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COUPONS FOR CATHODIC PROTECTION
Like most technologies, the Corrosion Control industry is always evolving. State-of-the-art anodes, power supplies and corrosion monitoring instrumentation have significantly increased the scope, effectiveness and reliability of CP systems.
The History
The first application of cathodic protection (CP) was traced to Sir Humphry Davy in 1824 when he succeeded in protecting copper sheathing against corrosion from seawater by using iron anodes. (How or if Sir Humphry Davy measured the copper sheathing-to-water potential is unknown.)
The “Challenge”
Despite the array of sophisticated measuring equipment available, there is still a challenge or difficulty measuring an accurate structure-to-electrolyte or “pipe-to-soil” (PSP) potential.
The typical PSP is measured with a high impedance voltmeter by connecting one test lead to the pipeline under test and the other test lead to a reference electrode. The reference electrode is placed in the soil (electrolyte) in proximity to the pipeline being tested.
Before 1975, placing a copper-copper sulfate reference electrode over a pipeline and measuring a PSP equal to or more negative than -0.850 volts would have been considered “full cathodic protection.” However, we have learned that depending on many variables, this procedure can result in significant errors due to IR drop.
IR drop is the voltage created by the CP current flowing in the (resistive) electrolyte, i.e., soil or water, between the reference electrode and the structure under test. This IR drop (voltage) is additive to the measurement at the pipe surface reading, which results in an indicated PSP higher (more negative) than actual. Without considering or measuring the IR drop, the resultant PSP measurement can have a significant error. To minimize the IR drop error, the reference electrode should be placed as close as possible to the pipe. Some consider this a “best practice,” but this may not always be possible.
The Cathodic Protection Coupon
A CP coupon is a device or assembly designed to eliminate, or at least minimize, IR drop errors when measuring PSPs. In its simplest form, a coupon is a piece of bare metal (alloy, size and shape vary) that has one or two insulated wires connected to the metal element of the coupon.
A typical CP coupon installation would position or bury the coupon adjacent to a cathodically-protected pipeline. The coupon wires are then routed to an above-ground test station where CP tests can be conducted. A portable digital multimeter (DMM) is used to measure the voltage between the coupon and a reference electrode.
There are four popular installation configurations for CP coupons:
1. The coupon is buried adjacent to and connected to the pipeline. The coupon now simulates a coating defect (holiday) on the pipeline and CP system being evaluated. In this condition, the coupon is cathodically- protected to the level of the pipeline since it is connected directly to the pipeline.
Page 2 of 6 2. The coupon is buried adjacent to the pipeline and the coupon wire is NOT connected to the pipe. In this
condition, the coupon will simulate a “native” or unprotected pipeline.
3. The coupon is buried adjacent to and connected to the pipeline. A current measuring shunt is usually installed in the coupon-to-pipe circuit. In this configuration, the coupon is used to measure AC current density.
4. The coupon is buried adjacent to and connected to the pipeline. A normally closed switch is installed in the coupon-to-pipe circuit. The coupon now simulates a coating defect (holiday) on the pipeline and CP system being evaluated. In this condition, the coupon will be cathodically-protected to the level of the pipeline while the switch is closed. When the switch is opened, an “instant disconnect” PSP can be measured. In this case, opening the switch will replicate an interrupter and the corresponding PSP is the same as using CP current interrupter.
Basic Applications for CP Coupons
Coupons are used for monitoring CP effectiveness by measuring CP potentials and/or AC current densities: • Measure a CP “ON” potential or a CP “OFF” potential
• Alternative CP criteria, either the polarized “OFF” potential or the 100-mV depolarization criterion • To replicate a “native” potential with a completely isolated coupon
• A direct connect coupon can be used to measure real-time AC current density in areas of induced AC
Evolution of the CP Coupon
Many pros and cons in using coupons have developed over the years resulting in different ideas on the best coupon design.
The evolution of coupons and their application has taken different paths, as evidenced in the examples that follow. Many CP experts insist that the size, surface area, shape, the installed location and installation methods must all be considered to obtain accurate data from the coupon. As a result, coupon designs are based on the philosophy of the coupon manufacturer.
Dr. Neil deGrasse Tyson (American Astrophysicist) said it best when he referred to “The great challenge of life. Knowing enough to think you are right, but not knowing enough to know you are wrong”. The great challenge in the CP industry is to accurately measure a pipe-to-soil potential to ensure protection of a structure for safety and reliability. Given the variables, evaluating the proper CP coupon for an installation can be challenging.
