| Primary Protection Function | Provides a strongly bonded, electrically insulating epoxy barrier against seawater, dissolved salts, and soil moisture. | Combines an epoxy primer, adhesive layer, and polyethylene outer layer for barrier protection plus mechanical resistance. | Protects exposed steel at coating defects, holidays, scratches, and damaged field-joint areas by supplying protective current. |
| Typical Marine Use | Suitable for shop-coated pipe, internal or external surfaces, moderate handling conditions, and applications where a relatively thin coating is acceptable. | Generally preferred for buried, submerged, shore-crossing, and offshore pipelines subject to impact, abrasion, and handling damage. | Commonly required for permanently submerged or buried carbon-steel pipelines, regardless of whether FBE or 3LPE is selected. |
| Typical Coating Thickness | Approximately 300–500 micrometres for many external pipeline applications; the specified value depends on service, pipe diameter, and applicable standard. | Commonly about 2.5–3.7 mm for external pipeline systems, with the final thickness selected according to diameter, installation method, and mechanical-risk level. | CP does not replace coating thickness requirements. A high-quality coating reduces required CP current and improves system reliability. |
| Temperature Capability | Often selected for continuous service temperatures around 80–110°C, depending on the qualified epoxy formulation and project specification. | Commonly selected for service temperatures up to approximately 80°C; higher-temperature polyethylene systems require specific qualification. | Design must consider the actual operating temperature because coating resistance, electrolyte resistivity, and current demand vary with temperature. |
| Mechanical Resistance | Good adhesion and chemical resistance, but the relatively thin layer can be more vulnerable to gouging and impact during lifting, transport, and installation. | High resistance to impact, abrasion, soil stress, and handling damage because of the thicker polyethylene outer layer. | CP cannot repair mechanical damage. Any exposed steel should be inspected and repaired before immersion, backfilling, or commissioning. |
| Flexibility and Bending | Good when properly qualified, but excessive bending or poor surface preparation can cause cracking, disbondment, or loss of adhesion. | Good overall protection, although minimum bend radius, polyethylene strain limits, and field-joint flexibility must be confirmed for the installation method. | Flexible anode cables and electrical connections must be protected from bending, abrasion, and pullout during installation. |
| Surface Preparation | Requires abrasive blast cleaning, normally to a near-white metal condition, followed by controlled surface profile, cleanliness, and preheating. | Requires the same high-quality blast cleaning for the epoxy primer, followed by controlled heating and extrusion or application of the adhesive and polyethylene layers. | Electrical continuity, coating holiday inspection, anode connection integrity, and isolation from unintended metallic structures must be verified. |
| Holiday Detection | High-voltage holiday testing is normally used after coating, with test voltage determined by coating thickness and the governing specification. | Holiday testing is required for the complete multilayer system; the test method and voltage must be compatible with the total coating thickness. | All detected holidays should be repaired before CP commissioning because defects increase local current demand and may accelerate coating disbondment. |
| Cathodic Disbondment Risk | Requires qualified resistance to cathodic disbondment, especially at coating defects and field joints. Qualification should reflect the project temperature and electrolyte. | The epoxy primer and adhesive system must be qualified for cathodic disbondment; the polyethylene layer alone does not provide electrochemical protection at defects. | Excessive negative polarization can damage coatings and promote disbondment. Protection potential limits must be established with the selected coating system. |
| Field-Joint Treatment | Field joints may use compatible liquid epoxy, heat-shrink systems, or other qualified repair systems, depending on the installation specification. | Field joints normally require a separately qualified multilayer repair system, such as epoxy-based primer with compatible adhesive and protective outer layer. | Field-joint electrical continuity and coating integrity should be checked before the pipeline is connected to the CP system. |
| Cathodic Protection Options | Compatible with galvanic sacrificial anodes or impressed-current CP, provided coating resistance and current-distribution requirements are met. | Compatible with sacrificial anodes or impressed-current CP; the lower coating defect rate can reduce initial and long-term current demand. | Use sacrificial anodes where simplicity and low maintenance are important; use impressed-current systems where current output must be adjustable over a large structure. |
| CP Design Inputs | Include exposed steel area, coating breakdown factor, seawater or soil resistivity, temperature, design life, and electrical continuity. | Use the same inputs, but account for the coating system’s expected breakdown factors during installation, operation, and ageing. | Design should include current density, anode capacity or rectifier output, anode spacing, shielding, stray-current risk, monitoring points, and future inspection access. |
| Inspection and Monitoring | Inspect surface preparation, coating thickness, adhesion, cure, holiday detection, repairs, and field-joint condition. | Inspect each layer, interface temperature, thickness, adhesion, holiday detection, impact damage, and field-joint condition. | Monitor pipe-to-electrolyte potential, anode or rectifier output, current distribution, reference electrodes, and coating-related current changes. |
| Recommended Selection | Choose when strong adhesion, chemical resistance, thinner coating build, and controlled shop application are the main priorities. | Choose when the pipeline faces severe marine handling, abrasion, rock contact, burial stress, or long-term mechanical exposure. | Specify CP together with the coating system for submerged or buried carbon-steel pipelines, then verify both systems during commissioning. |
| Key Standards to Consider | Use the project’s applicable pipeline coating standard, such as ISO 21809-2, together with approved qualification and inspection procedures. | Use the applicable multilayer polyethylene coating standard, such as ISO 21809-1, together with project-specific qualification and repair requirements. | Use the applicable CP design and operation requirements, such as ISO 15589-2 for offshore pipelines, and coordinate them with the coating specification. |
