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  1. 1. Guidance on Practice for Cathodic Protection GP 06-31 BP GROUP ENGINEERING TECHNICAL PRACTICES Document No. GP 06-31 Applicability Group Date Draft 16 December 2005
  2. 2. Draft 16 December 2005 GP 06-31 Guidance on Practice for Cathodic Protection Page 2 of 39 Foreword This is the first issue of Engineering Technical Practice (ETP) BP GP 06-31. This Guidance on Practice (GP) replaces heritage documents from the merged BP companies as follows: BP PTA CP-CP-UGP-C Corrosion Protection—Cathodic Protection—Electrical Bonding of Underground Pipe—Construction Specification. PTA CP-CP-00-C Corrosion Protection—Cathodic Protection—Steel Structures— Construction Specification. PTA CP-CP-00-E Corrosion Protection—Cathodic Protection—Engineering Specification. PTA CP-CP-BRZ-R Corrosion Protection—Cathodic Protection—Thermite Brazing— Recommended Practice. PTA CP-CP-TST-R Corrosion Protection—Cathodic Protection—Insulating Flanges and Unions Testing—Recommended Practice. PTA CP-CP-00-G Corrosion Protection—Cathodic Protection—Guide. Amoco A CP-CP-GAOP-E Corrosion Protection—Cathodic Protection—Design of Galvanic Anodes for Offshore Platforms—Engineering Specification. A CP-CP-00-C Corrosion Protection—Cathodic Protection—Steel Structures— Construction Specification. A CP-CP-00-E Corrosion Protection—Cathodic Protection—Engineering Specification. A CP-CP-00-G Corrosion Protection—Cathodic Protection—Guide. A CP-CP-BRZ-R Corrosion Protection—Cathodic Protection—Thermite Brazing— Recommended Practice. A CP-CP-TST-R Corrosion Protection—Cathodic Protection—Insulating Flanges and Unions Testing—Recommended Practice. A CP-CP-UGP-C Corrosion Protection—Cathodic Protection—Electrical Bonding of Underground Pipe—Construction Specification. ARCO Engineering Standard 400 Electrical - General. Copyright © 2005, BP Group. All rights reserved. The information contained in this document is subject to the terms and conditions of the agreement or contract under which the document was supplied to the recipient’s organization. None of the information contained in this document shall be disclosed outside the recipient’s own organization without the prior written permission of the Director of Engineering, BP Group, unless the terms of such agreement or contract expressly allow.
  3. 3. Draft 16 December 2005 GP 06-31 Guidance on Practice for Cathodic Protection Page 3 of 39 Table of Contents Page Foreword ........................................................................................................................................ 2 Introduction..................................................................................................................................... 5 1. Scope .................................................................................................................................... 6 2. Normative references............................................................................................................. 6 3. Terms and definitions............................................................................................................. 7 4. Symbols and abbreviations.................................................................................................. 10 5. The importance of cathodic protection ................................................................................. 11 5.1. Corrosion threats ...................................................................................................... 11 5.2. Hazards identification and mitigation......................................................................... 11 6. Competent person ............................................................................................................... 11 7. Cathodic protection principles .............................................................................................. 11 7.1. Corrosion control....................................................................................................... 11 7.2. Coatings ................................................................................................................... 12 7.3. Cathodic protection................................................................................................... 12 7.4. Polarization............................................................................................................... 14 7.5. Protection criteria onshore ........................................................................................ 14 7.6. Protection criteria offshore ........................................................................................ 14 7.7. Current density ......................................................................................................... 15 7.8. Under-protection and over-protection........................................................................ 16 7.9. IR drop...................................................................................................................... 16 7.10. Coupons ................................................................................................................... 16 8. Galvanic anode systems...................................................................................................... 17 8.1. Galvanic anodes ....................................................................................................... 17 8.2. Aluminium anodes .................................................................................................... 17 8.3. Magnesium anodes................................................................................................... 18 8.4. Zinc anodes .............................................................................................................. 18 8.5. Applications marine and offshore.............................................................................. 18 8.6. Applications onshore................................................................................................. 19 8.7. Connection................................................................................................................ 19 8.8. Installation................................................................................................................. 20 8.9. Risk of failure............................................................................................................ 20 9. Impressed current anode systems ....................................................................................... 21 9.1. Introduction............................................................................................................... 21 9.2. Applications marine and offshore.............................................................................. 21 9.3. Applications onshore................................................................................................. 23 9.4. Power sources.......................................................................................................... 25 9.5. Anode types.............................................................................................................. 26
  4. 4. Draft 16 December 2005 GP 06-31 Guidance on Practice for Cathodic Protection Page 4 of 39 9.6. Reference electrodes................................................................................................ 28 9.7. Cables and connections............................................................................................ 28 9.8. Installation................................................................................................................. 29 9.9. Risk of failure............................................................................................................ 29 10. Hybrid systems .................................................................................................................... 29 11. Retrofit systems................................................................................................................... 30 12. Advantages and disadvantages – galvanic and impressed current systems ........................ 31 13. Design objectives and considerations .................................................................................. 32 14. Safety and integrity .............................................................................................................. 34 Annex A (Informative) Basics of corrosion..................................................................................... 36 A.1 General................................................................................................................................ 36 A.2 Anodic and cathodic reactions ............................................................................................. 36 A.3 Electrolytes.......................................................................................................................... 37 A.4 Resistivity ............................................................................................................................ 37 A.5 Bimetallic corrosion.............................................................................................................. 38 A.6 Microbial corrosion............................................................................................................... 38 A.7 Stray current corrosion......................................................................................................... 39 A.8 Electrochemical series......................................................................................................... 39 List of Tables Table 1 - Corrosion protection with reference to the steel potential in seawater ............................ 15 Table 2 - Current densities required to protect carbon-steel.......................................................... 15 Table 3 - Galvanic anode materials............................................................................................... 17 Table 4 - Resistivity limitations for magnesium and zinc anodes................................................... 19 Table 5 - Electrochemical characteristics of impressed current anodes ........................................ 28 Table 6a – Galvanic anodes ......................................................................................................... 31 Table 6b – Impressed current ....................................................................................................... 32 Table A.1 - Soil resistivity.............................................................................................................. 38 Table A.2 - Galvanic/electrochemical series of metals .................................................................. 39 List of Figures Figure 1 - Typical magnesium anode array that has been horizontally installed............................ 20 Figure 2 - Typical distributed impressed current cathodic protection system................................. 23 Figure A.1 - Electrochemical corrosion schematic......................................................................... 37
  5. 5. Draft 16 December 2005 GP 06-31 Guidance on Practice for Cathodic Protection Page 5 of 39 Introduction Cathodic protection of facilities is a key component of integrity management. Corrosion protection of onshore and offshore buried and submerged metallic structures shall be supplemented with cathodic protection. This document explains the importance of cathodic protection and gives guidance on how it should be applied. Cathodic protection is mandated by “Getting Health Safety and Environment Right” and by the Group “Integrity Management Standard,” as follows: “All BP Operations shall ensure processes are in place to confirm that facilities, and equipment used in that operation, are fit for service with the goals of avoiding loss of containment and maintaining structural integrity throughout the full lifecycle of the facility and equipment in question. For this purpose, the SPA IM shall ensure that integrity management and process safety issues are identified and managed.” This document falls within the corrosion series of technical practices and is the base guidance document related to cathodic protection. The following documents provide more detailed guidance on design and good practices: Corrosion Protection GP 06-31 Guidance on Practice for Cathodic Protection. GP 06-32 Guidance on Practice for Cathodic Protection of Onshore Pipelines. GP 06-33 Guidance on Practice for Cathodic Protection of Onshore Structures and Equipment. GP 06-36 Guidance on Practice for Cathodic Protection - Maintenance and Monitoring. GIS 06-311 Guidance on Industry Standard for Cathodic Protection – Procurement.
  6. 6. Draft 16 December 2005 GP 06-31 Guidance on Practice for Cathodic Protection Page 6 of 39 1. Scope a. This document provides guidance on the use of cathodic protection to mitigate (i.e. control and minimise) external and internal corrosion of metallic structures, buried or immersed in an electrolyte, i.e. an aqueous or soil environment. b. This GP provides guidance for the management, implementation, and continuous improvement of a cathodic protection system for pipelines, equipment, and structures, with emphasis on those that are considered to be health, safety, and environmentally critical. c. This GP defines the elements required to develop and implement cathodic protection within the larger context of the Integrity Management (IM) Standard. d. Cathodic protection should be applied to a wide variety of production and process equipment, offshore and onshore, including (but not limited to) the following: 1. Offshore: a) Fixed and floating production facilities. b) Drilling fluid storage tanks. c) Subsea systems, e.g. well-heads, risers, mooring systems, flowlines and pipelines. d) Firewater and sea water pump caissons. e) Coastal/marine terminals, e.g. wharfs, jetties, mooring and breasting dolphins. 2. Onshore: a) Above and below ground storage tanks and vessels. b) Buried process and utility pipelines. c) Well casings. d) Shell & tube heat exchangers using cooling water. e) Structures. 2. Normative references The following normative documents contain requirements that, through reference in this text, constitute requirements of this technical practice. The latest edition of the normative document referred to applies. American Petroleum Institute (API) API RP 580 Risk Based Inspection & Integrity Management. BP Integrity Management (IM) Standard GP 06-36 Guidance on Practice for Cathodic Protection - Maintenance and Monitoring. GIS 06-311 Guidance on Industry Standard for Cathodic Protection – Procurement. GIS 06-601 Guidance on Industry Standard for Coating of Metal Surfaces and Equipment.
