Everyone in the pipeline integrity industry is faced with the same challenge right now, better data at a lower cost. The primary tools of the trade are all labor-intensive and subject to interpretation. External corrosion coupons are an inexpensive way to prove cathodic protection test data, ensure AC mitigation systems are working, and monitor high-risk locations like pipeline crossings or nearby foreign cathodic protection systems. Typically, these useful devices are under-utilized because they are used incorrectly in the field, or the data is not well understood. So, what data can they provide, and how can it be used to improve your pipeline integrity program?
Cathodic protection (CP) systems are a critical part of the pipeline integrity puzzle. Providing the right amount of cathodic protection current is critical to mitigating external corrosion without damaging pipeline coatings. Interpreting data from annual potential surveys and close-interval potential is often difficult because of the complexity of finding and interrupting all DC current sources in the area. Buried galvanic anodes, DC decoupling devices, and foreign operator CP systems all contribute to the problem. In the simplest form, an external coupon is just a sample of metal, closely approximating a coating holiday on the protected pipeline. For a coupon, the connection to the pipeline can be easily interrupted, eliminating all current flow to the coupon, and creating a representative polarized potential.
A well equipped coupon test station typically includes multiple coupons, allowing the user to measure coupon "current applied", "polarized", and "native" potentials. These measurements can then be compared to "current applied" pipeline potentials in situations where current interruption is difficult or impossible. These models will typically also incorporate a buried reference cell near the test coupon, to reduce the influence of foreign CP current flowing through the soil.
In addition to potential data, coupons are a useful tool in determining the amount of current flow being applied to the pipeline. If the surface area of the coupon is known, and the amount and direction of DC current is measured on the coupon lead, a current density can be calculated. This current density calculation is a great troubleshooting tool when dealing with possible DC interference situations at foreign crossings, or when determining cathodic protection system effectiveness in a complex facility where a mixed metal environment makes it difficult to meet the NACE SP- criteria for effective cathodic protection.
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Current density calculations are also an essential part of monitoring for potential AC corrosion. AC corrosion is a constant threat to pipelines co-located with high voltage power lines. AC current induced onto the pipeline seeks a path to the earth at coating holidays, resulting in rapid pitting corrosion. The best indicator of possible AC corrosion is an AC current density greater than 20 Amps per square Meter. A wealth of information is available on the best methods for monitoring and mitigating AC corrosion and other threats related to co-location with high voltage AC power, but the lowly external steel coupon is the only way to accurately determine AC current density. All pipeline integrity management programs should include methods and standards for AC current density measurement and record keeping.
Bringing new or different data into an integrity management process includes far more planning and detail than can be covered in a simple LinkedIn article. Here are some considerations
As the need for better compliance data AND the consequences of failure become more severe with time, it is increasingly important to evaluate the available technologies for monitoring and record keeping. While the above information is by no means a comprehensive evaluation of coupons as a data source, it hopefully provides enough information to spark further evaluation of external corrosion coupons as a data collection tool.
Jordan Groody is the Director of Technical Services for Bass Engineering Company, a proponent of better data collection, and an occasional contributor to technical and professional publications. For questions about the contentpresented or re-use rights, please contact .
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