Section 10 — Material Requirements

Corrosion Protection for Underground Piping

IPE Engineering Practice IPE-EP-10-3-3

Document number: IPE-EP-10-3-3 · Section: 10 — Material Requirements

SCOPE

• This Practice covers the procedures and practices to achieve effective control of external corrosion on underground piping or other buried metallic structures.
• Description of the recommended coatings, electrical isolation and cathodic protection is provided.
• Specific provisions for the application of cathodic protection to existing and newly installed piping is included.
• An Asterisk (*) indicates that a decision by the Owner’s Engineer or Owner is required, or that additional information is furnished by the Purchaser.
• Any deviation from this Practice must be approved by the procedure described in EP 1–1–3.

2.0 REFERENCES

The latest edition of the following standards and publications are referred to herein.

STANDARDS AND PUBLICATIONS

STANDARDS AND PUBLICATIONS (CONTINUED)

DEFINITIONS

• Anode – An electrode that is characterized by electron loss (oxidation). Antonym: cathode.
• Contractor – Company or business that agrees to furnish materials or perform specified services at a specified price and/or rate to the Owner.
• Cathodic Protection – A technique to prevent corrosion of a metal surface by making it cathodic to its environment.
• Continuity Bond – A metallic connection that provides electrical continuity.
• Corrosion – Deterioration of a metal by its chemical reaction with a non–metal.
• Current Density – The current per unit area.
• Electrical Isolation – The condition of being electrically separated from other metallic structures and the environment.
• Electro–Osmotic Effect – Passage of a charged particle through a membrane under the influence of a voltage. Soil may act as the membrane.
• Electrode Potential – The potential of an electrode as measured against a reference electrode. The electrode potential does not include any loss of potential in the solution due to current passing to or from the electrodes, i.e., it represents the reversible work required to move a unit charge from the electrode surface through the solution to the reference electrode.
• Electrolyte – A chemical substance or mixture, usually liquid, containing ions that migrate in an electric field. For the purpose of this Practice, electrolyte refers to the soil or liquid adjacent to and in contact with a buried or submerged metallic structure, including the moisture and other chemical contained therein.
• Foreign Structure – Any structure that is not intended as a part of the system of interests.

• Galvanic Anode – A metal which, because of its relative position in the galvanic series, provides sacrificial protection to metal or metals that are more noble in the series, when coupled in an electrolyte. These anodes are the current source in one type of cathodic protection.
• Galvanic Series – A list of metals and alloys arranged according to their relative potentials in a given environment.
• Half Cell Reference Electrode – See Reference Electrode.
• Holiday – A discontinuity of coating that exposes the metal surface to the environment.
• Impressed Current – Direct current supplied by a power source external to the electrode system.
• Insulating Coating System – All components comprising the protective coating, the sum of which provide effective electrical insulation of the coated structure.
• Interference Bond – A metallic connection designed to control electrical current interchange between metallic systems.
• IR Drop – The voltage across a resistance in accordance with Ohm’s Law.
• Isolation – See Electrical Isolation.
• Line Current – The direct current flowing on a pipeline.
• Manufacturer – The recipient of a direct or indirect purchase order for materials and/or equipment. In this context, a direct order is one issued to a Manufacturer by a Contractor or the Owner. An indirect order is one issued to a Manufacturer by a vendor (recipient of a direct order) for materials, fabricated components, or subassemblies.
• Owner – Refining Company.
• Polarization – The deviation from the open circuit potential of an electrode resulting from the passage of current. In this Practice, polarization is considered to be the change of potential of a metal surface resulting from the passage of current directly to or from an electrolyte.
• Purchaser – The party placing a direct purchase order. The Purchaser is the Owner’s designated representative.
• Reference Electrode – A device whose open circuit potential is constant under similar conditions of measurement.
• Reverse Current Switch – A device that prevents the reversal of direct current through a metallic conductor.
• Sacrificial Protection – Reduction or prevention of corrosion of a metal in an electrolyte by galvanically coupling it to a more anodic metal.
• Stray Current – Current flowing through paths other than the intended circuit.
• Stray Current Corrosion – Corrosion resulting from direct current flowing through paths other than the intended circuit.
• Structure–to–Electrode Voltage – (Also Structure–to–Soil Potential or Pipe–to–Soil Potential). The voltage difference between a buried metallic structure and the electrolyte that is measured with reference to an electrode in contact with the electrolyte.

