Corrosion coupons have been used since the 1930s in North America and Europe by corrosion pioneers. Cathodic protection coupons are a proven way to determine the polarization level of a structure and to determine the IR drop error that is always associated with structure to soil potential measurement. (Learn more about corrosion coupons in the article Corrosion Coupons: Why Relying on One Test Method Isn't Enough.)
In this article we will discuss the use of coupons as a method to measure cathodic protection (CP) levels and to determine if a CP system is effective or not. Moreover we will show different types of cathodic protection coupons and how they are installed to effectively give an indication of CP levels. But first we must explain what a CP coupon is.
What is a Cathodic Protection (CP) Coupon?
A CP coupon is a piece of bare metal that should be manufactured from the same material as the structure under test . The coupon is buried in the same electrolyte near the structure to be tested and connected to it through a test station. This allows the coupon to be connected to the structure's CP system and to be polarized in a similar manner as the structure. Because the coupon is a bare piece of metal, it can resemble a holiday in the coating of the structure under test. (Related reading: Practical Techniques for Cathodic Protection Potential Measurement.)
Why Coupons are Used to Measure a Structure's Polarization Level
In structure to electrolyte potential measurement, an inevitable error is always introduced, which is known as the IR drop error. This error depends on several factors such as burial depth, soil resistivity, coating conditions and the presence of stray currents. To eliminate this error, the instant off potential measurement method has been adopted to measure the true polarization potential level of a structure.
All current sources affecting the structure under test should be interrupted synchronously in order to obtain an accurate instant off potential measurement. In some cases this is not applicable or too difficult, and it would be easier to assess the CP level of the structure using the coupon instant disconnect potential. Some of the cases where the CP coupon can be used instead of the structure to electrolyte potential are when:
- The structure under test is under the influence of several impressed current sources that must be synchronously interrupted together. It may be difficult in the case of a complex piping network where a large number of current sources are operating and must be interrupted simultaneously. In this case CP coupons can be momentarily disconnected from the structure and it's instant disconnect potential measurement compared to relevant standards in order to assess it's CP status. Moreover, if the 100 mV shift polarization criteria is adopted, and there is a need to obtain the rest potential or native potential of a coupon, then the CP coupons can be disconnected for a long period of time, thus allowing it to depolarize completely. This can be easily done without the need to disconnect the CP system for an extended period of time.
- The presence of foreign rectifier units can't be interrupted or their location is unknown, resulting in an IR error that will be present in instant off potential measurements.
- The presence of directly connected sacrificial anodes can't be interrupted, resulting in an IR error in off potential measurements.
- The presence of stray currents might affect off potential measurements.
- Rapid IR spikes that immediately appear after current interruption can affect off potential measurement.
- Multiple pipelines in the same right of way interfere with one another that make individual measurement for each pipeline difficult.
Structure to Electrolyte Instant Off Potential Measurement versus CP Coupon Polarized (Off) Potential
Structure to electrolyte instant off potential measurement is an effective and proven method to determine the CP status of a buried or submerged structure. However, as mentioned in NACE SP0169 (Control of External Corrosion on Underground or Submerged Metallic Piping Systems), if on grade voltage measurements are used to determine the CP status of a structure while CP is applied then IR drops other than across the structure to electrolyte boundary must be considered. NACE SP0169 includes a number of ways this may be done, and NACE TM0497-2018-SG (Measurement Techniques Related to Criteria for Cathodic Protection on Underground or Submerged Metallic Piping Systems) includes a number of test methods used for these criteria. One of the methods already mentioned in NACE SP0169 is using CP coupons.
Although it might seem that the instant off potential measurement of a structure is the same as CP coupon polarized (off) potential, three differences still exist between the two measurements.
First, the structure to electrolyte potential measurement is affected by several factors, including:
- The number and distribution of holidays around and along the structure
- Variation in the specific conductance of the coating along and around the structure
- Different electrolyte conditions (soil type, moisture content, temperature, oxygen content) along the length of the pipeline
- Different current densities along the length of the pipelines that result in different levels of polarization
- Interference resulting from AC and DC stray currents, telluric currents, etc. (For more on this topic, see Corrosion and Electrical Interference in Buried Metallic Structures.)