Coupon Designs and Configurations
1 cm2, Flat Steel Coupon - This popular coupon style represents the area of a holiday determined to be the most representative size most vulnerable to AC corrosion. Initially designed to provide AC current density information, this coupon can provide both AC and DC potential information. Due to its relatively small surface area, proper installation is critical for proper contact-to-earth. High contact-to-earth resistance can result in very large errors when measuring AC current density.
10-cm2, Flat Steel Coupon – This coupon style is gaining popularity due to its larger surface area and a theory that the surface area is more forgiving in less than optimal installation techniques. As with the 1cm2 coupon, this coupon provides both AC & DC potential
information. This coupon has a distinct advantage as each milliamp of AC current measured is a direct numerical conversion to Amps per Square Meter when measuring AC current densities.
Page 3 of 6 10-cm2, Cylindrical Coupon - Some experts believe that this cylindrical configuration
provides a more precise voltage gradient “envelope,” providing a more accurate reading. It also can provide DC & AC potential information. Like the 10-cm2 flat
coupon, this coupon allows for direct AC current density conversion.
10-cm2, Hemispherical Coupon – The hemispherical coupon shape tends to be preferred by some experts over the flat or cylindrical coupon. Like the 10-cm2 flat
coupon, this coupon allows for direct AC current density conversion
10-cm2, Dual Hemispherical Coupon – This dual design allows for one of the coupons to be dedicated as a “native coupon.” This allows the second coupon to be used for CP potential or AC current density measurement. This coupon also allows for direct AC current density conversion.
Combination Steel Coupon and Reference Electrode – This configuration provides a choice between a 1 cm2 (outlined n NACE standard SP0177-2014), a 10 cm2 or a 100
cm2 coupon allowing for more consistent contact to the soil. This combination of
coupon and reference electrode, allows for faster installation and provides all CP readings in one device. The manufacturer claims the product can “thrive in all environments,” even those chemically detrimental to other reference electrodes and it will survive dry summers and freezing winters.
IR Free CP Coupon – This coupon is designed to take potential measurements that are substantially free from IR-drop error without interrupting current or disconnecting the coupon. The potential measuring port is in the geometric center of the rectangular coupon (away from IR drop fields in the environment). It is used in locations where interrupting currents or disconnecting the coupon to make measurements is not practical. Included is a 1-cm2 coupon
for AC current density measurements. These assemblies are available with an reference electrode installed or configured so that a portable reference electrode can be used.
Portable AC Current Density Coupon - This coupon is designed to take current density measurements where a stationary or permanent coupon is not installed. Provided with a 10-cm2 conical tip, the coupon is driven into the soil. Using a portable DMM, the AC
current density can be measured. To increase the accuracy and resolution of the measurement, the DMM should be connected in sequence.
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Precautions to Ensure Accurate Coupon Readings
Improper Installation - The biggest issue leading to coupon inaccuracy is improper installation. While design drawings will provide the general location for the coupon, there are usually no installation specifications. As a result, an installer may not understand the function or purpose of the coupon, potentially burying it somewhere along the pipeline with no consideration for proper contact-to-earth protocols. It is imperative that the coupon element have close, intimate contact with the surrounding soil to function properly. For example, if a rock is in contact with the coupon element, this obstruction will prevent accurate coupon readings.
Resistance or Meter Loading - For measuring PSP only, the surface area of the coupon is relatively unimportant. However, proper contact-to-earth is critical. A coupon with a high resistance-to-earth contact will yield a lower (less negative) PSP.
Improper contact can result in “meter loading.” This occurs when a meter is connected to a circuit under test. All meters require some amount of current to operate. When connected to a circuit, the meter imposes a load on that circuit. If the circuit is very sensitive (most coupons and reference electrodes), the initial voltage on the circuit prior to connecting the meter will be different (lower) due to the load imposed by connecting the meter. Depending on the input resistance or impedance of the meter, this error can vary greatly. Understanding and considering meter loading is as important as dealing with IR drop. See Appendix for an explanation of “Meter Loading”.
Coupon Surface Area - To accurately measure AC current densities, the technician must know the surface area of the coupon used for testing. The two most common coupon sizes are 1-cm2 and 10-cm2. As with all coupons,
especially current density coupons, proper installation is critical. Ensuring that the coupon achieves intimate contact to soil should result in an accurate current density measurement.
Most AC current density coupons are installed with a meter shunt installed in series. This allows the technician to measure the magnitude of current without disconnecting the coupon. Depending on the value of the shunt installed, (especially when used on the 1-cm2 coupon), the voltage drop across the shunt may be so low that getting
an accurate value is difficult. The best way to get the highest resolution is to connect the DMM, in series, with the coupon and measure the AC current value directly.