• Structure–to–Structure Voltage – (Also Structure–to–Structure Potential) – The difference in voltage between metallic structures in a common electrolyte.
• Tafel Segment, Tafel Line, Tafel Slope, Tafel Diagram – When an electrode is polarized, it frequently will yield a current potential relationship over a region which can be approximated by:

 i 

h  B log 

 i0 

where h = change from open circuit potential, i = the current density, B and io = constants. The constant (B) is also known as the Tafel slope. If this behavior is observed, a plot on semi– logarithmic coordinates is known as the Tafel line and the overall diagram is termed a Tafel diagram.

The Tafel segment is that portion of the diagram that appears as a straight line when current is plotted on the logarithmic scale and potential change is plotted on the linear scale. The beginning of the Tafel segment is that point on the curve where the current–potential relationship follows the straight line with increasing current increments and deviates from the straight line with decreasing current increments.

• Voltage – An electromotive force or a difference in electrode potentials expressed in volts.

DETERMINATION OF NEED FOR CORROSION CONTROL

• Metallic underground piping is subject to corrosion. Adequate corrosion control procedures should be adopted to assure metal integrity for safe and economic operation.
• Environmental, physical and economic factors shall be used to determine the type of corrosion control procedures that should be adopted for a particular installation.
• Environmental and physical considerations include: the projected corrosion rate of the piping, the nature and pressure of the product being transported, the location of the piping as related to population density/environmentally sensitive areas and stray current sources foreign to the surface.
• In order to estimate a corrosion rate the following methods are suggested:
• Study the corrosion history of the piping system or other systems of the same material in the same general area.
• Study the environment surrounding the piping system: resistivity, pH and composition. Then, determine the probable corrosion rate based on experience under similar conditions.
• Excavate and inspect piping in the area.
• Maintain records detailing any inspection, leak locations, soil studies, structure–to–soil potential surveys, line current studies and wall thickness surveys to determine areas of maximum corrosion rate.
• The cost of maintaining the piping in service for the expected service life, contingent costs of corrosion, and cost of the corrosion control are economic factors to consider.

• In addition to the direct costs of corrosion the following contingent costs should enter into the economic analysis:
• Public liability claims
• Property damage claims
• Damage to natural facilities, such as municipal or irrigation water supplies, forests, parks, etc.
• Clean–up of product lost to surroundings
• Plant shut–down and start–up costs
• Cost of lost product
• Loss of revenue due to service interruption
• Loss of contract or goodwill
• Loss of reclaim or salvage value of the piping system
• The typical costs for cathodic protection of underground piping and/or coatings supplemented cathodic protection include:
• Relocation costs for moving piping to avoid known corrosive conditions.
• Expense of reconditioning and coating the piping system.
• A higher purchase price for more corrosion resistant materials.
• Cost of selected or inhibited backfill.
• Cost to electrically isolate and install cathodic protection.
• Possible costs to correct corrosive conditions.

STRUCTURAL DESIGN OF CATHODIC PROTECTION SYSTEMS

• Person(s) qualified to engage in the practice of corrosion control should be consulted during all phases of the piping design and construction.
• Insulating devices consisting of flange assemblies, prefabricated joints, unions or couplings should be installed where electrical isolation of portions of the system is required to facilitate the application of corrosion control. The insulating device shall be properly rated for temperature, pressure, and dielectric strength. Installation of the insulating device shall be avoided in enclosed areas where combustible atmospheres are likely to be present. Typical locations where electrical insulating devices would be considered are as follows:
• Points at which facilities change ownership such as meter stations and wellheads.
• Connections to main line piping systems such as gathering or distribution system laterals.
• Inlet and outlet piping of inline measuring and/or pressure regulating stations.
• Compressor or pumping stations, either in the suction and discharge piping or in the main line immediately upstream and downstream of the station.
• In stray current areas.
• At the junction of dissimilar metals for protection against galvanic corrosion.
• At the termination of service life connections and entrance piping to prevent electrical continuity with other metallic systems.
• At the junction of a coated pipe and a bare pipe.
• The need for lightning and fault current protection at insulating devices should be considered. Cable connections from insulating devices to arrestors shall be short, direct, and of a size suitable for short term, high current loading.
• Where metallic casings are required as part of the underground piping system, the pipeline shall be electrically isolated from such casings.

