A few facts and fantasies about cathodic protection.
Because of this confusion, a number of "fantasies" still exist about cathodic protection. My own discussions with other engineers have brought some of these "fantasies" to my attention, usually resulting in long discussions, sometimes ending in arguments.
This article is an attempt to address some of these misunderstandings without delving too far into theory. Three commonplace, but erroneous, ideas were chosen for discussion. There are many more, but P&GJ wanted an article, not a miniseries, so here goes.
Fantasy #1: Cathodic protection puts an electric charge on the pipe to protect it from corrosion.
FACT: There is no buildup of electric charge anywhere in the circuit during cathodic protection. The rectifier creates a voltage difference between the pipe surface and the anode, causing current to flow in the soil and forcing chemical reactions to occur which prevent corrosion from occurring on the pipe.
All cathodic protection systems are electrical circuits. Just like other closed circuits, the electric charge does not accumulate anywhere; it simply travels around in a circle continuously. The CP circuit has two parts: the metallic part and the electrolyte or soil part. What makes cathodic protection confusing is that the current must change from electron current in the metallic part to an ion current in the soil part. This current conversion requires some explanation.
When people think of electric current, they almost always think of electrons. This is because we were taught in school that electricity is moving electrons. That is both true and false. Electric current by definition is any movement of electric charge. A voltage difference between two ends of a wire causes a flow of negatively charged electrons in the wire, which is an electric current. But in other materials, electrons may or may not carry the current. Any positive or negative charged particle could do the job.
Cathodic protection deals with current flow in both wires and electrolytes, such as water or soil. In electrolytes, the current is carried by charged atoms or molecules called ions, which may have either a positive or negative charge. Electric current in electrolytes takes the form of ionic drift.
The passage of current from the soil to the pipe is possible only through electrochemical reactions at the pipe and anode surfaces. Any surface which uses electrons to create chemical reactions is called an "electrode." In cathodic protection these electrode reactions which convert electron current into ion current are different at the anode and cathode (pipe) surface. The reactions that occur at the cathode protect the steel from corrosion. It is the protective action of these chemical reactions at the cathode surface that gives cathodic protection its name.
Charged ions created at the electrodes slowly migrate through the ground under the influence of the voltage difference between the anode and pipeline created by the rectifier. The actual motion of the ions can be very slow, with tens of thousands of years required for the ions to move a few feet.
The "pipe to soil" potential is used to evaluate the effectiveness of CP.
FACT: The "pipe to soil" potential is impossible to measure and is not used for CP measurements. The actual measurement used is the "pipe to reference electrode" potential. Reference electrodes are stable points of reference for potential measurements. Soil is not a stable reference electrode.
The definition of the term potential is "the work required to move a unit charge from point A to point B." In terms we are more familiar with, this means that potential is the voltage difference between two different things in two different places. There is no such thing as the potential of the pipe, only the difference in potential (voltage) between the pipe and something else. To measure a potential we need "something else" to compare to the pipe. This is where reference electrodes come in. Reference electrodes are the "something else" that is needed to make a potential measurement.
Reference electrodes are sometimes called "half cells" because they are the other half of a potential measurement cell, the first half being the pipe itself. To be useful, reference electrodes must be stable, to give a constant point for comparison. That way, any change in the measured potential must come from changes on the pipe. The "pipe to soil" potential cannot be measured because soil cannot be made into a stable reference electrode.
The standard potential criteria developed by NACE International to determine whether a pipeline under cathodic protection is adequately protected is based on the idea of "close" potential. The "close" potential is the potential between the pipe and a stable half cell placed extremely close to the pipe surface. This "close" potential is the most accurate for assessing protection. Unfortunately this is almost always impossible, as it would require a permanently installed reference at the pipe surface, or perhaps an excavation. Real measurements are made with the reference placed at the ground surface on top of the pipe. These measurements not only have a large separation distance from the pipe, but are also affected by other errors, mainly voltage gradients in the ground associated with current flow. Errors from current flow in the soil are called IR drop errors, and are the most significant cause of inaccurate measurements.
A good estimate of the "close" pipe to reference electrode potential can be made from a distant half cell by eliminating IR drop and other errors. This can usually be done by interruption of rectifiers and control of stray currents, but this is not always possible. In many cases an experienced CP technician is required to identify sources of error, compensate effectively and determine an accurate estimate of the "close" potential in order to evaluate protection.
"To protect a long section of pipe, we need a big rectifier."
FACT: The effective range of an impressed current CP system is rarely determined by the output of the rectifier. The main determining factors are the resistance of the groundbed, the size of the pipe, and the quality of the pipe coating. Selection of a rectifier comes after those variables have been determined.
The distance of pipeline that CP can protect from a single rectifier location is not directly determined by the capacity of the rectifier. Other factors will normally determine the number, spacing and size of rectifiers needed. The two most important are circuit resistance and the amount of bare steel surface on the pipe. Groundbed resistance is the most significant source of resistance in the entire CP circuit. Since groundbed output is determined by the rectifier DC voltage divided by resistance, if groundbed resistance is low, more current is available to protect the pipe for a longer distance.
The second most important resistance is that of the pipeline returning current to the rectifier. While this resistance is trivial over short distances, over many miles it can become substantial. For this reason, a large pipeline with a lot of steel and a small resistance per linear foot can be protected for a greater distance than smaller lines.
Pipe coatings are important for cathodic protection because a good coating will minimize the amount of bare steel, reduce the current demand and allow the available current from the rectifier to spread farther down the line. Poor coatings lead to excessive current consumption and reduce the effective distance of CP protection. The major cause of increasing CP requirements on old pipelines is the deterioration of the pipe coating with time.
In principle, cathodic protection is simple. We use a rectifier or galvanic anode to create a complete circuit between the anode and the pipe, throwing current through the ground (or water) and creating chemical reactions on the pipe surface which prevent corrosion. In practice, however, the details can get very complicated and confusing. To maintain good CP systems, misunderstandings should be resolved with the help of a qualified corrosion engineer.
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|Publication:||Pipeline & Gas Journal|
|Date:||Mar 1, 1997|
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