Introduction

In the field of pipeline integrity management, accurate pipe-to-soil (P/S) potential measurements are the cornerstone of effective cathodic protection (CP) systems. These readings determine whether underground metallic structures, such as pipelines, are adequately protected from external corrosion. However, traditional P/S assessments often fall short due to IR drop errors, interference, or the impracticality of interrupting CP across huge networks. This is where external corrosion coupons can be used as an excellent tool, enabling IR-free, reliable data without compromising system operations.

External corrosion coupons, small metal samples mimicking pipeline material, provide a non-invasive method to simulate coating defects and capture true polarized potentials. These devices align with industry standards such as NACE/AMPP SP0104 and TM0497. This article explores their functionality, applications, installation, and advantages for corrosion technicians seeking to enhance CP monitoring. Integrating coupons can elevate your corrosion control strategies, ensuring compliance with regulations and optimizing asset longevity.

What Are External Corrosion Coupons and Why Do They Matter for Pipe-to-Soil Potential Data?

External corrosion coupons are bare metal devices, typically carbon steel, installed in native soil backfill adjacent to pipelines at the same depth and elevation. They replicate the pipeline's exposure to environmental corrosives, allowing technicians to evaluate CP performance without direct access to the structure. Basic models feature a known surface area (e.g., 1-10 cm²) with two wires: one connected to the pipeline for current-carrying and another as a test lead, terminating at above-ground test stations.

These coupons matter because they address key limitations in conventional P/S measurements. Standard readings, taken with a copper/copper sulfate (CSE) reference electrode, can be distorted by additional IR drop (the voltage loss through soil resistance) leading to overestimated protection levels. In complex environments, like near power lines or rail systems, stray currents can further complicate accuracy. Coupons act as surrogate coating holidays, enabling "instant-off" or depolarized potentials that reflect true CP effectiveness.

According to NACE SP0104-2020, "The Use of Coupons for Cathodic Protection Monitoring Applications," coupons provide a method to simulate defects and verify protection without widespread interruption. In high consequence areas (HCAs), this is crucial for demonstrating due diligence. For technicians, coupons offer site-specific insights, reducing false positives and guiding targeted remediation.

Primary Uses of External Corrosion Coupons in Enhancing Pipe-to-Soil Potential Accuracy

External corrosion coupons excel in scenarios where traditional methods are insufficient, particularly for obtaining IR-free P/S data. Their applications support field operations across many categories.

Coupons are often paired with permanent reference cells and / or soil access tubes to increase their accuracy.

IR-Free Pipe-to-Soil Potential Measurements Without Interrupting CP

Conventional P/S surveys often require interrupting the entire CP system to eliminate IR drop, a process that can be logistically challenging. Coupons overcome this by allowing brief disconnection at the test station while keeping the main system active. Placed 4-12 inches from the pipeline, they serve as a controlled surrogate for measuring "instant-off" potentials.

DC and AC Current Density Measurements for Interference Detection

Beyond potentials, coupons facilitate current density assessments, critical for identifying overprotection, underprotection, or interference. High AC densities (>20-30 A/m²) at defects can accelerate pitting near high-voltage lines, a common issue with shared corridors.

Wired with shunts or by using RMS ammeters, coupons can measure magnitude and direction of DC or AC current flow. This data may be used to infer current density along the pipeline surface and identify current discharge due to interference.

A difference between pipeline instant-off and coupon instant-disconnect potentials may indicate stray currents from rail systems or nearby CP. Integrated with close-interval surveys (CIS) or direct current voltage gradient (DCVG), this provides a better understanding of the complete picture.

Collecting Depolarized Data to Validate CP Effectiveness

Depolarized data allows technicians to determine the actual level of CP polarization without full shutdowns. Disconnect the coupon for hours to days and monitor potential decay; looking for a stable depolarized potential.