  7. 7. Draft 16 December 2005 GP 06-31 Guidance on Practice for Cathodic Protection Page 7 of 39 Euro Norm (EN) EN 12473 General Principles of Cathodic Protection in Sea Water. EN 12954 Cathodic Protection of Buried or Immersed Metallic Structures - General Principles and Applications for Pipelines. Det Norske Veritas (DNV) DNV RP-B401 Recommended Practice Cathodic Protection Design. DNV RP-F103 Recommended Practice Cathodic Protection of Submarine Pipelines by Galvanic Anodes. International Organization for Standardization (ISO) ISO 15589-1 Petroleum and Natural Gas Industries - Cathodic Protection of Transportation Systems - Part 1: On-Land Pipelines. ISO 15582-2 Petroleum and Natural gas Industries - Cathodic Protection of Transportation Systems - Part 2: Offshore Pipelines. NACE International (NACE) NACE RP0169 Control of External Corrosion on Underground or Submerged Metallic Piping Systems. NACE RP0186 Application of Cathodic Protection for Well Casings. NACE RP0193 External Cathodic Protection of On-Grade Carbon Steel Storage Tank Bottoms. NACE RP0196 Galvanic Anode Cathodic Protection of Internal Submerged Surfaces of Steel Water Storage Tanks. NACE RP0285 Corrosion Control of Underground Storage Tank Systems by Cathodic Protection. NACE RP0286 Electrical Isolation of Cathodically Protected Pipelines. NACE RP0388 Impressed Current Cathodic Protection of Internal Submerged Surfaces of Carbon Steel Water Tanks. NACE RP0572 Design, Installation, Operation and Maintenance of Impressed Current Deep Groundbeds. NACE RP0575 Internal Cathodic Protection Systems in Oil-Treating Vessels. NACE RP0387 Metallurgical and Inspection Requirements for Cast Sacrificial Anodes for Offshore Applications. NACE TM0190 Impressed Current Test Method for Lab. Testing of Aluminium Anodes. NORSOK NORSOK Std M-503 Cathodic Protection. 3. Terms and definitions For the purposes of this GP the following terms and definitions apply: Anaerobic Absence of un-reacted or free oxygen in the electrolyte Anode The electrode at which oxidation or corrosion of some component occurs
  8. 8. Draft 16 December 2005 GP 06-31 Guidance on Practice for Cathodic Protection Page 8 of 39 Anode backfill Material with a low resistivity, which may be moisture-retaining, immediately surrounding a buried anode for the purpose of decreasing the effective resistance of the anode to the electrolyte Bond Metal conductor, usually of copper, connecting two points on the same or on different structures usually with the intention of making the points equipotential Buried structure Any metal construction built or laid beneath ground level or built on ground level and then covered with earth Calcareous deposit A layer consisting of a mixture of calcium carbonate and magnesium hydroxide deposited on surfaces being cathodically protected because of the increased pH adjacent to the protected surface Cathode The electrode of an electrolytic cell at which reduction is the principal reaction Cathodic protection (1) Reduction of corrosion rate by shifting the corrosion potential of the electrode toward a less oxidizing potential by applying an external electromotive force. (2) Partial or complete protection of a metal from corrosion by making it a cathode, using either a galvanic or an impressed current. Cell Electrochemical system consisting of an anode and a cathode immersed in an electrolyte. The anode and cathode may be separate metals or dissimilar areas on the same metal. The cell includes the external circuit, which permits the flow of electrons from the anode toward the cathode. Coating defect Deficiency in protective coating (e.g. flaws, holidays, porosity) Copper/saturated copper sulphate reference electrode (Cu/CuSO4) Reference electrode consisting of copper in saturated solution of copper sulphate Coupon Representative metal sample used to quantify the extent of corrosion or the effectiveness of applied cathodic protection Drain point Location of negative cable connection to protected structure through which protection current returns to its source Driving voltage Anode close circuit potential, minus polarized cathode (steel) potential, expressed in volts Earthing (Grounding) Earthing (Grounding) may be described as a system of electrical connections to the general mass of earth. The characteristic primarily determining the effectiveness of an earth (ground) electrode is the resistance, which it provides between the earthing (grounding) system and the general mass of earth. Electrical isolation The condition of being electrically separated from other metallic structures or the environment
  9. 9. Draft 16 December 2005 GP 06-31 Guidance on Practice for Cathodic Protection Page 9 of 39 Electrolyte Liquid, or the liquid component in a medium such as soil, in which electric current flows by the movement of ions Electrolyte resistivity Electric resistance of the electrolyte assuming the electrolyte is homogeneous, expressed in Ohm-m Electro motive force (e.m.f.) Naturally occurring open circuit electrical potential difference between the anode and the cathode Galvanic anode Electrode providing current for cathodic protection by means of galvanic action Groundbed System of buried or immersed galvanic or impressed current anodes Impressed current anode Electrode that supplies current for cathodic protection by an impressed current source Ion Atom or group of atoms, carrying a negative or positive charge Interference Any change of the structure to electrolyte potential which is caused by foreign electrical sources. IR drop Voltage due to any current, developed in an electrolyte, such as soil, between reference electrode and metal of structure, in accordance with Ohm’s law (U(voltage) = I (current) x R (resistance)) Isolating joint Electrically discontinuous connection between two lengths of pipe, inserted to provide electrical discontinuity between them, e.g. monolithic isolating joint, isolated flange Natural potential Structure to electrolyte potential measured with no cathodic protection applied OFF potential Structure to electrolyte potential measured immediately after synchronous interruption of all sources of applied cathodic protection current ON potential Structure to electrolyte potential measured with cathodic protection current flowing Permanent reference electrode Permanently buried or immersed reference electrode designed for long life and installed close to the structure pH Measure of the activity of hydrogen ions (H+) in a electrolyte/solution and, therefore, its acidity or alkalinity; acidic (0-7), neutral (7) or alkaline (7-14)
  10. 10. Draft 16 December 2005 GP 06-31 Guidance on Practice for Cathodic Protection Page 10 of 39 Polarization A shift in the potential of an electrode (structure) from the equilibrium value as a result of current flow through its surface Protected structure Structure to which cathodic protection is effectively applied Protection current Current made to flow into a metallic structure from its electrolytic environment in order to effect cathodic protection of the structure Protection potential The structure to electrolyte potential for which the metal corrosion rate is acceptable Sacrificial anode See galvanic anode Silver/silver chloride reference electrode (Ag/AgCl) Reference electrode consisting of silver, coated with silver chloride, in an electrolyte containing a fixed concentration of chloride ions Structure-to-electrolyte potential The measured potential of a structure/electrode in the soil/electrolyte relative to the potential of a reference electrode, at a point sufficiently close to, but not actually touching, the structure Stray current corrosion Corrosion resulting from direct current flow through paths other than circuit intended Sulphate reducing bacteria (SRB) Organisms found in most soils and natural waters which reduce sulphate to sulphide in the absence of oxygen. When sulphide is liberated, it reacts with iron in typically tubed culture medium to form iron sulphide, thus accelerating corrosion. Transformer rectifier A device which converts alternating current (a.c.) to direct current (d.c.). The d.c. voltage derived in this way is used as a power source for impressed current cathodic protection systems. 4. Symbols and abbreviations For the purpose of this GP, the following symbols and abbreviations apply: AgCl Silver chloride a.c. Alternating current CuSO4 Copper sulphate d.c. Direct current e.m.f. Electro motive force EPTG Exploration Production Technology Group
  11. 11. Draft 16 December 2005 GP 06-31 Guidance on Practice for Cathodic Protection Page 11 of 39 IM Integrity Management SRB Sulphate Reducing Bacteria SPA IM BP Operation Single Point of Accountability wrt with respect to 5. The importance of cathodic protection 5.1. Corrosion threats Corrosion related to lack of or insufficient cathodic protection (CP) can have catastrophic consequences on hydrocarbon production and treatment facilities. Corrosion damage causing loss of containment may seriously harm the environment, personal health and safety, production, revenue, as well as BP’s reputation both locally and globally. 5.2. Hazards identification and mitigation a. Corrosion hazards related to lack of or ineffective cathodic protection shall be systematically identified for BP operated facilities according to the requirements of the IM standard. b. A Hazard and Risk Register shall be developed for each BP Operation to provide clear links from the identified hazards to measures, systems, processes, and procedures implemented to manage/mitigate the risks. In this context, maintenance and inspection activities shall be planned and carried out according to internationally recognized Risk Based Methodologies, e.g. API RP 580. c. Approved recommended mitigations shall be implemented as reasonably practicable. 6. Competent person a. Design, installation and testing of a cathodic protection shall be carried out under the direction of a ‘Competent Person’ in this field. The competent person shall be of Chartered status, with relevant qualifications from a recognised body such as the National Association of Corrosion Engineers (NACE), the Institute of Corrosion (ICorr), or equivalent. b. Standard practice should be to utilize a specialist contractor, experienced in cathodic protection applications. If this is not possible or if the cathodic protection system forms part of a larger contract, the ‘Competent Person’ shall be involved by supervising, approving, or consulting continually during the contract. c. The requirements for Competent Personnel shall be in accordance with Section 2 - Competence of the IM standard. 7. Cathodic protection principles 7.1. Corrosion control a. For effective corrosion control, engineering principles and procedures should be used to minimise corrosion to an acceptable level by the most economical method in a lifetime perspective. It is rarely practical or economical to eliminate corrosion completely.
