How To Interpret Cathodic Potential Protection Survey Data

Cathodic potential protection surveys are essential for maintaining the safety and proper functioning of your infrastructure. Pipelines that are particularly prone to corrosion often have a cathodic protection (CP) system in place to slow the corrosion process. To keep tabs on the status of your pipeline, you should have an engineering expert perform a cathodic potential protection survey.

Once the survey is complete, you’ll have a set of data at your fingertips that details its findings. How do you interpret the data from a cathodic potential protection survey, though? Knowing how cathodic protection works—and how to tell when it isn’t working—can help you improve the longevity of your pipeline.

What Is Cathodic Protection?

If you have a metallic structure submerged in water or buried underground, its location makes it more susceptible to corrosion over time. Pipelines, in particular, need consistent monitoring to ensure their safety and continued functionality. A corroded pipeline will not transfer materials very effectively, and it poses a safety hazard to employees who work near it.

That’s where cathodic protection comes in. CP systems are designed to convert the metallic surface of the pipeline to the cathode of an electrochemical cell. They do this by applying direct current (DC) to that metal surface. In electrical terms, it becomes the passive portion of the circuit, reducing its corrosion potential and keeping the pipeline in good condition.

Galvanic Cathodic Protection

Galvanic anodes, also called sacrificial anodes, are easy to use and do not require an outside power source. They utilize the natural voltage potential, or difference, between the anode and the structure it protects to divert current away from the structure.

Impressed Current Cathodic Protection

If galvanic cathodic protection is not sufficient to impede corrosion on the structure, the addition of an external power source can help. This process is called impressed current cathodic protection. That outside power source increases the voltage differential between the anode and the protected structure so it can divert more current away and reduce corrosion potential.

What Is the Purpose of a Cathodic Potential Protection Survey?

If you’re wondering whether the CP system on your pipeline or other buried structure is working as intended, have an engineer perform a cathodic potential protection survey. The purpose of the survey is to test the current function of the system, monitor corrosion as it occurs, and locate potential areas of improvement.

What the Survey Entails

When you enlist an electrical engineer to perform a pipeline cathodic protection survey, they will use a combination of techniques to gather data. The engineer will visually observe the cathodic protection system, measure the current thickness of your pipe walls, and use internal inspection devices to measure the system’s efficacy.

One of the tools your consulting engineer will use is called a reference electrode, which is part of a piece of equipment called a half-cell. Reference electrodes have a stable and clearly defined potential, which makes them useful in measuring electrodes with an undefined potential—like your pipeline.

How Do You Gather Cathodic Potential Protection Survey Data?

As your electrical engineering expert examines your CP system, they will use a variety of tools to gather different data sets.

Reference Electrode

A reference electrode, as briefly discussed above, provides a well-defined potential against which you can measure unknown electrode potentials. Many reference electrodes utilized in these surveys use a combination of a stable electrode and a working electrochemical cell to provide these comparisons.

Field Connections

The reference electrode won’t measure much of anything if it isn’t connected to the structure you’re surveying. The engineer performing your survey will link the reference electrode to the buried pipeline by way of a voltmeter.

Voltmeter

Your engineer’s voltmeter reads the voltage differential between the reference electrode and your pipeline. The reference electrode connects to the voltmeter’s positive terminal, while the pipeline itself connects to the negative terminal.

What Are the Types of Cathodic Potential Protection Survey Data?

During the course of your cathodic potential protection survey, your consulting engineer will collect several relevant measurements.

On Potentials

A CP system’s on potential refers to its efficacy at limiting corrosion when the system is up and running. Whether your system diverts current away from your pipeline with galvanic anodes or an impressed current rectifier, you should have this potential measured once a year.

Instant Off Potentials

Will your CP system continue to work if the electrochemical cell is momentarily disconnected or interrupted in its current? Engineers measure instant off potentials to determine a pipeline’s overall polarized potential.

In addition, this measurement takes into account the CP current flowing through the surrounding soil, which can skew the data measuring your system’s electronegativity. Have a consulting engineer measure this potential at the same time they check your system’s on potential.

Depolarized Potentials

Also referred to as native potentials, these measurements are taken when your CP system has been turned off for a while or before it is re-energized. Depolarized potentials act as a baseline for cathodic protection; comparing depolarized and on potentials will show you how effectively your CP system is working. It’s best to take these measurements every five years.

What Do These Measurements Mean for You?

The data points outlined above can provide valuable information if you know how to interpret them correctly. Electrical engineers recommend that you have a cathodic potential protection survey done once a year in order to identify weak points and opportunities for improvement within your system.

The following measurements are signs that your pipeline’s CP system is in good working condition:

• The system’s instant off potential is at least as negative as -850mV (millivolts) when measured against a copper sulfate half-cell.
• The depolarized potential is at least 100mV more negative than the system’s measured instant off potential.

If your CP system’s measured potentials fall outside these parameters, your pipeline is at greater risk of corrosion. Your consulting engineer can confirm these findings by measuring the thickness of your pipe walls to see how far corrosion has progressed.

How Can You Apply the Data To Improve Cathodic Protection?

Knowing how to interpret the data and use it to your benefit can improve the longevity of your pipeline’s CP system. If there are areas for improvement, the data can point you in the direction of a solution.

Pipelines are protected most effectively with the minimum amount of CP current that’s actually required to divert corrosive activity away. Stay alert for excess current in your CP system to minimize interference with other structures and improve the system’s overall longevity.

The data from your cathodic potential protection survey can also inform you whether you need to replace insulating joints or remove impedances to electrical continuity. Partner with your consulting electrical engineer to learn which parts of your system need extra protection or attention.

If you’re wondering about the efficacy of your cathodic protection system, consult with an engineering expert to perform a survey of the system’s potential. Once the consultant has collected all the necessary data, interpret the cathodic protection system survey accurately to identify areas of improvement.

Dreiym Engineering is proud to employ a team of electrical engineers who specialize in cathodic protection surveys. Our specialists are trained to use measuring instruments accurately and provide actionable solutions to mitigate pipeline corrosion.