In mixed-metal piping systems coupons deliver polarized potentials that remain largely unaffected by varying surface conditions, such as soil chemistry or coating variations. This capability is highlighted in AMPP publications, including the article "Coupons for Cathodic Protection Evaluation of Mixed-Metal Piping" from Materials Performance magazine (September 2017). Additional guidance from standards like NACE SP0104 reinforces their use for depolarization testing, noting that these measurements represent both polarized and depolarized states to verify protection effectiveness.

Facilitates the calibration of DCVG % IR and ACVG dB Measurements

There is often concern with the relationship between indirect fault size measurements reported from coating holiday surveys and the actual physical area on the pipe surface. The %IR or dB can be measured at the coupon (which is a known size) and provide a reasonable indication of that relationship for holidays of similar depth, soil conditions, and physical size.

Installation Best Practices: Integrating External Corrosion Coupons into Pipeline Systems

Proper installation is essential to maximizing the effectiveness of external corrosion coupons in cathodic protection (CP) systems. Always consult the manufacturer's guidelines for specific requirements, such as allowing the coupon to acclimate or "weather in" for a designated period before establishing a connection to the structure. This ensures accurate representation of the structure's polarization behavior and minimizes initial measurement discrepancies.

Site Selection: Choosing Optimal Locations for Maximum Effectiveness

Selecting appropriate locations for CP coupons is fundamental to obtaining reliable and representative data on the structure's protection levels. The objective is to position coupons in areas that reflect the varying environmental and operational conditions affecting the pipeline. As outlined in NACE SP0104-2020, key considerations include:

Proximity and Environmental Similarity to the Structure: Coupons should be installed 4 to 12 inches laterally from the protected structure and immersed in the same electrolyte (soil or water) to experience comparable current density and polarization. For pipelines, placement adjacent to the lower half (between the 3 o'clock and 9 o'clock positions) is recommended to account for typical groundwater accumulation and real-world exposure.
Targeted Monitoring Areas: Focus on locations where CP performance may fluctuate, such as:

Midpoints between CP current sources.
Transitions in soil conditions (e.g., from dry, rocky terrain to moist, low-resistivity valleys).
Vicinity of compressor or pump stations, including suction and discharge sides.
Zones susceptible to stray currents, foreign CP systems, or telluric currents.
Bare or poorly coated segments, which may necessitate additional coupons due to heightened variability in voltage drop errors compared to well-coated sections.

Anode Influence and Shielding Considerations: Account for anode types (e.g., distributed galvanic anodes) and their placement, which can lead to uneven polarization. Install coupons to evaluate current distribution while avoiding configurations where the coupon might inadvertently shield CP current from the structure or receive disproportionate protection.
Quantity and Distribution of Coupons: In complex or heterogeneous environments, deploy multiple coupons to capture diverse conditions. For large-diameter pipelines, circumferential placements can help identify localized variations in soil resistivity, coating integrity, and electrolyte chemistry.

Coupons are designed to simulate small, exposed areas on the structure, such as coating holidays, thereby serving as indicators for potential corrosion risks. Comprehensive documentation of installation details, including site coordinates, environmental conditions, and rationale, is critical for long-term data interpretation and regulatory compliance.

Installing Coupons at New Test Stations

For new pipeline constructions or dedicated CP monitoring setups, incorporating coupons during the initial build phase allows for optimal precision and integration. Ensure the coupon is embedded in undisturbed native soil matching the pipeline's environment to achieve accurate representativeness.

Follow these step-by-step guidelines:

Assess and Prepare the Site: Identify a location based on risk assessment and NACE SP0104-2020 criteria. Excavate to the pipeline depth using safe techniques, such as hand digging, augering, or air vacuum excavation, to prevent damage to adjacent structures.
Position the Coupon: Place the coupon 4 to 12 inches laterally from the pipeline at the same depth and elevation, preferably in the lower half (3 o'clock to 9 o'clock positions). Ensure the coupon material (typically carbon steel) matches the pipeline to ensure consistent polarization characteristics.
Wire and Connect Components: Route insulated wires from the coupon to the test station, including one for current-carrying connection to the pipeline and another as a test lead. Incorporate a disconnect switch in the test station to facilitate instant-off measurements. Install a nonmetallic soil-access tube (e.g., PVC, at least 2" diameter) extending to grade for reference electrode insertion, reducing voltage drop (IR) errors.
Backfill and Compact: Refill the excavation with native soil in layered increments, compacting each layer to match the original soil density and eliminate air voids. This promotes stable electrical contact and prevents settlement that could compromise data accuracy.
Test, Commission, and Document: Perform initial potential measurements to establish baselines. Allow for any manufacturer-recommended weathering period before full connection. Document all details, including GPS coordinates, photographs, wiring diagrams, and environmental data, for ongoing reference and compliance.