- Bimetallic connections that might be deliberately or accidentally connected to a CP structure (Sign up for the Bimetallic Corrosion Basics download.)
Second, based on these factors the structure to electrolyte potential measurement is mainly the weighted average of all areas exposed to the electrolyte. The true polarized potential of an individual holiday, especially if it is a small holiday on a large structure, cannot be accurately determined based on the factors mentioned above.
Third, when a coupon is installed close to the structure and in the same electrolyte, the coupon receives the same level of CP current and reaches the same level of polarization as an adjacent area of the structure having the same resistance to the earth. This gives a better indication of the CP status of the structure compared to the instant off structure to electrolyte potential measurement, which can be affected by several factors as mentioned above. Moreover, coupons can obtain other information that is not easily obtained using structure to electrolyte potential measurements. Examples include the instant disconnect potential, the depolarization behavior and the current picked up by a CP coupon.
Cathodic Protection Coupon Design
Several types of coupons are available, including the following:
- Two wires coupons.
- Coupons with a built-in integral reference electrode. As an option a flexible conduit can extend above grade for irrigation purpose in case of dry conditions.
- Coupons with a stationary electrode permanently buried near the coupon.
- Dual coupons with the same geometry and size for measuring both CP and rest potentials.
There are some points that should be taken into consideration when purchasing or designing a CP coupon, such as the following:
- The associated fitting and soil access tube (if used) should be made of non-metallic materials.
- The diameter of the soil access tube should be large enough to allow the placement of a common reference electrode like the ones in the market.
- The lead wires should be terminated in an accessible test station to have easy access to them. In addition, the test station should be equipped with a disconnect switch or a similar feature in order to allow rapid disconnection of the coupon from the structure.
- The coupon materials should be of the same material or alloy as the structure under test. This will allow it to have the same polarization behavior as the structure and to have meaningful data from the CP coupon potential readings.
- The size of the CP coupon should be chosen based on the largest holiday size expected on the structure under test. Commercially available coupon sizes range from 650 mm2 to 10,000 mm2. For bare or poorly coated structures, a large coupon should be chosen; however, for a well-coated structure a smaller coupon size should be selected where only small holidays are expected.
- Coupon lead wires should be securely connected to the coupon such that a low-resistance electrical continuity is maintained throughout the design life. This can be done through silver soldering, exothermic welding, mechanical connection or any other suitable method. The connection should be isolated from the electrolyte by encapsulating it in a protective coating.
Where to Install Coupons
Coupons should be installed in areas where different soil resistivity, soil chemistry, moisture content, current density, coating conditions and temperature are expected to be found. An example of these locations are on top of a dry rocky hill, a low wet valley, mid span between CP current sources and at suction and discharge points of compressor stations. CP coupons should be installed in each of these locations in order to assess the effectiveness of impressed current CP system for each of these areas.
How to Install Corrosion Coupons
In order to acquire meaningful readings a coupon must have a good electrical contact between itself and the surrounding environment. This is accomplished by compacting the soil around the coupon to remove any air voids that will degrade the electrical connection between the coupon and the surrounding environment.
Several methods are available for coupon installation, including:
- Excavation activities during structure investigation
- Vacuum excavation
- Hand digging
- Installation of the coupon during the structure's installation
According to NACE SP0104-2014 (The use of coupons for cathodic protection monitoring applications), it is recommended that the lateral distance between the coupon and the pipeline shall range from 10 to 30 cm and shall be installed in the lower half of the pipe (between the 3 o'clock and 9 o'clock position).
Although cathodic protection (CP) coupons have been used by pioneers in the corrosion industry to assess the cathodic protection level of a structure, they haven't gained popularity as a method to determine the polarized potential of the structure. The more common technique is the instant-off structure to electrolyte potential measurement.
However, due to several errors inherent in this technique and the complexity of interrupting several current sources in complex piping networks, the corrosion industry is beginning to emphasize the use of CP coupons as an assessment technique to determine the CP level of a structure.
In this article we showed the errors that exist in structure to electrolyte potential measurement and the advantages of CP coupons compared to structure to electrolyte potentials. We then examined different designs and how and where they can be installed according to NACE standard practices.