ISO 8044, Corrosion of metals and alloys — Basic terms and definitions ISO 10012, Measurement management systems — Requirements for measurement processes and measuring equipment ISO 136
Symbols
D b backfill diameter ε electrochemical capacity of the anode material
E potential measured at the metal/electrolyte interface ΔE potential shift due to cathodic protection current measured against a remote reference electrode
E a design closed-circuit potential of a galvanic anode
E c design protection potential (minimum negative potential)
E cor free corrosion potential (also called natural potential)
E p protection potential f c coating breakdown factor f f final coating breakdown factor f i initial coating breakdown factor Δf average yearly increase in the coating breakdown factor
I af actual end-of-life individual anode current output
I cf total current demand for the cathodic protection of a specific pipeline section at the end of life (for maximum coating breakdown factor)
I f required end-of-life individual anode current output j current density for bare steel j c current density for coated pipelines k contingency factor
L length of the pipeline m total net anode mass m a individual net anode mass n number of anodes r co average coating resistance
R a total circuit resistance for a galvanic anode cathodic protection system (assumed to be equivalent to the anode resistance)
R a/b anode resistance relative to backfill
R b/s backfill bed resistance relative to the natural electrolyte ρ resistivity of an electrolyte
Abbreviations
ACVG alternating current voltage gradient
CIPS close interval potential survey
CSE copper−copper sulphate (saturated) reference electrode d.c direct current
DCVG direct current voltage gradient
PREN pitting resistance equivalent numbers
SCE saturated calomel reference electrode
Personnel responsible for the design, installation, commissioning, operation, measurement, monitoring, and maintenance of cathodic protection systems must possess the necessary competence for their specific tasks.
EN 15257 or NACE Cathodic Protection Training and Certification Programme constitute suitable methods of assessing and certifying competence of cathodic protection personnel.
Cathodic protection personnel must demonstrate their competence through certification that aligns with prequalification procedures, such as EN 15257 or the NACE Cathodic Protection Training and Certification Programme, or any other equivalent scheme.
General
The protection potential, \( E_p \), is defined as the pipe-to-electrolyte potential at which the corrosion rate for carbon steel and cast iron is below 0.01 mm per year This low corrosion rate ensures that the material will remain acceptable throughout its design life Consequently, the criterion for cathodic protection (CP) is established based on this condition.
E p is the protection potential criterion;
E IRfree refers to the potential at the metal/electrolyte interface, representing the potential that is unaffected by the IR drop in a corrosive environment This is also commonly known as the "polarized potential."
The protection potential of a metal depends on the corrosive environment (electrolyte) and on the type of metal used.
Application of potentials that are too negative can result in cathodic over-protection leading to coating disbondment and blistering and hydrogen embrittlement of some metals.
The IR-free potential, E IRfree , shall not be more negative than the limiting critical potential, E l
In such cases, the criterion for CP is given by the condition in Formula (2):
Protection potentials
The IR-free potential, E IRfree , shall meet the criteria given by Formula (1) and, if applicable, Formula (2) Table 1 presents free corrosion potentials, E cor , protection potentials, E p , and limiting critical potentials,
E l , for different metals in different environmental conditions.
Table 1 — Free corrosion potentials, protection potentials, and limiting critical potentials of common metallic materials in soils and waters (except seawater) measured against CSE Metals or alloys Environmental conditions
Carbon steels, low alloyed steels and cast iron
Soils and waters in all condi- tions except those hereunder described −0,65 to −0,40 −0,85 a
Soils and waters at T > 60 °C c −0,80 to −0,50 −0,95 a Soils and waters in aerobic conditions at T < 40 °C with
Soils and waters in aerobic con- ditions at T < 40 °C with ρ > 1 000 Ωãm
Soils and waters in anaerobic conditions and with corrosion risks caused by Sulfate Reducing
Austenitic stainless steels with PREN < 40
Neutral and alkaline soils and waters at ambient temperatures
Austenitic stainless steels with PREN > 40 −0,10 to + 0,20 −0,30 -
Martensitic or aus- teno-ferritic (duplex) stainless steels −0,10 to + 0,20 −0,50 e
All stainless steels Acidic soils and waters at ambi- ent temperatures −0,10 to + 0,20 e e
Copper Soils and waters at ambient temperatures
All potentials referenced in this article are IR free and relate to a copper/saturated copper sulfate reference electrode, with the equation ECu = EH – 0.32 V It is essential to consider any changes in the resistivity of the medium surrounding the pipeline throughout its lifespan To mitigate hydrogen embrittlement in high-strength non-alloyed and low-alloyed steels with a yield strength exceeding 550 N/mm², the critical limit potential must be documented or determined experimentally For temperatures ranging from 40 °C to 60 °C, the protection potential can be linearly interpolated between the values established for 40 °C (−0.65 V, −0.75 V, −0.85 V, or −0.95 V) and the value for 60 °C (−0.95 V) Additionally, the risk of high pH stress corrosion cracking escalates with increasing temperature If any martensitic or ferritic phases are present, such as from hardening, the risk of hydrogen embrittlement should also be assessed through documentation or experimental methods.