• Where casing seals are used, they shall be installed to resist the entry of foreign matter into the casing.
• Where electrical contact would adversely affect cathodic protection, piping systems shall be electrically isolated from supporting pipe stanchions, bridge structures, tunnel enclosures, piling, or reinforcing steel in concrete. However, piping can be attached directly to a bridge without isolation if insulating devices are installed on each side of the bridge to electrically isolate the bridge piping from adjacent underground piping.
• Where an insulating joint is required, a device manufactured to perform this function shall be used, or, if permissible, a section of nonconductive pipe, such as plastic pipe, may be installed. In either case, these shall be properly rated and installed in accordance with Manufacturer’s instructions.
• River weights, pipeline anchors, and metallic reinforcement in weight coatings shall be electrically isolated from the carrier pipe and installed so that coating damage will not occur.
• Metallic curb boxes and valve enclosures shall be designed, fabricated, and installed in such a manner that electrical isolation from the piping system will be maintained.
• Where a metallic wall sleeve is used, and where it is intended to maintain electrical isolation between the sleeve and the pipe, insulating type spacing materials shall be used.
• Underground piping systems shall be installed so that they may remain electrically isolated from all foreign underground metallic structures. Where practicable, a 12 inch minimum separation shall be maintained between all buried metallic structures at crossings. Where it is impractical to achieve the recommended separation, insulating materials between the structures or other methods used to maintain electrical isolation may be required.
• A minimum separation of 10 feet shall be maintained between pipelines and transmission tower footings, ground cables, and counterpoise. Regardless of separation, consideration should always be given to lightning and fault current protection of pipeline(s) and safety of personnel (see NACE Standard RP0177, latest revision).
• Consideration should be given to the electrical properties of non–welded pipe joints. Where it is the objective to ensure electrical continuity, this shall be achieved either by using fittings manufactured for this purpose or by bonding the mechanical joints in an approved, effective manner.
• New piping systems shall be coated, unless thorough investigations indicate that coatings are not required (See Section 11 of this Practice).

CRITERIA FOR CATHODIC PROTECTION

This section lists criteria for cathodic protection which, when complied with either separately or collectively, will indicate that adequate cathodic protection of a metallic piping system in its electrolyte has been achieved.

• General
• The objective of using cathodic protection is to control the corrosion of metallic surfaces in contact with electrolytes.

• The selection of a particular criterion for achieving this objective depends, in part, upon past experience with similar structures and environments wherein the criterion has been used successfully.
• The criteria in Section 6.2 of this Practice have been developed through laboratory experiment or have been empirically determined by evaluating data obtained from successfully operated cathodic protection systems. It is not intended that persons responsible for corrosion control be limited to these criteria if it can be demonstrated by other means that the control of corrosion has been achieved.
• Voltage measurements on pipelines are to be made with the reference electrode located on the electrolyte surface as close as practicable to the pipeline. Such measurements on all other structures are to be made with the reference electrode positioned as close as feasible to the structure surface being investigated. Consideration should be given to voltage (IR) drops other than those across the structure–electrolyte boundary, the presence of dissimilar metals, and the influence of other structures for valid interpretation of voltage measurements.
• No one criterion for evaluating the effectiveness of cathodic protection has proven to be satisfactory for all conditions. Often a combination of criteria is needed for a single structure.
• Criteria for Steel and Cast Iron Structures
• A negative (cathodic) voltage of at least 0.85 volts but less than 1.4 volts as measured between the structure surface and a saturated copper–copper sulfate reference electrode contacting the electrolyte. Determination of this voltage is to be made with the protective current applied.
• A minimum negative (cathodic) voltage shift of 300 millivolts, produced by the application of protective current. Note: The minimum negative voltage shift should be 400 millivolts under anaerobic conditions. The voltage shift is measured between the structure surface and a stable reference electrode contacting the electrolyte. This criterion of voltage shift does not apply to structures in contact with dissimilar metals.
• A minimum negative (cathodic) polarization voltage shift of 100 millivolts measured between the structure surface and a stable reference electrode contacting the electrolyte. This polarization voltage shift is to be determined by interrupting the protective current and measuring the polarization decay. When the current is initially interrupted, an immediate voltage shift will occur. The voltage reading after the immediate shift shall be used as the base reading from which to measure polarization decay.
• A structure–to–electrolyte voltage at least as negative (cathodic) as that originally established at the beginning of the Tafel segment of the E–log–l curve. This structure–to–electrolyte voltage shall be measured between the structure surface and a stable reference electrode contacting the electrolyte at the same location where voltage measurements were taken to obtain the E–log–I curve.
• A net protective current from the electrolyte into the structure surface as measured by an earth current technique applied to pre–determined current discharge (anodic) points of the structure.
• Criteria for Aluminum Structures
• A minimum negative (cathodic) voltage shift of 150 millivolts, produced by the application of protective current. The voltage shift is measured between the structure surface and a stable reference electrode contacting the electrolyte (See precautionary notes in 6.3.3 and 6.3.4 below).

