The risks of corrosion causing catastrophic events at power plants were discussed in “The case for cathodic protection” in POWER, February 2008. Though the oil and gas industries arestrictly regulated and must provide protection for all pipelines, that protection stops at the gates to a power plant. Some plant managers don’t realize they have no on-site protection; others may have corrosion protection systems but don’t know how they operate—or if they are even functional. Cathodic protection is the proven way to stop corrosion and prevent further oxidation on storage tanks and pipelines within a plant. So, educating yourself about the basics of cathodic protection (CP) is a vital responsibility of power plant staff.
Admittedly, cathodic protection maintenance lacks the attraction of maintaining rotating or fired equipment. But it is nonetheless as important. When a turbine sheds a blade or a boiler slags up, there are physical clues to the problem. Underground pipeline corrosion, on the other hand, is hidden from view and easy to overlook when a plant is busy producing electricity. Ignoring cathodic protection maintenance is like going to the dentist when the pain flares in a tooth only to find that the problem has been present for months or years. The result in both cases is greater trauma and cost.
For power plants with old CP systems or limited records, it is wise to have a CP survey completed by a reputable firm. Such a survey will provide evidence of the working level of a CP system (if anyexists); recommendations for bringing the survey up to National Association of Corrosion Engineering–approved operation; or certification that the system is operating according the manufacturer’s intent. Then plant managers and power industry executives can be satisfied with the safety of their plant and its people, or it can begin taking steps to protect those assets from calamity. POWER caught up with Ted Huck (email@example.com), vice president of sales and marketing at Matcor Inc. and cathodic protection expert, to discuss the fundamentals of cathodic protection.
Basic cathodic protection theory
POWER: What is cathodic protection?
Huck: Cathodic protection is a means to prevent the corrosion of steel.
POWER: What is steel corrosion?
Huck: Corrosion of steel is electrochemical. Steel under normal conditions naturally wants to react with its environment in a reaction process known as oxidation/ reduction. It is this reaction that converts steel into iron oxide, or what is commonly called rust. The electrochemical reaction is called a galvanic corrosion cell.
POWER: Do some things corrode faster than others?
Huck: Corrosion rates vary from location to location. Technically, the rate of corrosion is a function of current flow. For example, 1 amp of DC current will consume 20 pounds of steel in one year.
POWER: How does cathodic protection prevent corrosion?
Huck: Cathodic protection is the process of creating a flow of current from a source (the anode) to the structure being protected (the cathode) and shifting the electro-potential of the cathode to a more negative level. This shift in potential prevents the oxidation/reduction reaction from occurring and halts the corrosion.
POWER: How do I apply electric current to the steel surface to stop corrosion?
Huck: That is what we call cathodic protection system design. To use an analogy, to light up a building there are many different types and configurations of lights that could be used. Applying current to the steel structure is a lot like lighting up a building. Anodes placed around the structure are much like light bulbs: The anodes are designed to have current flow from the anode to the structure. How much current flows and how that current distributes are all critical design features.
Galvanic anode systems
POWER: What does “galvanic cell” mean?
Huck: The galvanic cell, named after Luigi Galvani, consists of two different metals connected by a salt bridge or a porous disk between the individual halfcells. It is also known as a voltaic cell or electrochemical cell—a definition shamelessly taken from Wikipedia.org. Batteries are galvanic cells waiting to be connected to some device.
POWER: What are anodes and cathodes?
Huck: “Anode” describes the more electro-negative of the two different metals, while “cathode” describes the more positive of the two metals. Corrosion occurs at the anodic sites.Sometimes, the anode and cathode are different areas of the same structure. The important thing to remember is that milliamps of current can consume a lot of steel over a long period of time.
POWER: What are galvanic anodes?
Huck: Galvanic anodes are specific metals (magnesium, aluminum, andzinc) whose natural potential is less than steel’s. Because these metals are more negative than steel, they can be used as anodes to protect buried steel structures. The anodes are often called sacrificial anodes because they are consumed in the process. A typical home water heater has a sacrificial anode to prevent corrosion of the heater shell.
POWER: So you simply bury enough magnesium, aluminum, or zinc anodes next to the buried structure (a gas pipeline,for example), and the anodes will be consumed while protecting my structure?
Huck: That’s the basic idea. Of course, it is not that simple, and galvanic anode are quite fickle. The designer has very little design flexibility with galvanic anode systems.
POWER: What do you mean by “very little design flexibility”?
Huck: Galvanic anode systems are limited by the natural potential of the anode material. For example, high-potential magnesium anodes are typically –1.75V, while protected steel is typically above –0.85V, so the differential between the anode and the protected steel is less than 1V DC. For DC circuits, Ohm’s law applies (V = IR), and V is fixed at less than 1 volt. How much current you can achieve will depend on the circuit resistance. If R is too high, then there will not be sufficient current output. If R is too low, then the current output may be too high and the anode may be consumed too fast.