To prevent disbondment and/or blistering of the coating, the limiting critical potential, E l , should not be more negative than −1,20 V (CSE) for the currently used pipeline coatings.
Alternative methods
Other methods
Alternative methods may be used if it can be demonstrated that the control of corrosion is achieved.
Criteria in the presence of a.c
In locations where a.c interference is possible, measurements of a.c voltage and/or current density shall be carried out to evaluate the level of the a.c influence.
When an alternating current (a.c.) voltage is applied to a pipeline, it is essential to adhere to the protection criteria outlined in Table 1, even though these criteria may not fully safeguard against a.c corrosion The IR-free potential, denoted as \$E_{IRfree}\$, must satisfy the requirement specified in Formula (1).
EN 15280 gives guidelines for the a.c corrosion likelihood and defines detailed criteria that may be applied.
7 Pre-requisites for the application of cathodic protection
General
For effective cathodic protection, it is essential that the pipeline or its designated section is electrically continuous Additionally, the pipeline must be properly coated and isolated from other structures and earthing systems If the pipeline cannot be electrically isolated, a justification must be provided.
NOTE coatings are essential for buried pipelines, offering primary corrosion protection while cathodic protection safeguards areas where the coating may be damaged These coatings not only minimize the overall cathodic protection current needed but also reduce the risk of interference with nearby buried structures.
Electrical continuity
To ensure effective electrical continuity in the pipeline, it is essential to maintain low longitudinal resistance Any components that could potentially increase this resistance should be short-circuited using cables or low-resistance metal bonds with an appropriate cross-sectional area.
To ensure electrical continuity in non-welded pipelines, it is essential to install permanent bonds across high-resistance mechanical connectors using dependable attachment methods Additionally, the continuity must be verified through resistance and potential measurements.
Bonding may be required across isolating devices for measurement or other purposes To ensure permanent electrical continuity, this bonding should be performed at a test station.
Electrical isolation
General
Metallic contacts or resistive contacts between the pipeline and other structures, or direct connection to earthing systems, should be avoided; otherwise, the corrosion risk can be increased.
Pipelines must be electrically isolated from various foreign structures, such as compressor stations, pump stations, pressure reducing and metering stations, water pumping and storage facilities, wellsites, offshore pipelines and structures, terminals, processing facilities, and at junctions with other pipelines.
Electrical isolation can also be installed to divide the system into sections, e.g in stray current areas.
Isolation should be achieved by installation of isolating joints (monolithic isolation joints or isolating flange kits) or non-conductive pipe sections.
In cases where electrical isolation cannot be achieved, the design of cathodic protection must ensure adequate current supply and effective distribution This is essential to guarantee that the pipeline receives proper cathodic protection while preventing any negative impact on surrounding structures.
If there is a contact between the protected pipeline and any other less electronegative structure, EN 14505 considers they constitute a complex structure and gives recommendations that can help in that case.
Locations
When determining the placement of electrical isolation points, it is crucial to take into account the potential for d.c stray current This necessitates thorough measurements and analysis of the electrolyte surface gradients, as outlined in EN 50162 for additional guidance.
Electrical isolation can be required
— between pipeline sections consisting of different metallic materials,
— at the extremities of the pipeline,
— at significant changes in electrolyte resistivity,
— on pipelines requiring different cathodic protection systems,
— on pipelines with different current density requirements,
— in areas influenced by stray currents, a.c., or telluric currents,
— on some pipeline networks to facilitate the cathodic protection maintenance or measurements, and
— at interfaces with unprotected structures or earthed equipment.