• A minimum negative (cathodic) polarization voltage shift of 100 millivolts measured between the structure surface and a stable reference electrode contacting the electrolyte. This polarization voltage shift is to be determined by interrupting the protective current and measuring polarization decay. When the current is initially interrupted, an immediate voltage shift will occur. The voltage reading after the immediate shift shall be used as the base reading from which to measure polarization decay (See precautionary notes in 6.3.3 and 6.3.4 below).
• Precautionary Note – Excessive Voltages: Notwithstanding the alternative minimum criteria in

6.3.1 and 6.3.2 above, aluminum, if cathodically protected at voltages more negative than –1.20 volts measured between the structure surface and a saturated copper–copper sulfate reference electrode contacting the electrolyte and compensated for the voltage (IR) drops other than those across the structure–electrolyte boundary, may suffer corrosion as the result of the build– up of alkali on the metal surface. A voltage more negative than –1.20 volts should not be used, unless previous test results indicate no appreciable corrosion will occur in the particular environment.

• Precautionary Note – Alkaline Soil Conditions: Since aluminum may suffer from corrosion under high pH conditions and since application of cathodic protection tends to increase the pH at the metal surface, careful investigation or testing should be made before applying cathodic protection to stop pitting attack on aluminum structures in environments with a natural pH in excess of 8.0.
• Criteria for Copper Structures

A minimum negative (cathodic) polarization voltage shift of 100 millivolts measured between the structure surface and a stable reference electrode contacting the electrolyte. This polarization voltage shift is to be determined by interrupting the protective current and measuring the polarization decay. When the current is initially interrupted, an immediate voltage shift will occur. The voltage reading after the immediate shift shall be used as the base reading from which to measure polarization decay.

• Criteria for Dissimilar Metal Structures

A negative (cathodic) voltage between all structure surfaces and a stable reference electrode contacting the electrolyte equal to that required for the most anodic metal should be maintained. Amphoteric materials, which could be damaged by high alkalinity, should be electrically isolated with insulating flanges or the equivalent.

• Alternative Reference Electrodes
• Other standard reference electrodes may be substituted for the saturated copper–copper sulfate reference electrodes. Two commonly used reference electrodes are listed below along with their voltage equivalent to –0.85 volts referred to a saturated copper–copper sulfate reference electrode:
• Saturated KCl calomel reference electrode: –0.78 volt
• Silver–silver chloride reference electrode used in sea water: –0.80 volt.
• In addition to these standard reference electrodes, an alternative metallic material or structure may be used in place of the saturated copper–copper sulfate reference electrode, if the stability of its electrode potential is assured and if its voltage equivalent referred to a saturated copper– copper sulfate reference electrode is established.
• Special Considerations

• Special cases, such as those involving stray currents and stray electrical gradients may exist which require the use of criteria different from those listed above. Measurements of current loss and gain on the structure and current tracing in the electrolyte have been useful in such cases.
• Abnormal conditions sometimes exist where protection is ineffective or only partially effective. Such conditions may include elevated temperatures, disbonded coatings, shielding, bacterial attack, and unusual contaminants in the electrolyte.