POWER: Can galvanic anodes be used for buried power plant piping applications?
Huck: Yes, they are often used for this purpose. They offer several advantages over impressed current systems, including simplicity of operation and no requirement for external power supply for operation.
POWER: What are the concerns with a galvanic system in a power plant application?
Huck: The big concern is the grounding system. In a power plant, the grounding is extensive, and all of the above-ground metallic structures must be grounded to meet code. Unfortunately, to make a galvanic CP system work, the buried piping must be electrically isolated from the grounding system. This involves installing isolating flange kits at every location where the piping transitions from above grade to below grade.
POWER: What happens if the system loses electrical isolation?
Huck: Grounding networks are designed to provide a low-resistance path to dissipate current. If electrical isolation between the buried piping systemand the grounding system is lost, the galvanic anodes’ output will increase significantly, and all the current that the anode produces will be taken preferentially by the grounding network, leaving little or no current to protect the piping systems. The anodes will not be able to protect the piping, and they will be quickly consumed.
POWER: How can a system lose electrical isolation?
Huck: Unfortunately, it is all too easy to lose electrical isolation. For very simple piping systems, isolation can often be maintained by simply testing the isolating flanges on a regular basis. Most power plants, however, do not have simple piping systems and do not regularly test for loss of isolation of buried piping.
POWER: Does that mean galvanic anode systems should not be used in power plants?
Huck: No, but they should not be installed and forgotten. Periodic testing should be performed, and the isolation flanges do require testing and occasional replacement. For most complex power plants, it is almost impossible to ensure electrical isolation, and in those cases galvanic anode systems should be avoided.
Impressed current systems
POWER: What is an impressed current system?
Huck: Unlike galvanic anode systems, impressed current systems have an external power supply that provides current to the CP system and does not rely on the natural potential difference between the anode and structure.
POWER: What is the external power supply and what does it do?
Huck: The external power supply is called a transformer/rectifier, or simply rectifier. Most commonly, rectifiers are hooked up to an AC power source and convert the AC power to DC power. (Solar and other power sources can also be used.) The DC power source drives the cathodic protection circuit with current flowing from the positive (anode) to the negative (the structure being protected).
POWER: What are the advantages of an impressed current CP system?
Huck: Unlike a galvanic anode system,which relies purely on the natural potential difference between the anode and the structure and is limited to less than 1 V difference, rectifiers can be sized to provide DC voltages ranging from a few volts to several hundred volts. To use an analogy, galvanic anode systems are like sailboats: They are limited in power.Impressed current systems are like speed boats: They are limited only by the size of the engine.
POWER: Do impressed current CP systems use the same types of anodes?
Huck: No, because they use an external power supply, the anodes can be selected based on their consumption and current discharge capabilities and are not limited to metals whose natural potential is less than that of steel. Impressed current anodes can be designed to provide more current and to last much longer than galvanic anodes.
POWER: What about the isolation of piping from grounding systems?
Huck: Unlike galvanic anode systems,impressed current anode systems can be designed to protect virtually any structure, regardless of whether or not there is grounding present. It is just a
matter of providing sufficient current capacity to allow for significant current losses to the grounding system and any other structures that might take current intended for the structures being protected.
POWER: How much current can be lost to the grounding system and other structures when trying to protect buried plant piping?
Huck: That depends on the type of system being proposed. In some cases, 90% or more of the current might be lost to other structures when trying to protect just the piping. The important thing to note, however, is that with a properly designed impressed current CP system it is possible to protect any structure, regardless of the size, soil resistivity, and presence of plant grounding.
Maintenance and testing requirements
POWER: Are galvanic CP systems maintenance-free, since they have no rectifiers?
Huck: A common misconception is that galvanic anode systems require no maintenance, whereas impressed current systems require more maintenance because they require an external power source. This is only partially true. Galvanic anode systems inside a power plant are always vulnerable to the devastating impact of loss of isolation. The loss of isolation not only renders them ineffective in providing cathodic protection to the intended structure, but it also results in their rapid consumption. Both galvanic and impressed current systems require regular testing and inspection.
POWER: How often should I test my CP system?
Huck: Annual testing is strongly recommended for both galvanic and impressed current CP systems.
POWER: What about the rectifiers?
Huck: The rectifiers used for CP systems are generally quite robust and are designed to last the life of the CP system (often 30-plus years). It is recommended that they be checked monthly to ensure that they are still turned on. This involves a visual check of the volt and
amp meters and requires no special training or skills.