Assessing Cathodic Protection Criteria

Cathodic protection (CP) coupons and soil corrosion probes (SCPs) can be effective tools in applying CP criteria under challenging operating environments. These situations include measuring off-potentials using coupons in cross-bonded systems or structures influenced by interference from CP systems not controlled by the main protected system operator. They can also include situations where CP potentials cannot be reliably measured and corrosion rate measurements using electrical resistance (ER) soil corrosion probes are used as an alternative to conventional CP criteria for buried pipelines.

The goal here is to examine the various scenarios where CP coupons and SCPs can be used, using field data from installations on buried pipelines, and also with reference to the requirements of the new ISO standard ISO/FDIS 15589-1 on different CP criteria for varying soil resistivities. In this process, it will also be useful to review selected information obtained from the recent NACE International Technical Committee TG-321 report on soil corrosion probes.

Applying CP criteria

CP coupons and soil corrosion probes (SCPs) can be effective tools in applying CP criteria under challenging operating environments, such as cross-bonded systems or in high resistivity soil environments. CP coupons are used for measuring polarized (“off”) or depolarized (100 mV) potentials on pipelines and other structures, and are useful in situations where these type of potentials cannot be measured easily. This is usually in situations such as cross-bonded systems or structures influenced by interference from CP systems not controlled by the main protected system operator; or in situations where it is not practical to simultaneously switch off numerous sources of CP current connected to the pipeline being tested.

SCPs are used to measure the metal corrosion rate, and are connected to the structure so that they receive the same level of CP as the structure. Thus even if the measured potential reading on the structure is not at the minimum required level, the corrosion rate measurement (mils per year, or microns) can be used as an alternative criterion to assess CP effectiveness. Low levels of polarization in high resistivity soils, or erroneous potential readings due to poor soil contact are examples of situations where SCPs can be useful.

The new ISO international standard for CP criteria, ISO/FDIS 15589-1, lists different CP potential criteria for various soil resistivities. Both coupons and probes can be used to implement this standard. Coupons can be used to measure polarized potentials, while SCPs can be used in cases where the required polarization cannot be achieved, but where it can be shown through corrosion rate measurements that there is effective protection on the structure.

Cathodic protection coupons

The concept behind the use of CP coupons is that the coupon provides an estimation of the potentials on the adjacent structure by approximating coating holidays in the vicinity of the coupon test station. Thus for structures of known (and reasonably good) coating quality, the coupon off-potential, measured by interruption of the CP current to the coupon, provides an estimate of the polarized potential of the structure.

However, if the estimation of the size of the coating holidays in the vicinity of the coupon is in error, the coupon potentials and polarization could be either too conservative (if there are less holidays on the structure), or in error (if there are more holidays on the structure), and therefore the level of CP on the structure would be less than that being indicated by the coupon. NACE International has issued a Recommended Practice (RP-0104-2004) detailing the use of CP coupons for CP potential measurements. NACE Test Method TM-0497-2002 Appendix C details the use of coupons for determining the adequacy of CP on pipelines.

Types of coupons

Different types and sizes of coupons are used based on coating holiday estimation, the type of structure, and its material of construction. The material of the coupon should be similar to the material of the structure it is connected to; e.g., carbon steel or ductile iron. Coupons are available in different shapes and configurations, and some versions integrate reference electrodes into the coupon housing. This enables the coupon to minimize possible IR drop effects in high current density or high soil resistivity locations. Since the coupon provides an estimation of the potentials on the adjacent pipeline, by approximating a coating holiday in the vicinity of the coupon test station, the size of the coupon should be representative of the possible coating defects on the pipeline at the location where the coupon is installed.

Examples of commercially available CP coupons are shown in Figures 1 and 2. The two coupons shown in Figure 1 have separate lead wires so that, if required, one can be connected to the CP circuit, while the other is not connected to the CP circuit. This enables the non-connected wire to be used as a “native” coupon to measure the free corrosion potential of the coupon. The unique feature of the coupon in Figure 2 is that it has a conductive membrane in the center of the coupon which allows potential measurements to be made from inside the PVC pipe, thereby minimizing the IR drop for current-on readings.