DESIGN OF CATHODIC PROTECTION SYSTEMS

• General
• The purpose of this section is to recommend procedures for designing cathodic protection systems that will provide effective corrosion control by satisfying one or more of the criteria listed in Section 6.0 of this Practice, and which will exhibit maximum reliability over the intended operating life of the systems. In the design of a cathodic protection system, the following items should be considered:
• Recognition of hazardous conditions prevailing at the proposed installation site(s) and the selection and specification of materials and installation practices which will assure the safe installation and operation of the cathodic protection system.
• Specification of materials and installation practices to conform with applicable codes, National Electrical Manufacturers Association (NEMA) Standards, and Recommended Practices of the National Association of Corrosion Engineers.
• Selection and design of the cathodic protection system for optimum economy of installation, maintenance, and operation.
• Selection and specification of materials and installation practices which will assure dependable operation throughout the intended operating life of the cathodic protection system.
• Selection of a system to minimize excessive protective currents or earth potential gradients, which can cause detrimental effects on pipe, coating, or neighboring buried or submerged metallic structures.
• Direction of cooperative investigations to determine mutually satisfactory solution(s) of interference problems (See Section 9 of this Practice).
• Major Objectives of Cathodic Protection System Design
• Provide sufficient current to the structure to be protected and distribute this current so that the selected criteria for cathodic protection are efficiently attained.
• Minimize the interference currents on neighboring underground structures (See Section 9 of this Practice).
• Provide a design life of the anode system commensurate with the required life of the protected structure, or provide for periodic rehabilitation of the anode system.
• Provide adequate allowance for anticipated changes in current requirements with time.
• Place anodes where the possibility of disturbance or damage is minimal.
• Suggested Record Keeping Items
• Piping system specifications and practices including the following:
• Route maps and atlas sheets

• Construction dates
• Pipe, fittings, and other appurtenances
• Coatings
• Castings

• Corrosion control test stations
• Electrically insulating devices
• Electrical bonds
• Aerial, bridge, and underwater crossings
• Piping system site conditions including:
• Existing and proposed cathodic protection systems
• Possible interference sources (see Section 9 of this Practice)
• Special environmental conditions
• Neighboring buried metallic structures (including location, ownership, and corrosion control practices)
• Structure accessibility
• Power availability
• Feasibility of electrical isolation from foreign structures
• Field survey, corrosion test data, and operating experience including:
• Protective current requirements to meet applicable criteria
• Electrical resistivity of the electrolyte
• Electrical continuity
• Electrical isolation
• Coating integrity
• Cumulative leak history
• Interference currents
• Deviation from construction specifications
• Other maintenance and operating data
• Field survey work prior to actual application of cathodic protection is not always required if prior experience or test data is available to estimate current requirements, electrical resistivities of the electrolyte, and other design factors.
• Types of Cathodic Protection Systems
• Galvanic anodes can be of materials such as alloys of magnesium, zinc, or aluminum. These are installed in the soil or water, either bare or packaged in special backfill. The anodes are connected to the pipe, either singly or in groups. Galvanic anodes are limited in current output by the pipe–to–anode driving voltage and the earth resistivity. Cathodic protection of large bare or poorly coated piping may not be attainable by using galvanic anodes.
• Impressed current anodes can be of materials such as graphite, high silicon cast iron, lead– silver alloy, platinum, or steel. These anodes are installed in the soil or water, either bare or in special backfill material. They are connected with an insulated conductor, either singly or in groups, to the positive terminal of a direct current source, such as a rectifier or generator. The pipeline is connected to the negative terminal of the direct current source.
• Considerations Influencing Selection of the Type of Cathodic Protection System
• Stray currents causing significant potential fluctuations between the pipeline and earth may preclude the use of galvanic anodes.

