Cathodic Protection of Aboveground Petroleum Storage Tanks

API RECOMMENDED PRACTICE 651 FOURTH EDITION, SEPTEMBER 2014

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Copyright © 2014 American Petroleum Institute

Foreword

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Suggested revisions are invited and should be submitted to the Standards Department, API, 1220 L Street, NW, Washington, DC 20005, standards@api.org.

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Contents

Page

1. Scope 1

2. Normative References 1

   1. Standards, Codes, Publications, and Specifications 1

   2. Other References 3

3. Terms and Definitions 3

4. Corrosion of Aboveground Steel Storage Tanks 8

   1. Introduction 8

   2. Corrosion Mechanisms 9

   3. Internal Corrosion 11

5. Determination of Need for Cathodic Protection 11

   1. Introduction 11

   2. Tank History 12

   3. Tank Pad and Soil Conditions 14

   4. Other Factors Affecting Cathodic Protection 18

6. Methods of Cathodic Protection for Corrosion Control 19

   1. Introduction 19

   2. Galvanic Systems 19

   3. Impressed Current Systems. 21

7. Design of Cathodic Protection Systems 23

   1. Introduction 23

   2. Influence of Replacement Bottoms, External Liners (Release Prevention Barriers), and Secondary Containment on Cathodic Protection System Design 24

   3. External Cathodic Protection 27

   4. Internal Cathodic Protection 32

8. Criteria for Cathodic Protection 33

   1. Introduction 33

   2. Protection Criteria 33

   3. Measurement Techniques 34

   4. Alternative Reference Electrodes 35

9. Installation of Cathodic Protection Systems 35

   1. Introduction 35

   2. Galvanic Anode Systems 35

   3. Impressed Current Systems. 36

   4. Corrosion Control Test Stations, Undertank Monitoring Methods, and Bonds 39

10. Interference Currents 42

    1. Introduction 42

    2. Sources of Interference Currents 42

    3. Detection of Interference Currents 42

    4. Control of Interference Currents 42

11. Operation and Maintenance of Cathodic Protection Systems 43

    1. Introduction 43

    2. Safety 43

    3. Cathodic Protection Surveys 44

    4. Cathodic Protection Records 45

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Contents

Figures

Page

1. Electrochemical Corrosion Cell 8

2. Oxygen Concentration Cell Caused by Rocks or Clay in Tank Pad 9

3. Example of Stray Current Corrosion of an Unprotected Tank Bottom 10

4. Galvanic Corrosion 11

5. Cathodic Protection with Galvanic Anodes 20

6. Impressed Current Cathodic Protection 22

7. Impervious External Liner Beneath Aboveground Storage Tank 24

8. New Steel Bottom on Top of Old Bottom 26

9. Current Requirement Test Setup 31

10. Potential Measurement Schematic 34

11. Typical Galvanic Anode Installation 36

12. Typical Shallow Anode Bed Installation 37

13. Commonly Installed Deep Anode Bed 38

14. Permanently Installed Reference Electrode and Test Station 40

15. Perforated Pipe Installed for Reference Electrode 41

Tables

1. General Classification of Resistivity 15

2. Partial Galvanic Series 20

3. Commonly Used Reference Electrodes 35

4. Permanently Reference Electrodes 40

vi

Introduction

Persons planning to construct an aboveground storage facility, replace existing aboveground storage tanks and associated piping systems, or cathodically protect existing aboveground storage tanks and associated piping should refer to applicable local, state, and federal fire, safety, and environmental regulations as well as the most recent edition of the following publications:

• API Standard 650;

  
  

• API Recommended Practice 652;

  
  

• API Standard 653;

  
  

• API Specification 12B;

  
  

• API Specification 12D;

  
  

• API Specification 12F;

  
  

• API Standard 2610;

  
  

• NACE RP0193;

  
  

• NACE RP0285;

  
  

• NFPA 30;

  
  

• NFPA 70; and,

  
  

• PEI RP200.

The appropriate government authority having jurisdiction should be consulted for regulations that apply to the area of installation prior to taking any action suggested in this recommended practice.

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Cathodic Protection of Aboveground Petroleum Storage Tanks

1. Scope

   1. The purpose of this recommended practice (RP) is to present procedures and practices for achieving effective corrosion control on aboveground storage tank bottoms through the use of cathodic protection. This RP contains provisions for the application of cathodic protection to existing and new aboveground storage tanks. Corrosion control methods based on chemical control of the environment or the use of protective coatings are not covered in detail.

      
      

   2. When cathodic protection is used for aboveground storage tank applications, it is the intent of this RP to provide information and guidance specific to aboveground metallic storage tanks in hydrocarbon service. Certain practices recommended herein may also be applicable to tanks in other services. It is intended to serve only as a guide to persons interested in cathodic protection. Specific cathodic protection designs are not provided. Such designs should be developed by a person thoroughly familiar with cathodic protection practices for aboveground petroleum storage tanks.

      
      

   3. This RP does not designate specific practices for every situation because the varied conditions in which tank bottoms are installed preclude standardization of cathodic protection practices.

      
      

2. Normative References

   1. Standards, Codes, Publications, and Specifications

The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

API Specification 12B, Bolted Tanks for Storage of Production Liquids

API Specification 12D, Field Welded Tanks for Storage of Production Liquids

API Specification 12F, Shop Welded Tanks for Storage of Production Liquids

API Recommended Practice 500, Recommended Practice for Classification of Locations for Electrical Installations at Petroleum Facilities

API Recommended Practice 575, Inspection of Atmospheric and Low Pressure Storage Tanks API Standard 620, Design and Construction of Large, Welded, Low-pressure Storage Tanks API Standard 650, Welded Tanks for Oil Storage

API Recommended Practice 652, Lining of Aboveground Petroleum Storage Tank Bottoms

API Standard 653, Tank Inspection, Repair, Alteration, and Reconstruction

API Recommended Practice 1615, Installation of Underground Petroleum Storage Systems

API Recommended Practice 1621, Bulk Liquid Stock Control at Retail Outlets

API Recommended Practice 1632, Cathodic Protection of Underground Petroleum Storage Tanks and Piping Systems

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