Figure 3 shows a typical CP coupon test station head. This includes terminals for the wire connected to the pipeline; an on-off switch installed between the pipeline wire terminal and the CP coupon wire connection terminal; another terminal for the “native” coupon; and a terminal for a reference electrode, if one is installed permanently.

Installation and connections

The preferred installation location of the coupon depends on the shape and size of the structure and also the type of CP system protecting the structure; i.e., close or remote anode beds, impressed current or galvanic anodes. Coupons must be installed close to the structure, and in a way that they receive the same CP current as the structure with no possible shielding or interference effects. A typical CP coupon connection method for a buried pipeline is shown in Figure 4, where the CP coupon is installed at the bottom of a PVC tube. The coupon is connected to the pipeline CP circuit through an “on/off” switch in the test station head. The switch is normally “on,” thereby providing CP current to the coupon. “Off” potential and depolarization measurements can be made by switching the CP current “off” using this switch. Coupon potentials can be measured by lowering or installing a reference electrode inside the PVC tube (also referred to as a “reference tube”), which reduces measurement errors due to high resistivity soil or CP currents circulating in the vicinity of the coupon.

Application

The ISO international standard for CP criteria, ISO/FDIS 15589-1:2003, specifies the CP protection criteria for buried pipelines. This standard allows for the assessment of IR free potentials from the instant “off” potentials measured on a coupon installed adjacent to, and at the same depth, as the pipeline. As mentioned above, this is particularly useful in situations where there is a requirement to measure polarized or depolarized potentials in cross-bonded systems or structures, influenced by interference from CP systems not controlled by the main protected system operator, or if it is not practical to simultaneously switch off numerous sources of CP current connected to the pipeline being tested.

Soil corrosion probes

ER probes increase resistance as a function of metal loss in the cross section of a probe, and indicate the cumulative metal loss of the sample. When used in soil-side applications, they are referred to as “soil corrosion probes” (SCPs). The use of corrosion probes for buried/soil-side applications is not yet as common as the use of CP coupons to determine CP effectiveness. There is increasing interest in the industry in using ER probes as an alternative tool to determine if sufficient corrosion prevention is being achieved on a structure, in cases where the required potential criteria may not be met. Certain countries and industries have taken the lead in using ER probes; e.g., buried pipelines in Australia, and soil side protection on aboveground storage tanks.

NACE Committee TG-321 has issued a Technical Report on the use of SCPs which outlines the state of the art in the use of these probes for applications on buried pipelines and structures. This report mainly addresses applications for ER SCPs attached to buried pipelines. The report also includes information on the use of Linear Polarization Resistance (LPR) probes for assessing soil corrosiveness, and the use of corrosion probes for reinforced concrete structures.

Types of probes

ER soil corrosion probes typically used for pipeline applications are of two types: a) ring type probes as shown in Figure 5, and b) flat probes shown in Figures 6, 7 and 8. The ring type probes are installed as part of a complete test station installation, similar to the CP coupon installation described above (Figures 1 and 4). The probe assembly consists of a circular probe element used for measuring the corrosion rate, which is installed at the bottom of a PVC tube.

The probe is constructed together with a concentric coupon ring that can be used to measure the native or polarized CP potentials. The “probe element” shown in Figure 5 is connected to the CP circuit, and provides the same level of CP as the pipeline that it is installed on. The coupon ring may or may not be connected to the CP circuit, depending on whether only native potentials are required to be measured, or additional CP polarized readings are to be taken. The connecting wires from the probe are terminated in a test head assembly (Figure 9) mounted on top of the PVC tube.

