Understanding Shielding and Its Impact on CP

Mechanism: The insulating barrier physically blocks the direct flow of CP current to coating defects (holidays) beneath it. Although current may attempt to flow through the soil or water filling the gap between the barrier and the metal, the longitudinal resistance of this narrow electrolyte path is often too high to allow sufficient current penetration.

Practical Rule of Thumb: Current generally cannot be forced through a space greater than about 3 to 10 times the thickness of the insulating layer, depending on soil or water resistivity. For example, if a rock is 1 inch thick and closely spaced to the pipe, current penetration beyond 3 to 10 inches of soil or water gap is unlikely.

Implications: Areas shielded by such barriers may remain unprotected, allowing corrosion to initiate and continue under the barrier despite the rest of the pipeline being protected.

Partial Shielding: If the insulating barrier is porous and absorbs moisture, becoming somewhat conductive, partial current flow may occur, providing some protection. However, this is unreliable and should not be counted on.

Shorted Pipeline Casings: Ideally at cased crossings the pipeline would be isolated, the annulus would be dry, and the end seals are intact. If the casing is in metallic contact with the carrier pipe, CP current tends to flow to the casing and is shielded from the pipe inside. Any water accumulation inside the casing increases the corrosion risk.

Reinforcing Wire in Weight Coatings: Metallic reinforcing wires embedded in concrete weight coatings can electrically contact the pipe, intercepting CP current. Even a single contact point can shield an entire length of pipe from protection.

Congested Underground Areas: In pump stations, tank farms, or other congested sites with multiple buried metallic structures, current may preferentially flow to other structures or grounding systems, creating potential gradients in the soil that reduce CP effectiveness on the target pipeline.

Mechanism: The metallic object provides a lower resistance path for CP current, effectively "stealing" current that would otherwise protect the pipe surface. This diversion reduces current density at coating defects and can leave areas vulnerable to corrosion.

Implications: Shielding by metallic diversion can cause localized corrosion, especially inside casings or near reinforcing wires, and can impose additional load on CP systems, reducing overall efficiency.

Preventing Current Flow to Coating Defects: Shielded areas do not receive sufficient protective current, allowing anodic reactions and corrosion to proceed.

Creating Localized Corrosion Hotspots: Corrosion under shielding can be aggressive and difficult to detect until significant damage occurs.

Increasing System Load and Reducing Efficiency: Diverted current to unintended metallic objects increases anode consumption and power requirements without improving protection.

Complicating Potential Measurements: Shielded areas may show misleading pipe-to-soil potentials, masking corrosion risk during surveys.

Close Interval Potential Surveys (CIPS): Attempt to identify areas with insufficient polarization that may indicate shielding.

Direct Current Voltage Gradient (DCVG) and Alternating Current Voltage Gradient (ACVG) Surveys: Attempt to locate coating defects and assess current distribution, highlighting shielded zones.

Resistance Measurements: Measuring resistance between the pipe and surrounding soil or between the pipe and metallic objects can reveal unintended electrical contacts causing shielding.

Visual Inspection at Excavations: Confirm presence of insulating barriers, metallic contacts, or water accumulation in casings.

Electrical Isolation Testing: Verify integrity of insulating spacers, end seals, and isolation joints to prevent metallic short circuits.

Avoid or Minimize Use of Casings: Where possible, avoid cased crossings. If necessary, ensure proper insulation between casing and carrier pipe using spacers and end seals.

Use Nonmetallic Reinforcing Materials: Replace metallic reinforcing wires in weight coatings with nonmetallic alternatives to prevent current diversion.

Proper Installation and Inspection: Ensure insulating barriers are not too close or continuous in a way that blocks current flow. Inspect and maintain isolation devices and seals.

Distribute CP Anodes Appropriately: Use close spaced anodes in congested areas to ensure overlapping areas of influence and uniform protection.

Repair or Replace Damaged Coatings: Attempt to minimize coating holidays that exacerbate shielding effects.

Electrical Bonding and Isolation: If appropriate, bond foreign metallic structures or electrically isolate them to control current paths. Proper testing needs to be completed.

Hidden Shielding: Shielding can occur in inaccessible locations, making detection and repair difficult.

Misinterpretation of Potential Readings: Shielded areas may show apparently adequate potentials, leading to false confidence in protection.

Inadequate Design for Congested Areas: Failure to account for multiple metallic structures can cause widespread shielding and potential corrosion.

Improper Installation of Isolation Devices: Missing or failed spacers and seals can create unintended metallic contacts.

Shielding is a critical factor that can undermine cathodic protection effectiveness by preventing protective current from reaching parts of the metallic structure.

It occurs due to insulating barriers or metallic diversions such as shorted casings, reinforcing wires, or congested underground metallic networks.

Shielding leads to localized corrosion, increased system load, and potential misinterpretation of protection status.

Detection requires specialized surveys, resistance measurements, and careful inspection.

Mitigation involves proper design, installation of isolation devices, use of nonmetallic materials, and targeted repairs.

Proactive management of shielding effects are essential for long term corrosion control success.

NACE SP0169 – Control of External Corrosion on Underground or Submerged Metallic Piping Systems

ASTM G8, ASTM G95 – Standard Test Methods for Cathodic Disbondment of Pipeline Coatings

NACE TM0115 – Laboratory Testing of Pipeline Coatings for Cathodic Disbondment Resistance

AMPP SP0502 – External Corrosion Direct Assessment (ECDA) Standard Practice