of cable or tubing can avoid this and also readily identify the normal monitoring locations. 9.4.8 If isolating devices are required, inspectio n and electrical measurements should be ma de to ensure that electrical isolation is effective and meets the requir ements for cathodic protection. 9.4.9 Electrical devices isolated from the liquid storage system for cathodic protection purposes should be provided with safety grounds in accordance with applicable electrical codes. 

### 10 Interference Currents 10.1 INTRODUCTION The purpose of this section is to identify sources of interfere nce currents and to recommend pr actices for the detection and co ntrol of these currents. It should be noted that the installation of a new impressed current cathodic pr otection system may cause int er ference with neighboring structures. CAUTION: Care shall be exercised when using directional boring techni ques to prevent undermining of the tank pad and to avoid damaging the tank bottom. Figure 15—Perforated Pipe Installed for Reference Electrode Tank

Perforated PVC pipe
Reference electrode Test meter

SandC ATHODIC P ROTECTION OF A BOVEGROUND P ETROLEUM S TORAGE T ANKS 31
Interference is the undesirable discharge of current from a structure caused by the appl ication of electrical current from a fo reign source. Interference is normally from a DC source, although AC can also cause interference problems. A more detailed descrip

tion can be found in 4.2.1. Consult the latest edition of NACE RP0193 for more inform ation on interfe rence currents. 10.2 SOURCES OF INTERFERENCE CURRENTS 10.2.1 Constant Current These sources have essentially constant direct current output. Th e most common sources are rectifie rs energizing nearby cathodi c protection systems.
10.2.2 Fluctuating Current These sources have a fluctuating direct curr ent output. The greatest sources of stray current are the electric railway, rapid t ransit system, underground mining electrical systems, and welding machines.
10.3 DETECTION OF INTERFERENCE CURRENTS During corrosion control surveys, personnel should be alert for electrical or physical observatio ns that could indicate interfe rence from a neighboring source. These include: a. a negative shift of the structure-to-soil potential on the aff ected structure at a point where current is picked up from the foreign direct current source, b. a positive shift of the structure-to-soil potential on the affe cted structure at a point where current may be discharged from the affected structure, and c. localized pitting in areas near or immediately adjacent to a foreign structure.
10.4 CONTROL OF INTERFERENCE CURRENTS 10.4.1 There are three fundamental approaches to resolving an interference problem: a. design that aims at minimizing exposure; b. bonding to provide a metallic return of current collected by a foreign structure; c. auxiliary drainage of the collected current by the use of galvanic anodes. 10.4.2 Interference problems can be prevented an d resolved by participation in local corrosion coordinating committees. When interference effects are observed, the commi ttee can often provide information on the source of the interference currents. If n o local committee exists, refer to NACE TPC 11 and/or contact NACE International fo r information regarding these committees. 

### 11 Operation and Maintenance of Cathodic Protection Systems 11.1 INTRODUCTION 11.1.1 The purpose of this section is to reco mmend procedures and practices for ener gizing and maintaining continuous, effec

tive, and efficient operation of cathodic protection systems. Electrical measurements and inspections are necessary to determine that protection has been established according to applicable criteria and that each part of the cathodi c protection system is operat ing properly. Conditions that affect protection are subj ect to change with time. Corresponding changes may be required in the cathodic protection system to maintain protection. Periodic measurements and inspection are necessary to detect changes in the cathodic protection system. Conditions may exist where oper

ating experience indicates that testing and inspections should be made more frequently than recommended herein. 11.1.2 Care should be exercised in selecting the location, number, and type of electrical measur ements used to determine the adequacy of cathodic protection. If tanks are empty, there may be large areas of the bottoms which are not in contact with the underlying soil. Potential surveys, in this case, may give misleading information. Bottom-to-electrolyte potential readings may indicate adequate cathodic protection for the portion of the tank bottom in contac t with the soil but when the tank is full and all of the tank bottom is in contact with the soil, protection may be insufficient. There

fore, potential surveys should be conducted with an adequate level in the tank to ma ximize contact of the tank bottom with the pad material.32 API R ECOMMENDED P RACTICE 651