  12. 12. Draft 16 December 2005 GP 06-31 Guidance on Practice for Cathodic Protection Page 12 of 39 b. Corrosion should be prevented by removing one or more of the conditions which allow corrosion to occur (see Annex A). c. The following corrosion control principles shall be adhered to when designing and installing cathodic protection systems. 7.2. Coatings The primary corrosion control principle for the use of coatings is to provide an isolating barrier between the metal and its surrounding electrolyte and to reduce the flow of corrosion currents (including stray currents). However, coatings unfortunately always contain defects and, in service, further flaws develop over a period of time. a. A combination of applying both a coating and cathodic protection results in the most practical and economical overall corrosion protection system and shall be the primary principle for corrosion protection of immersed and/or buried structures operated by BP. b. Most of the corrosion protection is provided by the coating while cathodic protection provides protection to deficiencies in the coating and ensures the integrity of the associated structure. As the coating degrades with time, the activity of the cathodic protection system develops to ensure continued efficient protection of the defects in the coating. The provision of a protective/insulating coating to the structure greatly reduces the current demand for cathodic protection of the metallic surface and the use of a well- applied and suitable coating increases the effective spread of cathodic protection current. c. The coatings that shall be used are those that have a high electrical resistance, shall be continuous, and shall adhere strongly to the surface to be protected. Furthermore, coating characteristics shall include: stability in the environment, abrasion resistance, and compatibility with the alkaline environment created by cathodic protection. d. GIS 06-601 shall be referred to for details of surface preparation before applying any coating system. Correct surface preparation is vital for coating systems and often governs service life of the coating system. The following documents provide details of coating systems that can be deployed on cathodically protected steel structures. • GP 06-40 Pipeline Coating Guide and Selection. • GP 06-63 Internal Coatings. 7.3. Cathodic protection 7.3.1. General The principle of cathodic protection is in connecting an external anode to the metal to be protected and the passing of an electrical d.c. current so that all areas of the metal surface become cathodic and, therefore, do not corrode. The external anode may be a galvanic anode where the current is a result of the potential difference between the two metals or it may be an impressed current anode where the current is impressed from an external d.c. power source. a. Cathodic protection shall be applied to coated immersed and buried structures, with the coating providing the primary form of corrosion protection. The current requirements are too excessive for uncoated systems. b. The protected structure shall be fully electrically continuous for cathodic protection to be applied effectively.
  13. 13. Draft 16 December 2005 GP 06-31 Guidance on Practice for Cathodic Protection Page 13 of 39 Highly resistive connections restrict the amount of current afforded to the structure section being protected and areas of discontinuity can corrode. Note: When cathodic protection is applied to a metal surface (cathode), the chemical reaction can form a scale or deposit which increases the electrical resistance and reduces the flow of current. Consequently, the current required to achieve cathodic protection initially is more than that required to maintain protection once polarisation has been achieved. Cathodic protection reduces the corrosion rate of a metallic structure by lowering its corrosion potential, bringing the metal closer to a passive state. The two main methods of achieving this goal are by using: • Galvanic anodes with potential lower than the metal to be protected (see Table A.2). • An impressed current provided by an external d.c. source. 7.3.2. Galvanic anodes When two metals are electrically connected to each other in an electrolyte, e.g. seawater, electrons flow from the more active metal to the other due to the difference in the electrical potential, the so called “driving force”. When the most active metal (anode) supplies current, it gradually dissolves into ions in the electrolyte and, at the same time, produces electrons which the least active (cathode) receives through the metallic connection with the anode. The result is that the cathode is negatively polarized and, hence, is protected against corrosion. To calculate the rates at which these processes occur, one has to understand the electrochemical kinetics associated with the complex sets of reactions that can all happen simultaneously on these metals. Galvanic anodes include the following main types: • Aluminium. • Magnesium. • Zinc. Aluminium anodes need catalysts, e.g. indium to be efficient as anode. Zinc anodes are anodic to iron in general, but the polarity reverses at temperatures above 60°C (140°F). Zinc anodes are generally specified to strict chemical composition; see GIS 06-311. 7.3.3. Impressed current anodes Cathodic protection can be applied if the metal to be protected is coupled to the negative pole of a d.c. source, while the positive pole is coupled to an auxiliary anode. Since the driving voltage is provided by the d.c. source, there is no need for the anode to be more active than the structure to be protected. In fact, it is possible to use anodes which can remain inert (non-consumable) during impressed cathodic protection. It is also possible to use semi-consumable anodes such as graphite and high silicon iron or consumable anodes such as scrap iron, for example. Items to be protected shall be electrically connected and should have a welded or brazed connection to an anode. For bolted or clamped assemblies without an all-welded brazed
  14. 14. Draft 16 December 2005 GP 06-31 Guidance on Practice for Cathodic Protection Page 14 of 39 electrical grounding, the electric continuity should be assured. Coating on contact surfaces shall be removed before assembly. 7.4. Polarization Polarization is often described as the ‘potential change’ of the structure from the natural potential observed as cathodic protection is applied. Polarization is the change in the potential of an electrode as the result of current flow. As more current is applied, the degree of polarisation increases. Once polarization has been achieved the rate of electrochemical reactions slows down. The degree of polarization at the structure to electrolyte interface is measured in terms of the potential difference between the surface of a structure and its electrolytic environment. 7.5. Protection criteria onshore a. On buried carbon steel structures, measured protection criteria shall be at the following or more negative level. 1. Without risk of SRB activated corrosion: –0,850 V wrt Cu/CuSO4 reference electrode. Carbon-steel is the most common material used for onshore structures. Theoretically, C-steel must reach –0,850 V wrt Cu/CuSO4 reference electrode to prevent corrosion. A structure at this level or more negative is considered to be cathodically protected. 2. In anaerobic conditions in which there is a risk of SRB activated corrosion: –0,950 V wrt Cu/CuSO4. b. The protection criteria shall be measured without the influence of current flowing through the electrolyte (see IR drop, clause 7.9, instant-OFF potential measurement). 7.6. Protection criteria offshore a. For submerged offshore structures, cathodic protection criterion is –0,800 V wrt Ag/AgCl/seawater. In an offshore environment where SRB occur in the seabed, the accepted criterion for cathodic protection is –0,900 V wrt Ag/AgCl/seawater reference electrode. b. High-purity zinc alloy may also be used as a reference electrode for measuring solution potentials in seawater applications. These types of reference electrodes are less accurate than Ag/AgCl/seawater but are mechanically more robust. Table 1 below shows corrosion, cathodic protection, and over polarization of steel as a function of electrode potential in an offshore environment. This table demonstrates how protection levels vary for different levels of potential on steel in seawater.
  15. 15. Draft 16 December 2005 GP 06-31 Guidance on Practice for Cathodic Protection Page 15 of 39 Table 1 - Corrosion protection with reference to the steel potential in seawater 7.7. Current density a. Detailed understanding of environmental conditions is required to determine cathodic protection design parameters and, in particular, the cathode current density in order to achieve and maintain adequate cathode polarization. The environmental factors which influence cathode current density requirements are presented in Annex A clause A.3. b. The key influencing factors are as follows: 1. Increased availability of oxygen at the surface of the metal directly increases current density. This may occur because of increased water flow or turbulence. Thus, current densities to structures in sea water, rivers, etc. are likely to vary continuously. 2. The pH of the environment has a great effect on current density. 3. The presence of coatings, marine fouling, and calcareous deposits have a significant effect on current density. c. Some typical values of current density for steel are shown in Table 2. d. The total anode current shall be determined from the area of the structure. Table 2 - Current densities required to protect carbon-steel Potential (V) Ag/AgCl/seawater Grade of corrosion protection Potential (V) Zn/seawater - 0,40 - - + 0,65 Intense corrosion - 0,50 - - + 0,55 Freely corroding - 0,60 - - + 0,45 - 0,70 - Some protection - + 0,35 Threshold of full protection - 0,80 - - + 0,25 For normal and aqueous conditions - 0,90 - High risk of HIC of high strength steels - + 0,15 - 1,00 - Some over polarization recommended for - + 0,05 SRB conditions - 1,10 - - - 0,05 Increasing over polarization Can affect adhesion of some coatings - 1,50 - Adverse effect on fatigue life - - 0,45 Environment Current density A/m 2 (A/ft 2 ) Acidic solutions 350 – 500 (33 – 46) Saline solutions 0,3 – 10 (0,03 – 0,9) Seawater 0,05 – 0,15 (0,005 – 0,014) Saline mud 0,025 – 0,05 (0,002 – 0,005)
  16. 16. Draft 16 December 2005 GP 06-31 Guidance on Practice for Cathodic Protection Page 16 of 39 7.8. Under-protection and over-protection Under-protection is not acceptable and appropriate measures shall be carried out to reinstate protection. Under-protection or more positive values than the polarization criteria results in inability to mitigate effectively against corrosion. Over-protection occurs when excessive current levels are applied by the cathodic protection system, promoting an increase in electrolyte pH at the metal surface. This can damage a coated system and is often called ‘cathodic disbondment’. At excessive negative levels, hydrogen is also evolved. Hydrogen can damage any coating present and can also result in hydrogen embrittlement of high tensile metals. This condition depends on the metal being protected, the environment, and the polarised potential. 7.9. IR drop Potential measurement between a metal surface and electrolyte interface can be considerably affected by potential drop due to the resistance of the surrounding electrolyte and the protection current flowing through it to the structure. This is known as the IR drop, and has the effect of making the measured potential appear more negative than the actual potential at the metal/electrolyte interface. The IR drop is directly related to the electrolyte resistivity and resistance of any coatings used. a. To record an accurate potential reading, excluding the IR drop, the reading shall be taken with the CP system switched off. The reading should be taken immediately (within one second) after the system is switched off, as a delay may result in the polarization film starting to decay. This recording is referred to as a polarized or ‘instant-OFF’ potential. b. Reference electrode should be placed as close as practical to the metal surface being measured. 7.10. Coupons Coupons are representative metal samples used to quantify the extent of corrosion or the effectiveness of applied cathodic protection. a. A coupon should be constructed from the same material used as the protected structure. The surface area of the coupon shall reflect the expected size of a typical coating defect. b. Two separate test leads shall be connected to the coupon. One test lead shall be connected between the coupon and the structure, and the other shall measure the potential of the coupon with respect to a reference electrode. c. By temporarily disconnecting the bond cable between the coupon and the structure, an “instant off” potential can be obtained for the coupon. d. On buried pipelines, coupons are often installed in conjunction with a permanent reference electrode. The reference electrode should be placed as close as possible to the coupon to minimise any errors in the measurement of potentials due to IR drop in the electrolyte. Note: If a large number of coupons are used, the current demand of the cathodic protection system could increase significantly. In such cases, the total surface area of the coupons shall be taken into account when designing the system.