The flat type probes are installed close to the pipeline, and the connecting cable terminates in a test station box that can be installed away from the pipeline, if required by site conditions. Another type of flat probe which has a relatively slim profile (Figure 8) has been developed for use under tape wraps.

Installation and connections

The probe connecting cables are terminated in a test station box which has terminals for the CP connection wires, an on-off switch, and a multi-pin socket for connection of the instrument used to take the probe measurement. A typical test station head terminal block arrangement is shown in Figure 9. The CP “connection wire” shown in Figure 6 is connected to the “probe lead” terminal, which is connected to the “pipe” terminal through the on/off switch. This switch is of the “normally on” type, therefore the probe is provided CP current at all times, except when “off” potential readings are being taken by operating the switch.

In the installation arrangement shown in Figure 10, an ER probe test station is installed adjacent to a CP potential measurement test station, and then connected to the CP source by making a connection to the CP test station lead wire, rather than directly to the pipeline. This simplifies the installation since a new wire connection to the pipe is not required. Another type of installation routes the cable from a flat ER probe (Figures 6, 7, 8) mounted adjacent to the pipe or under a wrap, directly to a terminal block in the CP test station box.

Application

SCPs are used to measure the metal corrosion rate and are connected to the structure so that they receive the same level of CP as the structure. Thus, even if the measured potential reading on the structure is not at the minimum required level, the corrosion rate measurements (mils per year, or microns) is used as an alternative criterion to asses CP effectiveness. SCPs are typically used for buried pipelines or congested plant piping, where CP potentials cannot be reliably measured and corrosion rate measurements using ER probes are used as an alternative to conventional CP criteria for buried pipelines.

A portable corrosion probe reader instrument is used to measure the cumulative metal loss on the corrosion probe. For probe potential measurements, the probe “off” potential is measured by interrupting the current flow to the probe through the on/off switch. For the calculation of the corrosion rate, a graph is made of metal loss versus time. The corrosion rate between any two measurement times is the difference in metal-loss measurements divided by the time difference and annualized; i.e., the slope of the metal-loss curve between the selected times. For a series of measurements, the average corrosion rate is calculated using regression analysis from the slope of the trendline as calculated by the least squares method. Most graphing programs, such as an electronic spreadsheet, have built-in capability to calculate slopes. For new installations, data is typically collected on a monthly basis until a corrosion rate trend can be established (approximately 12 months), after which data can be taken on a quarterly basis.

Conclusion

CP coupons and soil corrosion probes can be used as an alternative to pipeline potential measurements for determining CP effectiveness. CP coupons are useful in situations where there is a requirement to measure polarized or depolarized potentials in cross-bonded systems or structures influenced by interference from CP systems not controlled by the main protected system operator. They are also useful in situations where it is not practical to simultaneously switch off numerous sources of CP current connected to the pipeline being tested. CP coupons provide an estimation of the potentials on the adjacent pipeline, by approximating a coating holiday in the vicinity of the coupon test station. Thus, the size of the coupon should be representative of the possible coating defect sizes on the pipeline at the location where the coupon is installed.

ER probes measure the metal corrosion rate, and can be used in cases where the required polarization cannot be achieved, or cannot be measured accurately, but where it can be shown that there is effective protection on the structure through corrosion rate measurements. Visual observations through bell hole inspection of the pipeline can be done to corroborate the calculated corrosion rates.

International standard ISO/FDIS 15589-1 lists different CP potential criteria for various soil resistivities. Both coupons and probes can be used to implement the new ISO international standard for CP criteria. Coupons can be used for measuring polarized potentials, while probes can be used in cases where the required polarization cannot be achieved, but where it can be shown through corrosion rate measurements that there is effective protection on the structure.

Acknowledgments

The author wishes to express his appreciation to ES Technology and the Consulting Services Department of Saudi Aramco for permission to publish this article. Based on a paper presented at the NACE CORROSION 2007 Conference & Expo, March 11-15, 2007, Nashville, Tennessee.