• The effects of cathodic protection interference currents on adjacent structures may limit the use of impressed current cathodic protection systems.
• The lack of a source of external power may preclude the use of an impressed current system.
• On pipelines where stray currents are not present and where an external power source is available, the magnitude of protective current required is a dominant factor. The feasibility of protection with galvanic anodes can be established when current requirements, electrical resistivity of the electrolyte, and anode–to–pipe voltage have been reasonably estimated, calculated, or determined by field testing.
• The physical space available, proximity of foreign structures easement procurement, surface conditions, presence of streets and buildings, river crossings, and other construction and maintenance aspects should be considered.
• Future development of the right–of –way area and future extensions to the pipeline system should be considered.
• The costs of installation, operation, and maintenance.
• Factors Determining Anode Current Output, Operating Life, and Efficiency
• Various anode materials have different rates of deterioration when discharging a given current density from the anode surface in a specific environment. Therefore, for a given current output, the anode life will depend on the anode material as well as the anode weight and the number of anodes in the cathodic protection system. Established anode performance data may be used to calculate the probable deterioration rate.
• Data on the dimensions, depth, and configuration of the anodes and electrolyte resistivities may be used to calculate the resultant resistance–to–electrolyte of the anode system. Formulas and graphs relating to these factors are available.
• Proper design of a galvanic anode system must consider pipe–to–anode potential with resultant current output and, in special cases, anode lead wire resistance.
• Where galvanic anodes are installed individually or in large numbers, there are many situations where it is impractical to individually design each installation. In such cases, the use of statistical techniques is suggested to determine the anode design that will best provide cathodic protection in a high percentage of installations. Sampling techniques can be used to establish the success of such anode designs as a percentage of the total installations.
• Galvanic anode performance in most soils can be improved by using special backfill material. Mixtures of gypsum, bentonite, and salt cake (sodium sulfate) are most commonly used.
• The number of impressed current anodes required can be reduced and their useful life lengthened by the use of special backfill around the anodes. The most common materials are coal coke, calcined petroleum coke, and natural or manufactured graphite.
• In the design of an extensive distributed anode impressed current system, the voltage and current attenuation along the anode connecting wire should be considered. In such cases, the design objective is to optimize anode system length, anode spacing and size, and conductor size in order to achieve efficient corrosion control at the extremities of the protected structure.

• Where it is anticipated that entrapment of gas actions could impair the ability of the impressed current groundbed to deliver the required current, suitable provisions shall be made for venting the anodes. An increase in the number of anodes may reduce gas blockage.
• Where it is anticipated that electro–osmotic effects could impair the ability of the impressed current groundbed to deliver the required current output, suitable provisions shall be made to ensure adequate soil moisture around the anodes. Increasing the number of impressed current anodes may further reduce the electro–osmotic effect.
• Design Drawings and Specifications
• Suitable drawings shall be prepared to designate the overall layout of the piping to be protected and the location of significant items of structure hardware, corrosion control test stations, electrical bonds, electrical insulators, and neighboring buried or submerged metallic structures.
• Layout drawings shall be prepared for each impressed current cathodic protection installation, showing the details and location of the components of the cathodic protection system with respect to the protected structure(s) and to major physical landmarks. These drawings should include right–of–way information.
• The locations of galvanic anode installations shall be recorded on drawings or in tabular form, with appropriate notes as to anode type, weight, spacing, depth, and backfill.
• Specifications shall be prepared for all materials and installation practices that are to be incorporated in construction of the cathodic protection system.

INSTALLATION OF CATHODIC PROTECTION SYSTEMS

• General
• The purpose of this section is to recommend procedures that will result in the installation of cathodic protection systems that achieve protection of the structure when design considerations recommended in Sections 5.0 and 7.0 of this Practice have been followed.
• All construction work on cathodic protection systems shall be performed in accordance with construction drawings and specifications. The construction specifications shall be in accordance with recommended practices in Sections 5.0 and 7.0 of this Practice.
• Construction Supervision
• All construction work on cathodic protection systems shall be performed under the surveillance of trained and qualified personnel to verify that the installation is made in strict accord with the drawings and specifications. Exceptions may be made only with the approval of qualified personnel responsible for corrosion control.
• All deviations from construction specifications shall be noted on as–built drawings.
• Galvanic Anodes Inspection and Handling
• Packaged anodes shall be inspected and steps taken to assure that backfill material completely surrounds the anode. The individual container for the backfill material and anode should be intact. If individually packaged anodes are supplied in waterproof containers, that container must be removed before installation. Packaged anodes should be kept dry during storage.

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