  17. 17. Draft 16 December 2005 GP 06-31 Guidance on Practice for Cathodic Protection Page 17 of 39 8. Galvanic anode systems 8.1. Galvanic anodes a. Galvanic (sacrificial) anodes protect the structure they are connected to by corroding in an electrolyte. They are made of alloys based on magnesium, zinc, and aluminium and are more electro-negative than carbon-steel (see Table A.2) (notice exception in clause 7.3) and do not require an external power source. b. Anode current output shall be determined for the complete life-cycle of the structure, to ensure that the anode design meets initial and final cathode current demand requirements. c. Galvanic anodes should be used in low resistivity environments, e.g. less than 100 ohm-m (330 Ώ-ft), where the small driving voltages between the anodes and Carbon-steel are able to produce useful current outputs. d. The active surface of a sacrificial anode shall never be painted or isolated from the electrolyte. The electrical output of an anode is given by current capacity which is expressed in Ah/kg (Ampere-hour/kg) or kg/Ay (kg/Ampere-year) [A-hr/lb (Ampere-hour/pound or lb/A-yr (pound/Ampere-year)]. Table 3 shows typical ranges of values for closed circuit potential current capacity and consumption rates for some types of sacrificial alloys. e. Efficiency shall be determined by a number of factors including nature of the environment, operating current density and metallurgical microstructure. It is apparent that if the cathode reaction rate on the anode is low, then the efficiency is high. For instance, severe pitting and intergranular attack may result in a piece of the anode becoming detached without complete consumption of the electric charge in that piece. f. Anode alloy shall comply with GIS 06-311. Table 3 - Galvanic anode materials 8.2. Aluminium anodes Aluminium is normally stable due to the formation of a thin protective oxide. To disrupt the physical integrity of the protective film, several aluminium anode alloys have been developed. Alloying elements found to induce activation include Hg, In, Sn, Ga, Bi, Zn, Cd, Mg and Ba. Commercially successful aluminium anodes mainly contain zinc (0,5-4%) with either indium (0,01-0,05%) or mercury (0,030-0,050%). Alloy Common Environment Potential - Reference Electrode (V) Capacity (Ah/kg) [A-hr/lb] Consumption rate (kg/Ay) [lb/A-yr] Aluminium 2 – 6% Zn, 0,01 – 0,03% In Seawater/ sea-bed mud –1,10 (Ag/AgCl/seawater) 1 800 – 2 500 (816 – 1 134) 4,8 – 3,5 (10,5 – 7,7) Magnesium 1.5% Mn 6% Al, 3% Zn Soil/fresh water Soil/fresh water –1,7 (Cu/CuSO4) –1,5 (Cu/CuSO4) 1 200 (544) 1 200 (544) 7,3 (16,1) 7,3 (16,1) Zinc – Type 1 US Mil spec 1800K 0,5% Al, 0,07% Cd, max. 0,005% Fe Zinc – Type 2 0,005% Al, 0,003% Cd, max. 0,001 4%Fe Seawater/ sea-bed/ brackish water Brackish water/ backfills/ soil –1,05 (Ag/AgCl/seawater) –1,00 (Ag/AgCl/seawater) 760 – 780 (345 – 354) 740 – 760 (336 – 345) 11,5 – 11,2 (25,4 – 24,7) 11,8 – 11,5 (26,0 – 25,4)
  18. 18. Draft 16 December 2005 GP 06-31 Guidance on Practice for Cathodic Protection Page 18 of 39 a. Due to environmental concerns about the use of mercury, BP operated facilities shall only use aluminium anodes alloyed with zinc and indium. When coupled to a more noble metal in a chloride containing environment, the aluminium anode corrodes by pitting rather than uniform dissolution; still, the efficiency is normally greater than 90%. For aluminium-zinc-indium (Al-Zn-In) anodes, iron is an impurity that can severely inhibit anode performance, ref. the specified concentration limitations in GIS 06-311. b. Aluminium is basically used as an anode for seawater applications. However, as it does not stay active when buried in soil, aluminium anodes should not be used for onshore cathodic protection applications. 8.3. Magnesium anodes Magnesium anodes should theoretically be able to provide a large driving potential. However, in practice it does not, the low efficiency (50-60%) has been attributed to hydrogen evolution at local cathodes and complex surface chemistry at the anode surface. The theoretical current capacity for a magnesium anode is approximately 2 200 Ah/kg (998 A-hr/lb) while actual is in the range of 1 200 Ah/kg (544 A-hr/lb). a. Alloying elements (Al, Zn, Mn) should be added to reduce the rapid activation of Magnesium anodes. b. Magnesium alloy anodes, because of their large driving voltage, should be used in soils, water tanks, and similar high-resistance environments. c. In high conductive environments such as seawater, magnesium anodes shall not be utilised due to risk of overprotection and the high consumption rates. d. Magnesium alloy anodes should only be used where frequent replacement is practical. 8.4. Zinc anodes a. Because zinc is nobler than iron above 60°C (140°F), zinc anodes shall only be used for ambient temperature applications. Zinc anodes should be specified to strict chemical compositions because of harmful impurities which impair the performance of the anode; reference the specified concentration limitations in GIS 06-311. To improve the efficiency of zinc anodes, they are alloyed with aluminium and cadmium which combine with the iron to form less harmful intermetallics. As a result, anode efficiencies greater than 90% can be achieved. b. Zinc anodes, because of their driving voltage, may be used for the protection of coated pipelines in high conductivity electrolytes, such as in seawater, and at ambient temperatures. c. Zinc anodes should also used in applications in which spark hazards need to be avoided as in storage tanks containing flammable hazards. 8.5. Applications marine and offshore a. Aluminium or zinc anodes should be utilised for marine and offshore applications. However, aluminium anodes should be the preferred option because aluminium has about 3 times higher current capacity than zinc; see Table 3. b. Zinc anodes should only be considered for systems operating at ambient temperature. c. Magnesium should not be used offshore owing to the alloy’s high driving potential and consumption rate in seawater.
  19. 19. Draft 16 December 2005 GP 06-31 Guidance on Practice for Cathodic Protection Page 19 of 39 d. Driving potential of aluminium and zinc alloys is considered to be low, and therefore minimises risk of cathodic disbondment of high performance coatings. There is also reduced risk of thermite or incentive sparking of these alloys when in contact with steel. 8.6. Applications onshore a. Galvanic anodes of magnesium or zinc anodes should be considered for protection of onshore high quality coated applications. However, galvanic anodes should only be used for small diameter pipelines or for short lengths of larger diameter pipelines in low resistivity soils, water, swamps, or marshes. b. The anodes may also be used for protecting buried tanks or the bases of above ground storage tanks and other types of storage vessels. c. Galvanic anodes may be used to provide temporary protection of newly laid pipelines or for localised (hot-spot) protection to supplement impressed current systems. d. Selection of anode alloy for a particular application shall be restricted by the resistivity of the electrolyte. Table 4 offers guidance on use of magnesium and zinc anodes, and maximum resistivity restrictions. Table 4 - Resistivity limitations for magnesium and zinc anodes Magnesium (6%Al-3%Zn) Magnesium (high purity) Zinc – Type 1 Zinc – Type 2 < 50 ohm-m (164 Ώ-ft) < 100 ohm-m (328 Ώ-ft) < 5 ohm-m (16 Ώ-ft) < 30 ohm-m (98 Ώ-ft) e. For most onshore applications in soil, pre-packaged anodes with specially formulated backfill should be used, as specified in GIS 06-311. f. Only magnesium alloys should be used in potable water, due to non-toxic corrosion products. 8.7. Connection a. Good electrical continuity shall be maintained between the structure and the galvanic anode system. b. Insert design shall accommodate anode weight and the forces likely to be encountered during its lifetime, including impact, storm damage, wave action, and ice flows. c. Stand-off and flush mounted anode types should be welded directly to the structure surface or to steel doubler plates pre-welded to the structure. d. Bracelet anodes should be fitted around a pipeline and bolted or welded together. 1. The electrical continuity between the anode bracelet insert and the pipeline should be provided by cable tails pre-attached to the insert. 2. Each bracelet should have two bonding leads attached. 3. Cable tails should be connected to the pipeline or riser using thermite welds or pin brazed connections. e. For onshore pipelines, the anode cable tail should be brought into a test post at the surface and be connected to a pre-attached cable from the pipeline. Alternatively, bracelet anode inserts can be supplied with a steel extension bar that is welded to a doubler plate pre-installed on the pipeline. f. Connection of the anode insert to the structure or cable shall become part of the protected structure and receive protective current.
  20. 20. Draft 16 December 2005 GP 06-31 Guidance on Practice for Cathodic Protection Page 20 of 39 8.8. Installation a. Galvanic anodes shall be installed in such a way that an even current distribution is achieved and allows each element of the structure’s surface to reach the protection potential. b. The underside of stand-off anodes should be installed 300 to 350 mm (12 in to 14 in) from the cathode surface. c. Flush mounted anodes should only be used for fabrications in which space limitation prevents use of stand-off anodes or if the cathode current densities are low. d. For onshore pipelines, galvanic anodes should be installed separately or in small groups at intervals of 30 m to 200 m (100 ft to 700 ft). Spacing depends on the current demand from the pipeline and current provided by individual anodes. e. Anodes should be buried just below the pipeline, close to the pipeline as shown in Figure 1 (the anodes can also be vertically installed). Figure 1 - Typical magnesium anode array that has been horizontally installed Note: Galvanic anodes should be spaced more closely in areas of high current demand. f. If additional current demand is required at remote locations from impressed current anodes and groundbeds, galvanic anodes should be used to supplement protection. g. For storage tanks sat on the ground, external cathodic protection of the tank base should be provided with galvanic anodes spread evenly around the tank perimeter base and installed at 2 m to 3 m (7 ft to 10 ft) depth. h. For internal protection of storage tanks, anode distribution shall depend on the type of tank floor. 8.9. Risk of failure If monitoring reveals insufficient protection, appropriate corrective measures shall be carried out as soon as reasonable. If the cathodic protection design has not been appropriately calculated, the structure may not achieve full polarization or meet the required design life of the system.