11.1.3 If cathodic protection devices are shut off while working on storage tanks, the system should be re-energized as soon as possible to avoid corrosion damage during extensive maintenance periods.
11.2 SAFETY 11.2.1 All impressed current systems shall be desi gned with safety in mind. Care shall be taken to ensure that all cables are pro

tected from physical damage and the possibility of arcing. 11.2.2 Rectifiers and junction boxes shall meet regulatory requiremen ts for the specific location a nd environment in which they are installed. Such locations shall be determined by reviewing local, state, federal, and prevailing industrial codes. Consideration should be given to locating is olating devices, junction bo xes, and rectifiers outside of hazardous areas in case sparks or arcs occur during testing. 11.2.3 In order to prevent arcing, care shall be exercised when working on breakout piping attached to tanks with cathodic pro

tection applied. When cathodic protection systems are turned off, sufficient time shall be allowed for depolarization before op en ing connections. Bonding cables shall be used when parting breakout piping joints. Additional guidance regarding arcing due to static electricity, stray currents or lightening can be obtained from API RP 2003.
11.3 CATHODIC PROTECTION SURVEYS 11.3.1 General 11.3.1.1 Prior to energizing a new cathodic protec tion system, measurements of the native structure-to-soil potential should be made. Immediately after any cathodic protecti on system is energized or repaired, a surv ey should be conducted to determine that it operates properly. An initial survey to verify that it sati sfies applicable criteria should be conducted after adequate pola rization has occurred. Polarization to a steady state may take several months after the system is energized. This su rvey should include one or more of the following types of measurements: a. structure-to-soil potential; b. anode current; c. native structure-to-soil potentials; d. structure-to-structure potential; e. piping-to-tank isolation if protected separately; f. structure-to-soil potential on adjacent structures; g. continuity of structures if protected as a single structure; h. rectifier DC volts, DC amps, efficiency, and tap settings. 11.3.1.2 Annual cathodic protection surveys are recommended to ensure the effectiveness of cathodi c protection. The electrical measurements used in the survey may include one or more of the measuremen ts listed in 11.3.1.1.
11.3.2 Inspection, Testing, and Maintenance of Cathodic Protection Facilities 11.3.2.1 Inspection and tests of cathodic protection facilities should be made to ensure their proper operation and maintenance. 11.3.2.2 All sources of impressed current should be checked at intervals not exceeding two months unless specified otherwise by regulation. Evidence of proper function may be current outpu t, normal power consumption, a signal indicating normal opera

tion, or satisfactory electrical state of the protected structure. A satisfactory co mparison between the rectifier operation on a bimonthly basis and the rectifier operation during the annual survey implies the protected status of affected structures is sim ilar. This does not take into account possible effects of foreign current sources. 11.3.2.3 All impressed current protective facilities should be inspec ted annually as part of a preventive maintenance program to minimize in-service failure. Inspections should include a check of th e general condition, includ ing electrical shorts, groun d connections, rectifier me ter accuracy, efficiency, and circuit resistance. 11.3.2.4 The effectiveness of isolating devices and continuity bonds should be evaluated during the periodic surveys. This can be accomplished by onsite inspection or by evaluating cathodic protection test data. 11.3.2.5 The tank bottom should be ex amined for evidence of corrosion whenever access to th e bottom is possible. This may be during repairs or modifications, or in conjunction with insp ections required by API Std 653. Examination for bottom-side cor

-C ATHODIC P ROTECTION OF A BOVEGROUND P ETROLEUM S TORAGE T ANKS 33
rosion may be achieved by making coupon cutouts or by nondestr uctive methods such as ultrasonic inspections or electromag

netic flux leakage. 11.3.2.6 Remedial measures should be taken where periodic tests and inspections indicate that protection is no longer adequate according to applicable criteria. Thes e measures may include the following: a. repair, replacement, or adjustment of cathodic protection system components; b. providing supplementary cathodic protection where additional protection is necessary; c. repair, replacement, or adjustment of continuity and interference bonds; d. elimination of acciden tal metallic contacts; e. repair of defective isolation devices; f. resolution of in terference currents.
11.4 CATHODIC PROTECTION RECORDS 11.4.1 Data pertinent to the design, installation, operation, maintena nce, and effectiveness of corro sion control measures should be documented in a clear, concise, and workable manner. 11.4.2 In determining of the need for cathodic protection, items listed in 5.2 should be recorded. 11.4.3 In designing cathodic protection systems, the following should be recorded: a. design and location of isolating devices, test leads and other test facilities, and details of other special corrosion contro l mea

sures taken; b. results of current requirement test s, where made, and procedures used; c. native structure-to-soil potent ials before curre nt is applied; d. results of soil resistivity tests at the site , where they were made, and procedures used; e. name of person conducting surveys. 11.4.4 In installing corrosion control facilities, the following should be recorded. a. Impressed current systems. 1. location and date placed in service; 2. number, type, size, depth, backfill, and spacing of anodes; 3. specifications of rectifier or other energy source; 4. interference tests and the parties participating in resolution of any interference problems. b. Galvanic an ode systems. 1. location and date placed in service; 2. number, type, size, depth, backfill, and spacing of anodes unless part of a factory installed system. 11.4.5 A record of surveys, inspections, and test s described in 11.3.1 and 11.3.2 should be maintained to demonstrate that appli

cable criteria for cathodic protection have been satisfied. 11.4.6 In maintaining corrosion control facilities, the following information should be recorded: a. repair of rectifiers and other dc power sources; b. repair or replacement of anodes, connections, and cable; c. maintenance, repair, and replacement of coating, isol ating devices, test leads, and other test facilities. 11.4.7 Records sufficient to demonstrate the need for corrosion cont rol measures should be retained as long as the facility involved remains in service. Records related to the effectiveness of cathodic protection should be retained for a period of fiv e years unless a shorter period is specifically allowed by regulation.Invoice To ( ❏ Check here if same as “Ship To”) Name: Title: Company: Department: Address:
City: State/Province: Zip/Postal Code: Country: Telephone: Fax: E-Mail:



### ❏ Payment Enclosed 

### ❏ P.O. No. (Enclose Copy) 

### ❏ Charge My IHS Account No. 

### ❏ VISA 

### ❏ MasterCard 

### ❏ American Express 

### ❏ Diners Club 

### ❏ Discover Credit Card No.: Print Name (As It Appears on Card): Expiration Date: Signature:

Quantity Product Number Title Total

Subtotal Applicable Sales Tax (see below)

Rush Shipping Fee (see below) Shipping and Handling (see below) Total (in U.S. Dollars) ★ To be placed on Standing Order for future editions of this publication, place a check mark in the SO column and sign here:

Pricing and availability subject to change without notice. Date:

SO ★ Unit Price 

### ❏ API Member (Check if Yes) Ship To (UPS will not deliver to a P.O. Box) Name: Title: Company: Department: Address:
City: State/Province: Zip/Postal Code: Country: Telephone: Fax: E-Mail:

Mail Orders – Payment by check or money order in U.S. dollars is required except for established accounts. State and local taxes, $10 process ing fee, and 5% shipping must be added. Send mail orders to: API Publications, IHS, 15 Inverness Way East, c/o Retail Sales, Englewood, CO 80112-5776, USA. Purchase Orders – Purchase orders are accepted from established accounts. Invoice will include actual freight cost, a $10 processing fee, plus st ate and local taxes. Telephone Orders – If ordering by telephone, a $10 processing fee and actual freight costs will be added to the order. Sales Tax – All U.S. purchases must include applicable state and local sales tax. Customers claiming tax-exempt status must provide IHS wit h a copy of their exemption certificate. Shipping (U.S. Orders) – Orders shipped within the U.S. are sent via traceable means. Most orders are shipped the same day. Subscription updates are sen t by First-Class Mail. Other options, including next-day service, air service, and fax transmission are available at additional cost. Call 1-800 -854-7179 for more information. Shipping (International Orders) – Standard international shipping is by air express courier service. Subscription updates are sent by World Mail. Normal delivery is 3-4 days from shipping date. Rush Shipping Fee – Next Day Delivery orders charge is $20 in addition to the carrier charges. Next Day Delivery orders must be placed by 2:00 p.m. MST to ensure overnight delivery. Returns – All returns must be pre-approved by calling the IHS Customer Service Department at 1-800-624-3974 for information and assistanc e. There may be a 15% restocking fee. Special order items, electronic documents, and age-dated materials are non-returnable. Effective January 1, 2007. API Members receive a 30% discount where applicable. The member discount does not apply to purchases made for the purpose of resale or for incorporation into commercial products, training courses, workshops, or other commercial enterprises. Available through IHS: Phone Orders: 1-800-854-7179 (Toll-free in the U.S. and Canada) 303-397-7956 (Local and International) Fax Orders: 303-397-2740 Online Orders: global.ihs.com 

# 2007 Publications Order Form

# There’s more where this came from.

The American Petroleum Institute provides additional resources and programs to the oil and natural gas industry which are based on API® Standards. For more information, contact: • API Monogram® Licensing Program Phone: 202-962-4791 Fax: 202-682-8070 • American Petroleum Institute Quality Registrar Phone: 202-962-4791 (APIQR®) Fax: 202-682-8070 • API Spec Q1® Registration Phone: 202-962-4791 Fax: 202-682-8070 • API Perforator Design Registration Phone: 202-962-4791 Fax: 202-682-8070 • API ISO/TS 29001 Registration Phone: 202-962-4791 Fax: 202-682-8070 • API Training Provider Certification Program Phone: 202-682-8490 Fax: 202-682-8070 • Individual Certification Programs Phone: 202-682-8064 Fax: 202-682-8348 • Engine Oil Licensing and Certification System (EOLCS) Phone: 202-682-8516 Fax: 202-962-4739 • API PetroTEAMTM (Training, Education and Meetings) Phone: 202-682-8195 Fax: 202-682-8222

Check out the API Publications, Programs, and Services Catalog online at www.api.org.

Helping You Get The Job Done Right. ®01/07Product No. C65103