By using a 10-cm2 coupon and the DMM set on AC milliamps (mA), the numerical value displayed is a direct
conversion to Amps per Square Meter (A/M2). Example: the DMM displays 30.2 mA AC, which now can be
converted to 30.2 Amps per Square Meter. No mathematical conversion is needed in this case.
Coupon Installation Caveats
Soil Backfill - The soil environment surrounding the pipeline must be used to backfill the coupon to provide the most accurate coupon interpretation. To achieve proper contact-to-earth, the backfill material must be carefully selected and prepared. Example: If a sand bedding is used to backfill the pipeline, then the same sand should be used for the coupon installation.
Similarly, if native soil was used to backfill the pipeline, this same soil should be used to backfill the CP coupon. This will provide the most consistent and complete contact with the coupon surface.
Note: the soil should be screened to select only the fine grains of soil to ensure close contact with the coupon element.
Location - The installation location of the selected coupon(s) must be specified to ensure that the measurement obtained is representative of what the CP designer intended.
Stationary Reference Electrode - Any CP reference electrodes installed in conjunction with the CP coupon should be installed in a similar manner (as referenced above).
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Questions & Answers
Question #1 – How long does it take for a “native” coupon to reach an equilibrium or stable potential?
Answer #1 – A “native” coupon may take six months (or longer) to reach an equilibrium (stable) potential. Much depends on the soil and moisture conditions.
Question #2 – When can I connect the CP coupon to the pipeline?
Answer #2 - Like a native coupon, it is important to allow a CP coupon to reach an equilibrium before it is connected to the CP system. The amount of time needed will vary depending on soil, oxygen, moisture content, etc. Routine monitoring will eventually show a more positive potential shift. Once stabilized, the coupon is at equilibrium. Only when this occurs should the coupon be connected to the pipeline. Once connected, it should remain connected 24/7, except when under test.
Question #3 – When can I use the CP coupon for an “instant disconnect” potential test?
Answer #3 - This will vary depending on the soil conditions and the CP current density in the area. As a general guideline, a CP coupon should be connected to the pipeline for minimum of 72 hours before conducting an “instant disconnect” potential test.
Question #4 – Why do I get a lower potential on the coupon when measuring to a stationary reference electrode as compared to the potential reading to the same coupon using my portable reference electrode?
Answer #4 - It is likely that the stationary reference electrode has dried and has a high resistance-to-earth. Therefore, the coupon and/or the stationary reference electrode cannot produce the current required to properly operate the electronics in the multimeter. This observable fact is referred to as “meter loading” and often misunderstood or overlooked by technicians. See Appendix for an explanation of “Meter Loading.”
Question #5 - How do I know if I have a meter loading issue?
Answer #5 - There are several specialty meters and meter accessories that can detect and correct for meter loading. They fall into three categories:
1. Variable Input Impedance DMM – This instrument allows for changing the input impedance while measuring a voltage. If the voltage increases when the meter is switched to a higher impedance, then meter loading exists. Depending on the range of the meter’s variable impedance selections, an accurate reading can be obtained in most cases
2. Potentiometric Voltmeter Instruments - This technology has been in service for over 70 years. While awkward to use, it has many advantages over a variable impedance DMM. Potentiometric measuring instruments can be a stand-alone meter assembly or an accessory that is connected between the standard DMM and the high resistance electrode circuit. This instrument can provide up to 2 billion ohms input resistance.
3. Electronic Impedance Buffer Circuit – This is an accessory device that connects between the standard DMM and the high resistance electrode circuit. Simple to use (a single button), this device provides the equivalent of over 5 billion ohms input resistance, virtually eliminating meter loading.
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Summary
There remains much to do and learn regarding the design, installation methods and specifications for coupons. To complicate matters, coupons are often installed with associated stationary cathodic protection reference electrodes. This multiplies the probability of introducing additional errors to the final potential reading.
The probability of meter loading being a factor in obtaining inaccurate readings is relatively high. Conditions that can exacerbate good contact-to-earth include:
• High resistance backfill materials, i.e., sand or simply very dry soil. Regardless of the chemical properties in a soil, if it is dry it will be high resistance. Over time, in arid environments, backfill material can simply dry out.
• Incomplete contact to the soil due to poor installation. Due to their relatively small surface contact, the 1-cm2 coupons would be greatly affected.
How can a corrosion technician use and trust the results of the data collected from coupons?
• First, it is important that the coupons and reference electrodes are properly installed.
• Second, it is important that the technician understand the function, purpose and limitations of the components under test.
• Third, meter loading is an issue that must be considered and addressed. If it is ignored, it is not a matter of if, but when this issue will lead you astray.
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