  21. 21. Draft 16 December 2005 GP 06-31 Guidance on Practice for Cathodic Protection Page 21 of 39 If anodes become detached from the structure or electrical continuity lost, this could result in a reduction in protection levels. Incorrect or deviating alloy composition can result in complete passivation of the anode under certain operating conditions. 9. Impressed current anode systems 9.1. Introduction An impressed current system provides protection using an adjustable d.c. source, with the negative terminal of the power supply connected to the steel structure (drain point) and the positive terminal connected to the anodes. a. Impressed current systems shall be designed to satisfy statutory electrical and hazardous area requirements. As operating voltages can be large by comparison with galvanic anodes, fewer larger capacity anodes are utilised. Typically, galvanic anodes designed for operating in seawater have a current output of 1 amp to 5 amps, whereas corresponding impressed current anodes have an output of 10 amps to 100 amps. b. The current delivered by the d.c. power source shall be controlled throughout the design life of the cathodic protection system to achieve and maintain an adequate protection potential over the whole steel surface of the structure. c. In addition to a d.c. power source (transformer rectifier), the following components shall be part of an impressed current system: 1. Anodes - to pass the direct current from the transformer rectifier via the electrolyte to the structure to be protected. 2. Reference electrodes - to measure the structure potential and control the output of the transformer rectifier. 3. Power cabling - which carries the direct current from the transformer rectifier to the anodes. 4. Signal cabling - which transmits potential signals from the reference electrode to the transformer rectifier. 5. Cable routing components - to facilitate the safe passage of cables between anodes, electrodes and the transformer rectifier. 9.2. Applications marine and offshore A wide range of marine and offshore structures can be protected using impressed current cathodic protection systems. An impressed current system generally consists of one or more transformer rectifiers, several anodes and a number of fixed reference electrodes. a. Transformer rectifiers with automatic potential control should be used when the environment, structure configuration, and service conditions vary frequently because that can result in large fluctuations in the current demand required to maintain polarization. Note: Fluctuations in current demand can be caused by several factors including: variations in salinity as for estuaries, change in electrolyte flow rates, large fluctuations in submerged steel areas caused by tidal variations or wetted surface areas of mobile units depending on their loading condition.
  22. 22. Draft 16 December 2005 GP 06-31 Guidance on Practice for Cathodic Protection Page 22 of 39 b. If environment and service conditions remain relatively constant and large fluctuations in current demand do not occur, manual control transformer rectifiers may be used. c. Specific areas may require a multi-zone control system in order to optimise current distribution to the cathodic protection demand. To optimise current distribution, the protection of each zone should be considered individually with its own impressed current system. d. The current output of the transformer rectifier shall be distributed to the structure by means of a number of anodes. The size, quantity, and locations of anodes shall be calculated to achieve an even distribution of protection current and in accordance to the structure’s current demand. Note: Measures shall be taken to ensure good isolation between the anodes and the structure. e. Due to the higher current output of impressed current anodes compared to galvanic anodes, they should be located whenever possible remote from the steel surface (1,5 m (5 ft) minimum) to be protected. If this is not practical, a special dielectric shield should be applied to the adjacent support surface to prevent localised high current densities, which may induce local over-polarization with potentials outside acceptable limits. Note: Over-polarization can induce both hydrogen and chlorine evolution. These effects can cause damage to high strength steels and disbondment of paint coatings and be a health and safety hazard in confined spaces. f. Dielectric shields may consist of thick applied coatings, prefabricated plastic, or elastomeric sheets, etc. 1. The material should be considered for the service intended and the life of the system. 2. The dielectric material should be resistant to chlorine, hydrocarbons, and deleterious chemicals, which can cause disbonding or degradation of the shield material. g. To measure the performance and the effectiveness of an impressed current system, reference electrodes shall be used to assess structure-to-electrolyte potentials. When using permanent reference electrodes in seawater for potential control of automatic transformer rectifiers, these should be zinc or silver/silver chloride (seawater), see clause 9.6. Note: Reference electrodes shall be isolated from the structure. They have a limited life and procedures for regular calibration and maintenance shall be established. For bare steel structures, reference electrodes are best placed at positions remote from anodes, where the effect of the cathodic protection is least, to ensure that structure-to-electrolyte potential at all points is within set limits. h. For coated structures, it is important to ensure that structure-to-electrolyte potentials do not become sufficiently negative to cause coating damage. Additional reference electrodes shall, therefore, be located close to anode positions to ensure over-protection does not occur. i. Tests should normally be made using portable equipment to confirm set limits can be maintained over the full service conditions, and to verify the corresponding structure to electrolyte potentials as measured by the permanent reference electrodes.
  23. 23. Draft 16 December 2005 GP 06-31 Guidance on Practice for Cathodic Protection Page 23 of 39 9.3. Applications onshore a. A large range of onshore structures may be protected using impressed current systems. Figure 2 below shows a typical distributed impressed current cathodic protection system. b. For well-coated and electrically continuous buried steel pipelines, many kilometres (miles) may be protected from a single power source. The normal source of current is a mains supply transformer rectifier. c. If mains power is not available, another source of supply (such as a diesel-driven, solar generator, wind generator, or thermoelectric unit) may be used. For onshore applications, transformer rectifiers may usually be manually controlled, as the environment and service conditions remain relatively constant with minimal variation in current demand. d. The current output of the transformer rectifier shall be distributed to the structure using a number of anodes. For onshore applications, these should be buried in groups, which are connected in parallel and surrounded in coke breeze. There are several different types of groundbeds: 1. Shallow horizontal. 2. Shallow vertical. 3. Deep well or borehole. 4. Distributed shallow horizontal/vertical single anodes. Note: For deepwell or borehole groundbeds, a plastic vent pipe should be installed to prevent any gas build-up. Figure 2 - Typical distributed impressed current cathodic protection system e. The type and location of anode groundbeds in relation to the structure to be protected should be carefully considered. If the separation is too small, then local over-protection can occur resulting in damage to the pipe coating and poor distribution of protective current. Alternatively, if the groundbed is too remote, cabling and power costs increase unreasonably. f. Groundbeds should normally be sited a minimum of 100 m (330 ft) from the structure to achieve even current distribution. Complex structures, such as multiple buried lines in
  24. 24. Draft 16 December 2005 GP 06-31 Guidance on Practice for Cathodic Protection Page 24 of 39 process plant or refineries, may require a distribution of smaller groundbeds individually placed much closer to the structure. Note: Anode groundbeds should be sited where the soil resistivity is as low as possible and compatible in a suitable position in relation to the structure to be protected. g. Buried cable connections shall be fully encapsulated and watertight. If possible, connections shall be made above ground in a connection pillar or anode junction box. h. Surface connection boxes should be designed to resist tampering, theft, or vandalism. i. The effectiveness of the cathodic protection system shall be monitored on a regular basis. j. In order to be able to measure pipe-to-soil potentials, it is necessary to make electrical contact with the pipe. 1. An insulated conductor shall be attached to the pipeline, and brought to a test post or surface terminal box. 2. The test posts are typically located at one kilometre intervals along the pipeline route. 3. Additional locations may be required in certain critical areas such as pipe sleeves or where exceptionally aggressive soil conditions exist. k. Additionally, if there are areas along the pipeline route that are inaccessible during normal operation, the placement of permanent reference electrodes should be considered. A further advantage of permanent reference electrodes is that readings can be taken from exactly the same location each time, ensuring consecutive readings for the structure are directly compatible. For onshore monitoring purposes, portable copper/copper sulphate reference electrodes are typically used to measure pipe-to-soil potentials. Permanent reference electrodes, by their nature, have to last the lifetime of the structure. Again, copper/copper sulphate electrodes are used for this purpose but are usually much larger and more robust. The electrodes use gels to prevent drying out during the service life and sometimes have a membrane to prevent contamination of the copper/copper sulphate with foreign matter. l. For external cathodic protection of buried tanks or above ground storage tank bases, the same general principles explained for pipelines shall apply. m. However, if groups of tanks are in close proximity or in confined locations such as petrochemical refineries, other foreign structures may have to be considered. 1. Closely distributed anode systems may be preferred as an alternative. 2. These typically consist of single anodes of mixed metal oxide or silicon iron, installed in a sealed metallic tube or canister, which is filled with coke breeze. 3. These shall be evenly distributed around the periphery of the tanks and buried directly in the ground. n. Another technique used for protecting the external area of above ground storage tank bases is to use mixed metal oxide strip or wire anodes. 1. These shall be located directly beneath the base of the tank within the clean sand foundation. 2. The mixed metal oxide strip shall be laid in a grid formation directly under the tank base to a depth of typically 0,3 m (1 ft), whereas the mixed metal wire system is laid in loops or circles at the same depth of burial.