Adding Coupons to Existing Test Stations

For operational pipelines, adding coupons to established test stations provides a cost-effective means to upgrade monitoring capabilities without extensive disruption.

Follow these step-by-step guidelines:

Assess and Plan the Site: Verify the pipeline's depth, location, and condition using line locators to mitigate risks. Review existing test station wiring and select an adjacent position.
Excavate Carefully: Employ localized excavation methods, such as hand shoveling or air vacuum, to access the required depth without fully exposing the pipeline. Minimize the disturbance to a small pit or augered hole for coupon placement.
Position the Coupon: Insert the coupon 4 to 12 inches laterally from the pipeline at matching depth, ensuring it resides in undisturbed native soil for environmental consistency.
Wire and Connect Components: Route insulated wires through existing conduits to the test station, configuring one for current-carrying connection and another as a test lead. Add or retrofit a disconnect switch if feasible, and install or extend a nonmetallic soil-access tube (e.g., PVC, at least 1-inch or 2.5 cm diameter) for reference electrode access, shielding measurements from surface interferences.
Backfill, Test, Commission, and Document: Refill with native soil in compacted layers to eliminate voids and restore site integrity. Conduct initial potential tests, observe any weathering period, and comprehensively document the process, including GPS data, updated wiring schematics, and baseline readings.

Incorporating Buried Permanent Reference Cells: Enhancing Measurement Stability

To achieve greater precision in potential measurements, integrate buried permanent reference cells (e.g., copper-copper sulfate [Cu/CuSO4] electrodes) alongside coupons. These cells offer stable, long-term benchmarks unaffected by surface weather or seasonal variations.

Follow these step-by-step integration guidelines:

Assess and Select the Site: Identify a position adjacent to the coupon, maintaining at least 20" separation to avoid ion leakage that could alter local soil chemistry.
Excavate and Position the Assembly: During coupon installation, create a dedicated space for the reference cell at the same depth, ensuring firm soil contact for reliable performance.
Wire and Connect Components: Route the reference cell's insulated lead wire to the test station, securing all connections and applying protective encapsulation where necessary.
Backfill and Calibrate: Refill with native soil in compacted layers to prevent voids. Record initial potentials.
Document and Maintain: Log details, including serial numbers, exact positions, and calibration data.

Conclusion: Empower Your Cathodic Protection Strategy with External Corrosion Coupons

External corrosion coupons are indispensable for achieving accurate pipe-to-soil potentials, transforming CP monitoring from reactive to proactive. By facilitating IR-free measurements, current density assessments, and depolarized data, they address critical challenges in pipeline corrosion control. Their integration ensures regulatory compliance, safety, and cost savings.

Roberts Corrosion Services, LLC

Established in 2011, Roberts Corrosion Services, LLC delivers comprehensive, turn-key cathodic protection and corrosion control solutions nationwide. Our expertise includes CP design and inspection, installation and repair, surveys and remedial work, deep anode drilling, full laboratory analysis for samples and corrosion coupons, and custom CP rectifier manufacturing.

While our initial focus was on the Appalachian Basin area in West Virginia, we complete field work all over the US. We are a licensed contractor in many states and can complete a wide range of services, including advanced internal corrosion fluid sampling and field testing.

Our biggest strength is in our flexibility for our clients. Solutions and Results.

Let us know how we can help.

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Location: Ellenboro, West Virginia

(304) 869-4007

External Corrosion Coupons