In the case of distribution systems, electrical isolation shall be installed at the interface with customer facilities.
Isolating joints
The mechanical design, materials, dimensions, and construction of the isolating joints shall meet the requirements of ISO 13623, EN 14161, EN 1594 or EN 12007-3, as applicable.
Monobloc isolation joints should be used wherever possible They can be installed above-ground, in a pit or buried.
Monobloc isolation joints should be electrically tested before installation.
NOTE 1 Electrical tests are proposed in Annex F.
NOTE 2 Installing the isolation joint above ground provides the advantage of easier visual, electrical, and ultrasonic inspection On the other hand, buried isolating joints are less exposed to mechanical or fire hazards, voluntary or involuntary damage, and can prevent the product inside the pipeline from freezing (sometimes referred to as “frost-proof grounding”).
NOTE 3 Installing an isolating joint in a pit provides the advantages of a buried isolation joint but with the added risk of gas entrapment For this reason, local regulations can prevent installation of an isolation joint in a pit.
An above-ground isolating joint is essential for ensuring that cathodic protection covers the entire buried section of a pipeline connected to above-ground facilities In cases where the isolation joint is buried, the pipeline operator must implement additional corrosion protection measures for the isolated section, ensuring its integrity and safety.
To prevent internal corrosion in pipelines transporting multiphase fluids with a high water content, it is advisable to install an isolating joint on vertical or angled transition sections.
Buried isolation joints shall be externally coated with materials that are compatible with the coating applied to the pipeline.
Isolating flanges can deteriorate due to exposure to weather, dirt, and moisture To prevent this degradation, it is essential to use flange protectors or viscoelastic compounds to safeguard against dirt and moisture ingress.
A major cause of failure of isolating flanges is poor installation techniques To reduce this risk, the manufacturer’s instructions should be followed or factory preinstalled kits utilized.
— over-tightening of the flange bolts (isolating flanges require a lower tightening torque than flanges without isolating gaskets),
— incorrect alignment of flange faces, and
— improper surface preparation of flange faces.
Isolation materials must be engineered to endure specific service conditions, including the transported medium, temperature, pressure, and mechanical stress, while also possessing suitable dielectric strength Additionally, key characteristics of the isolating flange kit encompass mechanical properties such as flexural and tensile strength, isolation efficiency, and resistance to water absorption.
Isolating joints shall be installed in such a manner as to eliminate the risk of accidental shorting.
To prevent damage from high voltages resulting from lightning strikes or alternating current fault currents from power lines, it is essential to implement protective devices such as suitable isolating spark gaps, surge protective devices, and effective electrical earthing.
Isolating joints installed in areas classified as hazardous in accordance with EN 60079-10-1 shall conform to the certification and operational requirements of the hazardous area.
Isolating joints shall be provided with accessible test facilities.
Internal corrosion risks at isolating joints
Pipelines transporting fluids with a distinct water phase face a significant risk of internal corrosion, particularly near isolating joints This corrosion occurs on the anodic side, where the internal surface of the pipe has a less negative potential The extent of this corrosion is primarily influenced by the fluid's conductivity and the voltage differential across the isolating joint.
To reduce the risk of internal corrosion, it is essential to apply a lining made of an electrically isolating material on the side of the pipeline that exhibits a more negative internal potential.
The internal lining length of a pipeline increases with higher electrolyte conductivity, larger pipeline diameters, and greater voltage across the isolating joint In the absence of prior experience, it is essential to determine the appropriate lining length through calculations or tests.
The internal coating length can be established by evaluating the internal ohmic resistance of the fluid, which can be done through simulation software or calculations based on Ohm's law This assessment must be documented and should take into account the fluid resistivity, the potential drop across each section of the isolating joint, and the absence of defects in the external coating on the anodic side of the pipeline.
NOTE 1 Based on feedback and experience, a value of 100 Ω for the internal ohmic resistance is commonly used for the evaluation of the length of the internal coating.
NOTE 2 For long lengths of lining, an entire pipe length lined in a factory is generally used.
In practice, if both sides of isolating joints are lined, the length of the lining on the cathodic side shall be at least the calculated length.
NOTE 3 When custom made asymmetrical lined isolating joints are used, the risk incurred by installing the longer liner on the wrong side of the isolating joints can be significant.