  25. 25. Draft 16 December 2005 GP 06-31 Guidance on Practice for Cathodic Protection Page 25 of 39 3. Titanium strips called conductor bars with factory encapsulate cables shall be spot welded to the mixed metal oxide strip anodes at different locations and provide the positive feed from the transformer rectifier. 4. The mixed metal oxide wire anodes should be provided with pre-attached factory encapsulated feeder cables. o. A number of permanent reference electrodes should also be installed beneath the tank base to ensure that set limits of cathodic protection are maintained throughout the design life of the system. The advantage of using an anode system located directly beneath the tank is uniform current distribution to the base of the tank. This minimises interference or loss of current to foreign buried structures that may be in close proximity. p. Impressed current systems may be used to internally protect potable or firewater storage tanks. Note: Only magnesium anodes shall be used in potable water tanks. q. Distributed anode systems shall generally be used within tanks that are either suspended/ tethered from the roof of the tank or surface mounted. 1. Anode systems that are surface mounted shall require dielectric shields to prevent localised over-polarization and assist in achieving uniform current distribution. 2. Tanks being protected shall be vented to prevent possible build up of hydrogen and chlorine. r. With onshore cathodic protection systems, if there is going to be an extended period before the system can be energised, a temporary galvanic anode system shall be considered. 9.4. Power sources a. The power source for impressed current systems shall be a transformer rectifier with a.c. supply provided by a local electricity supply. b. If local a.c. supply is not available, alternative power sources should be considered. These include solar-electric, thermo-electric, wind, and diesel generators. c. The following shall be considered before specifying the transformer rectifier: 1. Available a.c. supply. 2. Output control – manual or automatic (constant current/constant potential). 3. Outdoor or indoor use – safe or hazardous area. 4. Type of mounting – pole, plinth or wall. 5. Type of cooling – air or oil. 6. Measuring devices – voltmeter, ammeter, etc. 7. Number of d.c. positive and negative terminals. 8. Need for provision of current interrupter. 9. Need for a.c. and/or d.c. surge protection. 10. Electrical and safety requirements for the equipment. 11. Type of environmental protection and housing. 12. Identification and rating plate details.
  26. 26. Draft 16 December 2005 GP 06-31 Guidance on Practice for Cathodic Protection Page 26 of 39 d. Transformer rectifiers shall be suitable for continuous operation throughout the design life of the impressed current system. The transformer rectifier shall be able to deliver current equal to or greater than 1,25 times the sum of calculated total electrical current demands for the zones or parts of the structure they are intended to protect. e. The output voltage of the transformer rectifier should take into account the resistance of the electric circuit comprising of cables and anodes and the back e.m.f. between the anode and the cathode. The back e.m.f. is the naturally occurring open circuit potential difference between the anode and the cathode in seawater or soil. 9.5. Anode types a. Impressed current node materials used for seawater applications should include the following: 1. Mixed metal oxide – coated titanium. 2. Platinised titanium/niobium. 3. Lead-silver-antimony. 4. Silicon iron with chrome. 5. Magnetite. b. Of the above materials, only mixed metal oxide and platinised titanium/niobium anodes should generally be used for offshore applications; these anode materials have the following properties: 1. Low rate of consumption. 2. Low anode polarization. 3. Ability to operate at high current densities. 4. Good electrical continuity. 5. High mechanical strength. 6. Good resistance to abrasion and erosion. 7. Readily fabricated into useful forms. The production of precious metal anodes requires specialised manufacturing equipment and techniques, and demands the implementation of stringent quality assurance and quality control standards to ensure the predicted in-service performance is achieved. Note: Anode material and substrate determine the maximum operating voltage and anodic current density which can be utilised. Impressed current materials suffer deterioration, dependant on the magnitude of anodic current density and applied voltage. Typical electrochemical characteristics of impressed current anodes are given in Table 5. c. Anodes shall have factory encapsulated cable connections. 1. The electrical connection between the anode lead cable and the anode body shall be made watertight and mechanically secure. 2. Cable and connection insulating materials shall be resistant to chlorine, hydrocarbons and other deleterious chemicals. d. Anode materials used for onshore, buried applications shall include the following: 1. Mixed Metal Oxide – coated titanium.
  27. 27. Draft 16 December 2005 GP 06-31 Guidance on Practice for Cathodic Protection Page 27 of 39 2. Silicon iron with chrome. 3. Magnetite. 4. Graphite (impregnated with linseed oil). 5. Scrap Iron. e. Generally, onshore anodes shall be installed in a horizontal bed or vertical column of carbon (coke). The coke breeze or carbonaceous backfill serves two purposes: 1. Prolongs anode life. 2. Acts as an anode and allows construction of a long large surface area anode giving a low resistance to earth (ground). f. The environmental impact of anode material dissolution and the breakdown of the conductive carbonaceous backfill shall be considered. The most commonly used anodes onshore are mixed metal oxide in rod, tube, wire or mesh forms and high silicon iron with chrome in rod or tubular forms. Note: Scrap iron or steel is inexpensive and easily fabricated but difficult to maintain integrity of electrical connection and has a high consumption rate and, therefore, rarely used. g. Essential design requirements of anodes, are given below: 1. Anode material shall be sufficient to meet the design life requirements based on proven anode wear rates. 2. Anode design shall have a low electrical resistance to emit the maximum current without exceeding permissible anode voltages. 3. At maximum current output, the permissible anode current density shall not be exceeded. 4. Throughout the anode design current output range, the anode shall not induce excessively negative cathode potentials, which may cause damage to the structure or disbondment of protective coatings. Note: The active surface of an impressed current anode should never be painted or isolated from the electrolyte.
  28. 28. Draft 16 December 2005 GP 06-31 Guidance on Practice for Cathodic Protection Page 28 of 39 Table 5 - Electrochemical characteristics of impressed current anodes 9.6. Reference electrodes a. Reference electrodes for marine or offshore applications shall be used to measure the steel- to-electrolyte potential and may be used to control the output current delivered by the cathodic protection system. 1. These are generally zinc or silver/silver chloride (seawater) electrodes. 2. Zinc electrodes are more robust; whereas, silver/silver chloride (seawater) electrodes are more accurate. b. For onshore applications in which permanent potential monitoring is required to control current output, copper/copper sulphate electrodes, which are specifically designed for long-term buried applications, should be used. c. Location of the reference electrodes is very important, especially when used to control the system. Locations should be determined by calculations or experience to ensure the potential of the structure is maintained within the design set limits. d. There should be no direct electrical contact between the reference electrodes and the steel structure. e. If permanent reference electrodes are not required to control current output of an impressed current system, portable reference electrodes may be used periodically to provide a general indication of the effectiveness of the system. Note: This technique is less accurate as the location of the reference electrode is usually remote from the structure, resulting in errors in potential measurements due to IR drop in the soil or seawater. It is also difficult to replicate the exact location when taking potential measurements throughout the design life of the system. 9.7. Cables and connections a. Cables shall be protected to avoid any risk of mechanical damage, as this can seriously affect the performance of an impressed current system. b. Electrical termination between the anode lead cable and the anode shall be watertight and mechanically secure. Anode Materials Consumption Rate (g/Ay) [oz/A-yr] Maximum Current Density (A/m²) [A/ft 2 ] Maximum Voltage (V) Mixed Metal Oxide– seawater Mixed Metal Oxide– buried 0,000 6 to 0,006 (0,000 02 to 0,000 2) 0,006 to 0,008 (0,000 2 to 0,000 3) 400 to 1 000 (37 to 93) 80 to 100 (7,4 to 9,3) 8 8 Platinised Titanium– seawater 0,004 to 0,012 (0,000 1 to 0,000 4) 500 to 1 000 (46 to 93) 8 Platinised Niobium– seawater 0,004 to 0,012 (0,000 1 to 0,000 4) 500 to 1 000 (46 to 93) 50 Lead-Silver – seawater 15 to 75 (0,53 to 2,65) 150 to 250 (14 to 23) 24 Silicon iron-chrome – buried 100 to 300 (3,5 to 10,6 10 to 30 (0,9 to 2,8) 50 Magnetite – buried 2 to 3 (0,07 to 0,11) 80 to 100 (7,4 to 9,3) 50 Graphite – buried 200 to 500 (7,1 to 17,6) 5 to 15 (0,5 to 1,4) 50 Scrap Iron – buried 9 000 (317) 5 (0,5) 50
  29. 29. Draft 16 December 2005 GP 06-31 Guidance on Practice for Cathodic Protection Page 29 of 39 c. When determining the cross-section of a cable conductor, the voltage drop for the length of the cable shall be taken into consideration. Note: The maximum current rating specified for a given cable conductor size, shall never be exceeded. d. Dedicated cables shall be used for permanent reference electrodes. 1. These should be screened to prevent any electrical interference. 2. These cables shall not be located next to power cables, on cable trays, or in the same conduit system. 9.8. Installation a. Installation of impressed current cathodic protection systems should be carried out under the supervision of a competent person, to ensure the installation is in accordance with the relevant drawings, specifications, and procedures. b. The impressed current system should be installed as simultaneously as the structure to be protected is being installed. 9.9. Risk of failure Generally the risks of failure to impressed current systems are much greater compared to a galvanic system. The type of installation and attachment devices used for impressed current anodes and groundbeds is, therefore, critical with respect to mechanical damage as few anodes or groundbeds are involved at relatively high output currents. The loss of an impressed current anode or groundbed may significantly reduce the performance of the cathodic protection system. If monitoring reveals insufficient protection, appropriate remedial actions shall be timely carried out to reinstate integrity. 10. Hybrid systems a. The term “hybrid system” applies to a cathodic protection system using a combination of galvanic and impressed current anodes to protect a structure. Note: Most onshore, marine, and offshore structures, which are protected by impressed current systems, also have a number of galvanic anodes installed. b. Galvanic anodes should generally be used to: 1. Provide temporary protection to the structure before the power supply to the transformer rectifiers can be energised. 2. Supplement protection levels where parts of the structure are shielded or remote from impressed current anodes or anode groundbeds. Note: The d.c. positive supply from the transformer rectifier is always connected to the anode and the negative to the structure to be protected. If ever the connections are reversed, this causes accelerated corrosion of the structure.
