In cathodic protection (CP) work, accurate data is the foundation for ensuring buried or submerged metallic structures like pipelines, tanks, and reinforced concrete remain protected from corrosion. Traditional structure-to-electrolyte potential measurements have been the go to method for assessing CP effectiveness, but they often include errors from voltage drops (IR drops) in the soil or across coatings. These errors can stem from reference electrode placement, varying soil resistivity, structure burial depth, coating conditions, stray currents, or even the CP current itself. Enter CP coupons: a tool that simulates a small bare area on the structure, allowing technicians to measure polarized potentials with reduced interference.

CP coupons have a history dating back to the 1930s when corrosion control pioneers in North America and Europe began using them to gauge polarization levels and confirm IR drops. Over time, research sponsored by the pipeline industry has validated their reliability. NACE SP0104-2020, "The Use of Coupons for Cathodic Protection Monitoring Applications," provides a standardized approach to designing, installing, and interpreting data from these coupons. Originally published in 2004 and revised in 2020, this standard focuses primarily on buried pipelines but includes appendixes for underground storage tanks (USTs), aboveground storage tanks (ASTs), water tanks, reinforced concrete, cased pipelines, and AC interference monitoring.

For corrosion field technicians, understanding this standard is crucial. It equips you with methods to evaluate CP compliance without always interrupting current sources, which can be impractical in complex systems with multiple rectifiers, foreign CP influences, or direct connected galvanic anodes. Coupons help minimize errors from long line currents, telluric activity, or stray DC/AC sources, leading to more reliable assessments of protection levels. By following SP0104-2020, you can confirm if structures meet criteria in NACE SP0169, such as -850 mV polarized potential or 100 mV polarization shift, while avoiding common pitfalls in traditional surveys.

This article breaks down the standard's key elements, offering practical insights on how to apply coupons in your daily work. We'll cover design, installation, monitoring techniques, and specialized applications, helping you enhance accuracy in CP evaluations and ultimately extend asset life.

CP coupons are small metal samples, typically made from the same material as the protected structure, installed in the same electrolyte and connected electrically to simulate a coating holiday or bare area. Once connected to the CP system via a test station, the coupon receives similar current and polarizes like the structure. Technicians can disconnect it to measure instant off potentials, depolarization, or current pickup, providing a snapshot of local protection without CP system interruptions.

A basic setup includes a CP coupon for polarized measurements and optionally a native coupon to establish baseline potentials in open circuit conditions. The standard emphasizes that while coupon potentials aren't identical to structure wide readings; due to factors like holiday distribution, varying electrolytes, or interference; they offer a precise, error minimized view of local conditions. For instance, in areas with uninterruptible CP currents (galvanic anodes or foreign systems), coupons eliminate IR drops by isolating the measurement point.

Key advantages for technicians include:

The standard outlines when coupons add value: in multi-rectifier zones, foreign CP interference, direct anode connections, telluric/long-line currents, stray influences, or when using polarization criteria. They're also useful for bare/poorly coated structures, multiple pipelines in rights-of-way, AC affected lines, or grounded station piping. However, coupons may not represent areas under disbonded coatings that shield CP current.

In practice, technicians can use coupons to comply with SP0169 criteria by calculating IR drops as the difference between on and instant off potentials. For example, if a pipeline's on-potential is -1.2 V but instant-off is -0.9 V, the 0.3 V drop must be considered. Coupons simplify this by providing direct polarized values.

Designing a CP coupon system starts with matching the coupon to the structure's material (carbon steel for pipelines) and anticipated holiday size. Common designs include:

Size matters: Commercially available coupons range from 1 in² to 16 in², simulating small holidays on coated structures or larger areas on bare ones. Larger coupons may introduce structure like errors, while shape affects resistance to earth; cylindrical ones polarize differently than flat plates.

Key features to include:

Technicians benefit from these designs by reducing measurement variability. For example, in variable electrolytes, a soil access tube allows reference electrode placement near the coupon, cutting errors from distant grade level readings.

Location selection is driven by the need to represent CP effectiveness accurately. Coupons must be in the same backfill and electrolyte as the structure, typically 4-12 in away, below the 3:00-9:00 position on pipes for bottom side simulation.

Consider environmental variations: Install in rocky hills, wet valleys, mid spans between CP sources, or near compressor stations where resistivity, chemistry, or temperature differ. For bare structures, more coupons are needed due to greater error susceptibility.

Avoid shielding: Face bare sides away from the structure, orient perpendicularly to minimize current blockage. In anode influenced areas, place midway for representative densities.

For technicians, strategic placement reveals "worst case" spots or confirms uniform protection, aiding troubleshooting in interference zones.

Installation methods vary: During construction, place coupons directly; for existing structures, use excavation, augering, vacuum digging, or hand methods. Connect via test stations with nonmetallic conduits.

Steps include:

In larger pipelines, consider circumferential placement for local variations. For high resistivity sites, add bentonite mixes (without contaminating coupon electrolyte).

Proper installation ensures long term data integrity, helping technicians avoid false positives from poor contact or voids.

During backfill, screen and compact soil, adding minimal water to prevent resistivity changes. Avoid voids from freezing or subsidence.

For soil access tubes, seal cable entries to avoid IR issues. In foreign structure proximity, ensure no unintended connections.

These precautions maintain similarity between coupon and structure environments, ensuring accurate CP assessments.

Monitoring involves high impedance voltmeters, data loggers, or automated readers (see Table 1 from standard for equipment by measurement type). Key readings:

Address IR errors: Test with current on/off and coupon connected/disconnected to quantify drops (Appendix D procedure). If instant off and disconnect match, IR is minimal.

Interpret considering site factors: Coupons sample locally, so account for anodes, stray currents. Initial discharge on new coupons may occur. Disconnect until stabilized.

USTs (Appendix A): Install near tank bottom, center for "worst case." Use during construction or retrofit; consider anode placement for density variations.

ASTs (Appendix B): Place under floor, mid radius, or near ring walls. Handle high resistivity pads; use for IR heavy setups.

Reinforced Concrete (Appendix C): "Rebar probes" at mesh depth; bond for potentials, isolate for rates.

Voltage Drop Calculation (Appendix D): Detailed 8-step procedure to isolate IR in disconnect readings.

Cased Pipelines (Appendix E): Monitor annulus with probes for isolation, rates, humidity/oxygen. Install via ends or vents.

AC Coupons (Appendix F): Locate at power line crossings; measure AC potential/density with loading context.

NACE SP0104-2020 empowers corrosion technicians with a framework for using CP coupons to achieve precise, error reduced monitoring. By mastering design, installation, and interpretation, you can confidently assess protection levels, comply with criteria, and mitigate risks in challenging environments.

Established in 2011, Roberts Corrosion Services, LLC delivers comprehensive, turn-key cathodic protection and corrosion control solutions nationwide. Our end-to-end expertise encompasses design and inspection, installation and repair, surveys and remedial work. We provide drilling services for deep anode installations and a full laboratory for analysis of samples and corrosion coupons, as well as custom CP Rectifier manufacturing.

While our initial focus was on the Appalachian Basin area, we complete field work all over the US. We are a licensed contractor in many states and can complete a wide range of services.

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

Let us know how we can help.

Website

LinkedIn

Location: 39.251882, -81.047440

(304) 869-4007