The lining should be of a type that does not deteriorate when in contact with the internal fluid, especially in the case of a lining exposed to salt water.
An effective alternative to internal lining is the installation of a sacrificial steel pipe spool piece adjacent to the isolating joint on the anodic pipeline side, which has a less negative internal potential Regular inspections of the spool piece are essential to track internal corrosion, ensuring timely replacement to prevent potential leakage.
Contacts between metallic structures
To prevent direct or resistive contact between reinforcing steel and the protected pipeline, it is essential to use insulating wall entry fittings and apply a high-quality coating to the pipeline Additionally, the potential for a shielding effect must be taken into account.
When a pipeline crosses a bridge or enters a tunnel, it is essential to install isolating joints at both ends to ensure safety The section of the pipe between these joints may be equipotentially bonded to the structure to protect against harmful touch potentials To maintain effective cathodic protection for the pipeline on either side of the bridge or tunnel, an equipotential bond is necessary to connect the pipeline across these structures.
If the pipeline crosses a bridge in a service trough containing an electrolyte (e.g sand), the pipe may be cathodically protected by galvanic anodes (e.g magnesium ribbon) and not connected to the bridge.
In the absence of isolating joints, it is crucial to ensure that the pipe is electrically isolated from the metallic components of the bridge or tunnel, particularly for metal bridges that support direct current (d.c.) traction systems.
Electrical earthing system
Electrical earthing of devices installed on the protected pipeline can be required for safety reasons or pipeline earthing can be required to mitigate the effect of induced electrical voltages.
An earthing system must be compatible with the cathodic protection system When regulations permit, this compatibility can be achieved by incorporating appropriately rated d.c decoupling devices into the earthing circuit While local earthing with zinc or galvanized electrodes directly connected to the pipeline is an option, it may still pose potential future risks to the effectiveness of the cathodic protection.
NOTE 1 These adverse effects on cathodic protection effectiveness are due to one or both of the following:
The resistance of local earth compared to remote earth can be significantly lower than the resistance caused by coating defects, leading to a decrease in cathodic protection (CP) current at these defects.
— zinc or galvanized steel potential can drift with time towards less negative potentials.
If earthing is being installed to mitigate the effect of a.c voltages on the pipeline, the earthing locations should be established through a specific study and detailed design.
NOTE 2 EN 15280 and EN 50443 provide guidance on the corrosion and safety aspects related to a.c influences. Detection and control of electrical interference shall be in accordance with Annex B.
Connecting earthing to electrically operated equipment, such as valves or pumps, can reduce the pipeline's resistance to earth, potentially hindering cathodic protection To ensure effective cathodic protection, several remedies can be implemented, adhering to relevant safety regulations These include isolating the electrically operated equipment from the protected pipeline, using isolating joints to separate the pipeline section connected to the equipment, installing isolating transformers, employing fault current circuit breakers with local earthing made of galvanized steel, zinc, or magnesium, and utilizing d.c decoupling devices to maintain electrical isolation between the equipment and the main earthing system.
Lightning and overvoltage protection
Pipelines can be affected by overvoltage and current strikes made by lightning or by nearby external power lines (e.g earth short circuits).
To protect the isolating joint against overvoltage, an isolating spark gap should be connected across the isolating joint.
To reduce the impact of electrical strikes and interference, it is essential to install isolating spark gaps, surge protective devices (SPDs), or DC decoupling devices between the pipeline and the earthing system, in accordance with national or local regulations and safety standards.
The effectiveness of overvoltage protection is significantly impacted by the cable length, cross section, and type It is essential to align these factors with the electrical characteristics of the isolating spark gap or surge protective device (SPD) To optimize protection, the cable length should be minimized, and any loops in the installation should be kept as small as possible.
NOTE To choose the type of isolating spark gaps, the following parameters are typical for cathodic protection applications for a 50 Hz case:
— d.c spark over voltage: from 500 V to 1 kV;
— 100 % lightning impulse spark over voltage (1,2/50 às): ≤1,30 kV for both class 1 and class 2 isolating joints, or ≤2,2 kV for class 1 only (see Annex F);
— nominal discharge current (8/20 às): 100 kA;
— lightning impulse current, I imp , (10/350 às): 100 kA (class H), 50 kA (class N);
— rated short-duration power frequency withstand voltage (50 Hz):