  30. 30. Draft 16 December 2005 GP 06-31 Guidance on Practice for Cathodic Protection Page 30 of 39 11. Retrofit systems a. Retrofit cathodic protection systems shall be considered when the design life of an existing cathodic protection system is exceeded and can no longer maintain the desired levels of protection or the system fails prematurely. b. Retrofit systems generally apply to marine and offshore structures. Galvanic anodes or impressed current systems may be used to supplement or replace existing protection systems. c. Bracelet anode assemblies comprising of flush mounted or stand-off galvanic anodes may be retrofitted to offshore structures using divers or ROVs. When using these types of anode assemblies, it is essential to achieve good, long-term electrical continuity between the steel hoops of the anode bracelet and the structural member. d. Galvanic anodes mounted on sleds may also be used to supplement protection of offshore structures and submarine pipelines. Electrical continuity between the anode sled and the structure shall be provided using minimum two armoured cables. e. Impressed current systems may also be retrofitted to offshore structures or pipelines. These systems are usually deployed as sleds, or tethered/pod configurations on the seabed, which are remote to the structure or pipeline. By comparison to a galvanic sled, an impressed current anode assembly has a much higher current output. f. Location of seabed impressed current anode assemblies shall be evaluated to avoid any interaction with foreign metallic structures in the vicinity. Typically, impressed current anode assemblies should be located between 50 m and 100 m (115 ft to 330 ft) from the structure to be protected to ensure uniform current distribution is achieved. Note: Underwater cable connections shall be avoided for impressed current installations.
  31. 31. Draft 16 December 2005 GP 06-31 Guidance on Practice for Cathodic Protection Page 31 of 39 12. Advantages and disadvantages – galvanic and impressed current systems Table 6a – Galvanic anodes Galvanic Anode Advantages Disadvantages Environment Installation Power source Anodes Control Maintenance Damage Connection Hazards Depending on the alloy, they can be used onshore and offshore. Straightforward installation. Independent of any power source. Less likely to affect neighbouring structures, because the output at any point is low. Tendency for current to be self adjusting. Generally maintenance-free. Robust, not very susceptible to mechanical damage. May be bolted or welded directly to the surface of structure to be protected. Connections are cathodically protected. Magnesium anodes can be used in potable water tanks. Not practical for use in high resistivity conditions. Restricted to well-coated structures because limited current available. Often bulky. Large quantity of anodes required for uncoated structures. Hydrodynamic loadings may be high. Anodes may restrict water flow in water system, e.g. for pump casing systems offshore. Anodes may be required at a large number of positions. Lifespan varies with conditions, so replacements may be required at different times. Magnesium should only be used in confined spaces when well vented and never in areas containing hydrocarbons. Al and Zn anodes should never be used in potable water tanks.
  32. 32. Draft 16 December 2005 GP 06-31 Guidance on Practice for Cathodic Protection Page 32 of 39 Table 6b – Impressed current 13. Design objectives and considerations The basic design objectives and considerations are in general common to both galvanic or impressed current cathodic protection systems for offshore or onshore applications. The principle design objectives are that the cathodic protection system shall: a. Be applied to buried and immersed properly coated metallic structures designed to protect the structures in their entirety for the design life. b. Comply with applicable codes and standards, e.g. certifying authority, local government and ETPs. If design codes or standards are not specified, the designer shall contact EPTG representative for advice. c. Ensure that potentials are as uniform as possible over the whole structure and that the system does not induce deleterious interactive effects on associated structures or pipelines. It is prudent that the structure that requires protection is electrically continuous. d. Consider and remediate effects on pipelines from parallel or crossing high-voltage power lines as large a.c. voltages can be induced and cause damage. Impressed Current Advantages Disadvantages Environment Installation Power source Anodes Control Maintenance Damage Connection Hazards Use less restricted by high resistivity conditions. Good flexibility. Can be applied to a wide range of structures. Controllable current output requiring fewer anodes due to higher current output. Controlled current output caters for changing conditions. Generally requires a small number of anodes. Lighter and designed to have minimum effect on water flow. Simple controls, which can be made automatic to maintain potential within close limits. Inspection can be maintained at relatively few points of the structure. Large capacity, long life systems. Fewer connections required. Can be flush mounted to the structure, preventing turbulence or water flow restriction May cause over-protection, coating disbondment or hydrogen induced cracking of high tensile steels. Requires high level of detail design and installation expertise. External power source necessary, with continuous power supply. D.c. polarity must be checked during commissioning, because misconnection with reversed polarity will accelerate corrosion. Effects on other structures that are near anode locations of protected structures should be assessed (but any interaction may be readily corrected). Monitoring and control required at regular intervals. Though designed for long life, requires regular inspection, monitoring and control. Lighter anodes less resistant to mechanical damage, therefore, loss of anodes more critical. Connection more complex requires high integrity insulation. Susceptible to water ingress at anode termination points, resulting in premature failure. Requires high integrity insulation on connection to positive side of rectifier, which are in contact with soil or water, otherwise it will corrode. Diver risk from electric shocks. Impressed current anodes must be switched off when divers are in the vicinity. Over- protection can result in coating disbondment and hydrogen induced cracking of high strength steels.
  33. 33. Draft 16 December 2005 GP 06-31 Guidance on Practice for Cathodic Protection Page 33 of 39 e. Ensure pipeline installations are not in direct contact with other foreign metallic structures, e.g. pumping stations, dissimilar metals, other cathodic protection system, storage tanks and electrical earths (grounds), as this would constitute a large current drain on the cathodic protection system. Electrical isolation may be required to limit the effects of interaction or stray currents by the installation of: 1. A monolithic isolating joint which is a factory pre-fabricated joint that is welded directly into a pipeline. Should be used for locations where repair/replacement is extremely difficult, such as high-pressure lines or offshore installations. 2. A flange isolation kit which consists of an insulation flange gasket, insulating sleeves placed over the studs, and insulating washers under the nuts. The insulation kit should, however, not be applied if the interior liquid is conductive. 3. An isolating joint which is a flange connected non-metallic or internally lined spool piece with a minimum length of 10x the pipeline’s outer diameter. The isolating joint should be used when the pipeline is carrying a conductive electrolyte/liquid. 4. Pipelines entering buildings or valve chambers shall be electrically isolated from concrete steel reinforcement by applying a suitable non-conductive material over the pipe part that passes over the concrete. The same requirement applies for above ground sections of pipelines which are supported on steel or concrete racks or bridges. f. Ensure pipelines crossings under roads or railway tracks, normally through steel casings, are provided with additional insulating material, at least twice the normal thickness, for the crossing section plus 3 m (10 ft) in each direction. g. Ensure if there is a possibility of lightning activity, lightning protection is provided to protect cathodic protection equipment such as output terminals of d.c. voltage sources and isolating joints. h. Consider having electrical earthing (grounding) devices installed for safety reasons on the structure to be protected and for pipelines evaluate if earthing (grounding) is required to mitigate the effect of induced electrical voltages. 1. If electrical safety-earthing (grounding) is required, it shall be made compatible with the CP system by installing polarization cells or diode circuits, suitably specified and rated for the purpose, in the earthing (grounding) circuit or by installing separate earthing (grounding) zinc or galvanised steel electrodes, buried in low-resistivity backfill and not in direct electrical continuity with other earthing (grounding) systems. 2. If earthing (grounding) has to be installed to mitigate the effect of a.c. induced voltages on the pipeline, this should be done at the locations where the anticipated or measured voltages to ground are highest, and where the pipeline is exposed and can be touched by personnel. i. For marine and offshore structures, provide additional corrosion protection to metallic surfaces in the tidal and splash zones of the structure, as cathodic protection is often not totally effective in these areas. To ensure these areas of the structure are adequately protected, a suitable paint coating system should be applied. j. Consider the secondary effects of the cathodic protection system. The application of cathodic protection may give rise to the development of alkalinity or the evolution of hydrogen at the protected surface. The effects which might occur are indicated below: 1. Alkalinity can cause deterioration of some paints by saponification. This can be minimised by avoiding the use of very negative potentials/overprotection and by using paints that are less susceptible to such damage.
  34. 34. Draft 16 December 2005 GP 06-31 Guidance on Practice for Cathodic Protection Page 34 of 39 2. Alkalinity generates, in the case of seawater, a white calcareous deposit (chalking). This is beneficial since the current density needed to maintain cathodic protection is reduced. 3. Hydrogen evolved at very negative potentials may create an explosion hazard in enclosed spaces. 4. Hydrogen embrittlement of high tensile steels or stainless steels is a possible danger. 5. Paint/coatings may be disbonded by cathodic protection. 6. By installing a cathodic protection system to an existing steel structure that has not been previously protected, may initially cause rust or scale to detach from the surface during the initial period of operation. a) This could result in blocked water passages for a short period. b) If steel surfaces have been seriously corroded, then the removal of rust that is plugging holes may cause leaks to become apparent during this period. 7. Chlorine may be evolved at the anodes of an impressed current cathodic protection installation if the electrolyte contains chlorides. This could create a hazard or cause excessive wear rate of some types of impressed current anode materials. k. Apply the relevant articles of the following international standards: 1. EN 12473, 12954. 2. DNV RP-B401, RP-F103. 3. ISO 15589-1, 15582-2. 4. NACE RP0169, RP0186, RP0193, RP0196, RP0285, RP0286, RP0388, RP0572, RP0575, RP0387, TM0190. 5. NORSOK Std M-503. 14. Safety and integrity a. The cathodic protection system shall comply with safety standards and regulations. b. A major risk with marine and offshore cathodic protection systems is the possible entanglement of divers or ROV’s umbilicals on anodes or the anode supports. If possible, anode supports should be designed to exclude any sharp edges or corners or protruding assemblies. c. If diver intervention or inspection work is required on a structure with impressed current systems, the applicable transformer rectifier providing current to anodes in the area that the diver will be working shall be switched off. If a diver should inadvertently make contact with an active anode element, they may suffer an electric shock. d. For onshore impressed current systems, the transformer rectifier d.c. voltage shall not exceed 50 volts for safety reasons. e. If there is a possibility of a.c. induced currents on a structure or pipeline, appropriate remedial actions shall be executed to safeguard personnel. f. Galvanic or impressed current anodes containing toxic elements shall not be used to protect potable water tanks. g. Impressed current systems shall not be used for the protection of tanks containing hydrocarbons. This also applies to galvanic anodes which have alloying elements that could cause incentive sparking if hit by a foreign metallic object or dropped from a height.
  35. 35. Draft 16 December 2005 GP 06-31 Guidance on Practice for Cathodic Protection Page 35 of 39 Over-polarization of the structure can cause the evolution of hydrogen gas at the surface of steel and stainless steels. This can cause hydrogen induced embrittlement and cracking. The material hardness and microstructure are important factors in this aspect. h. Another hazard generated by hydrogen evolution is the build up of gas in confined spaces, which may present a risk of explosion. To avoid these hazards, the following measures should be taken: 1. The structure to electrolyte potential shall be kept less negative than the threshold value at which hydrogen evolution is not significant. 2. Include adequate venting to prevent the build up of hydrogen. 3. Magnesium alloy anodes should not be installed in areas where hydrogen build up may occur. Electrochemical reactions at the surfaces of impressed current anodes in seawater can result in the evolution of chlorine gas, which is highly toxic and corrosive. If this gas is allowed to collect in confined spaces, it may present a hazard to personnel and materials. The long-term integrity of both galvanic and impressed current cathodic protection systems is dependant upon regular maintenance and monitoring throughout the design life of the system to ensure the correct levels of corrosion mitigation is maintained. For further details on this subject, refer to document, GP 06-36.
  36. 36. Draft 16 December 2005 GP 06-31 Guidance on Practice for Cathodic Protection Page 36 of 39 Annex A (Informative) Basics of corrosion A.1 General a. Metal that has been extracted from its primary ore (metal oxides or other free radicals) has a natural tendency to revert to that state under the action of oxygen and water. This action is called corrosion and the most common example is the rusting of steel. Corrosion is an electro-chemical process that involves the passage of electrical currents on a micro or macro scale. b. Where metal is lost, it converts to positive ions and loses electrons through the metal. The electrons travel through the metal to an area where they react with the environment to balance the charge by forming negative ions in solution. c. This corrosion process is initially caused by: 1. Difference in natural potential in galvanic (bimetallic) couples. 2. Metallurgical variations in the state of the metal at different points on the surface. 3. Local differences in the environment, such as variations in the supply of oxygen at the surface (oxygen rich areas become the cathode and oxygen depleted areas become the anode). A.2 Anodic and cathodic reactions a. The corrosion area where metal is lost by conversion to positive ions is the anode. The area where electrons react with the environment is the cathode. b. Corrosion takes place in an electrochemical cell providing the following four events occur (see Figure A.1). 1. An anodic reaction, e.g. oxidation which produces free electrons, which pass within the metal to another site on the metal surface (the cathode). • M Mn+ + ne- __ Metal dissolution A common example is: • Fe Fe 2+ + 2e- __ Iron dissolution 2. A cathodic reaction, e.g. reduction which consumes the electrons produced at the anode. In acid solution the cathodic reaction is as follows: • 2H+ + 2e- H2 (gas) __ Hydrogen reduction In neutral solution the cathodic reaction involve the consumption of oxygen: • O2 + 2H2O + 4e- 4OH- (alkali) __ Oxygen reduction 3. The presence of a conductive electrolyte to allow flow of ionic current between the anode and the cathode. 4. A metallic connection/path that completes the corrosion circuit allowing electric current flow from the anode to the cathode.
  37. 37. Draft 16 December 2005 GP 06-31 Guidance on Practice for Cathodic Protection Page 37 of 39 Figure A.1 - Electrochemical corrosion schematic A.3 Electrolytes a. An electrolyte is a chemical substance or mixture, usually liquid, containing ions that migrate in an electric field. Electrolytes encountered in relation with cathodic protection are water-based, typically: 1. Seawater, brackish or fresh water. 2. Water in soil, clay, mud, concrete, etc. b. Corrosiveness of the electrolyte depends on physical conditions and constituents, for example: 1. pH, temperature, pressure, etc. 2. Oxygen content. 3. Bacteria concentration. 4. Salinity. Wet concrete does not usually corrode steel due to the high alkalinity and passivation of the steel surface. Corrosion occurs, however, if the concrete gets contaminated with chlorides or the alkalinity reduces. A.4 Resistivity a. Soil acts as a reservoir for water and soluble salts, and the corrosiveness can be assessed by measuring the electrical resistance of the soil. Resistivity measurements provide a good indication of the aggressiveness of the soil (see Table A.1):
  38. 38. Draft 16 December 2005 GP 06-31 Guidance on Practice for Cathodic Protection Page 38 of 39 Table A.1 - Soil resistivity Resistivity Corrosiveness Typical soil conditions Below 5 ohm-m (16 Ώ ft) Very aggressive 5 -10 ohm-m (16 – 33 Ώ-ft) Aggressive 10 - 30 ohm-m (33 – 98 Ώ–ft) Moderately corrosive 30 -100 ohm-m (98 – 328 Ώ-ft) Mildly corrosive Above 100 ohm-m (328 Ώ-ft) Unlikely to be corrosive b. The electrolyte resistivity is essential for cathodic protection design and controls the amount of current that can be passed into the electrolyte from the anode/groundbed. There are exceptions to the table above. For example, a sandy soil in low marshy area is wet and exhibits low resistivity. Therefore, it may be more corrosive than many clay soils. The resistivity of seawater may also vary with depth and global location. A.5 Bimetallic corrosion a. Bimetallic (galvanic corrosion) refers to corrosion damage induced when two dissimilar materials are coupled in a corrosive electrolyte. It occurs when two (or more) dissimilar metals are brought into electrical contact under water. Galvanic corrosion is often caused by inappropriate design. b. The driving force for corrosion is the potential difference between the different materials. The relative nobility of a material can be predicted by measuring its corrosion potential. The well known galvanic series, see Table A.2, lists the relative nobility of certain materials versus common reference electrodes. To avoid metal contact and corrosion between dissimilar metals, an isolating joint are normally fitted to electrically separate the two metals. A.6 Microbial corrosion a. Microbial corrosion (MIC) refers to corrosion that is influenced by the presence and activities of micro organisms and/or their metabolites. Spectacularly rapid corrosion failures have been observed in soil due to microbial action. b. Anaerobic conditions may be created in the micro-environmental regime, even if the bulk conditions are aerobic. The pH conditions and availability of nutrients also play a role in determining what type of micro organisms can thrive in a soil environment. c. SRB reduce sulphate to sulphide, which usually shows up as hydrogen sulphide (can be recognized from its “rotten egg” odour) or, if iron is available, as black ferrous sulphide. Most common strains of SRB grow best at temperatures from 25°C to 35°C (77°F to 95°F). A few thermophilic strains capable of functioning efficiently at more than 60°C (140°F) have been reported. d. SRB have been implicated in the corrosion of cast iron and steel, ferritic stainless steels, 300 series stainless steels (also very highly alloyed stainless steels), copper nickel alloys, and high nickel molybdenum alloys. Clay Chalky Sandy
  39. 39. Draft 16 December 2005 GP 06-31 Guidance on Practice for Cathodic Protection Page 39 of 39 A.7 Stray current corrosion Stray current corrosion occurs when current flows though paths other than the intended circuit. Stray current corrosion is usually associated with d.c. systems. a. Stray current corrosion is normally mitigated by eliminating or reducing the stray current. This is usually done by increasing the metal/electrolyte interface. b. A.c. corrosion can be induced by proximity and parallelism to high voltage overheads power lines. This can be caused by either a long-term imbalance in the transmission system or high voltages near earthing (grounding) systems, resulting from lightning strikes and faults. Interference between the pipeline and power supply phases causes induced voltages and currents. Induced a.c. voltages can be very dangerous and induced a.c. currents can cause very rapid corrosion, particularly on well coated pipelines. Mitigation is therefore important to reduce both a safety and corrosion risk. A.8 Electrochemical series The electrochemical series (see Table A.2) shows the natural potentials of metals (those commonly referred to related to cathodic protection) with respect to standard reference electrodes. Table A.2 - Galvanic/electrochemical series of metals Material Potential (volts) vs. Ag/AgCl seawater Potential (volts) vs. Cu/CuSO4 ACTIVE Anodic Electro- negative Magnesium anode alloy Magnesium anode alloy Zinc, e.g. Zinc anode Aluminium, zinc, indium anode Carbon steel (clean & shiny) Carbon steel (rusted) Cast iron Lead, e.g. crimp connection C-steel in concrete (corroding) C-steel in concrete (not corroding) Mill scale on steel Copper, Brass, Bronze, e.g. cabling Austenitic Stainless Steel Carbon, Graphite, Coke, e.g. anode backfill –1,70 –1,50 –1,05 –1,05 to –1,10 –0,45 to –0,75 –0,15 to –0,45 –0,45 –0,45 –0,30 –0,15 –0,15 –0,15 –0,05 to +0,20 +0,35 –1,75 –1,55 –1,10 –1,10 to –1,15 –0,50 to –0,80 –0,20 to –0,50 –0,50 –0,50 –0,35 –0,20 –0,20 –0,20 –0,10 to +0,15 +0,30 NOBLE Cathodic Electro- positive

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