CATHODIC PROTECTION – AN INTRODUCTION
by Javeed Shaikh and Yatin Dambal
Corrosion
Corrosion is a naturally destructive phenomenon that occurs when some metals are exposed to the environment. The reaction between air, moisture and the metal substrate gives rise to specific chemical reactions that cause the metal to convert to its more chemically-stable oxide, hydroxide, or sulfide form. In iron-based metals, such as steel, corrosion comes in the form of iron oxides, also known as rust. For electrochemical corrosion to occur, three ingredients must be present: an anode, a cathode and an electrolyte. The anode and cathode are usually connected via a continuous electrical path while both are immersed in the same electrolyte. During this process, the anode experiences corrosion, while the cathode remains unaffected.
Figure 1. A typical electrochemical cell shows electrons flowing from the anode to the cathode through an electrical connection. (Source: Alksub at the English Wikipedia / CC BY-SA)
There are various methods to prevent and control corrosion. One of these is known as cathodic protection (CP). This technique works by connecting the metal to be protected to a more easily corroded “sacrificial metal.” This sacrificial metal corrodes preferentially (acting as the anode) while the more valuable metal object under consideration (acting as the cathode) remains protected. In this article, we will explain how this sacrificial protection method works and describe its various applications.
Understanding Galvanic / Bimetallic Corrosion
To understand how cathodic protection works, we must first appreciate the basics of bimetallic corrosion, also known as galvanic corrosion. Bimetallic corrosion, as its name implies, is a unique type of corrosion that occurs between the pairing of two metals. When a metal is immersed in an electrolyte, it adopts an electrode potential that represents the metal’s ability to be oxidized or reduced. The potential difference is a direct result of the difference in electrode potential between the two dissimilar metals. The metals positioned higher on the table are considered to be anodic (more electronegative), while the metals placed lower on the table are more cathodic (more electropositive). The further apart the contacting metals are in the galvanic series, the greater the potential difference between the metals, thus the more severe the corrosion at the anode. The electrode potential of various metals is displayed on a list known as the galvanic series. (See An Introduction to the Galvanic Series: Galvanic Compatibility and Corrosion for more information.)
Cathodic Protection (CP) and Its Method of Operation
While the design of cathodic protection systems can be sophisticated, their operation is based on the concept of bimetallic or galvanic corrosion described earlier. By understanding the principles of this type of corrosion, we can purposely pair metals together to ensure that one cathodically protects the other. In other words, if we want to protect a particular metallic structure, we can create conditions where this metal becomes the cathode of an electrochemical cell. By electrically connecting the metal to be protected to a more anodic (electronegative) metal, we can ensure that the anode sacrifices itself by corroding preferentially over its cathodic counterpart.
In some cases, external power sources can be used to supply additional electrons to the electrochemical process, which can increase the effectiveness of cathodic protection.
Cathodic protection systems are employed in numerous industries to protect a broad range of structures in challenging or aggressive environments. The oil and gas industry, in particular, uses cathodic protection systems to prevent corrosion in fuel pipelines, steel storage tanks, offshore platforms, and oil well casings. In the marine industry, this protection method is also used on steel piles, piers, jetties and ship hulls. Another common type of cathodic protection, known as galvanizing, is commonly used to protect steel members and structures. (To learn more, read Galvanization and its Efficacy in Corrosion Prevention.)
Types of Cathodic Protection (CP)
As mentioned previously, cathodic protection works by intentionally forming a galvanic cell with another sacrificial metal. This can be achieved by employing two distinct types of cathodic protection: Sacrificial cathodic protection and impressed current cathodic protection.
Sacrificial cathodic protection
In sacrificial cathodic protection systems, the sacrificial anode is connected directly or indirectly to the metal to be protected. The potential difference between the two dissimilar metals generates adequate electricity to form an electrochemical cell and drive galvanic or bimetallic corrosion.
This type of protection is commonly used in the oil and gas industry to protect the structural steel members of offshore rigs and platforms. Here, aluminum bars (or another suitable metal) are mounted directly on steel sections to assume the role of the sacrificial metal. Steel water heaters, tanks and piles are also cathodically protected using a similar method.
Figure 2. Schematic of a pipeline being protected by a sacrificial anode using passive cathodic protection methods. Notice how there is no external power source involved.
Another common example of passive cathodic protection is hot-dipped galvanized steel. During this process, steel members or structures are immersed in a molten zinc bath that coats the object. When the steel is removed from the molten zinc, it reacts with air and moisture to form a protective layer known as zinc carbonate, which creates a galvanic cell with the steel. When the steel member is scratched or damaged, such that the substrate is exposed, the surrounding zinc coating acts as the sacrificial anode and corrodes preferentially to protect the exposed steel. This type of protection continues until the nearby zinc is depleted.
In order for Sacrificial cathodic protection to work, the anode must possess a lower (that is, more negative) electrode potential than that of the cathode (the target structure to be protected). The table below shows a simplified galvanic series that is used to select the anode metal. The anode must be chosen from a material that is lower on the list than the material to be protected.
Impressed current cathodic protection (ICCP)
In large structures, it may not be feasible to use passive cathodic protection methods. The number of sacrificial anodes required to deliver enough current to provide adequate protection can either be unrealistic or impractical. To address this, an external power source is used to assist in driving the electrochemical reactions. This technique is known as impressed current cathodic protection (ICCP). ICCP systems are ideal for protecting lengthy structures, such as underground pipelines. The flanges of connecting pipes are usually insulated using isolation kits to separate the pipes into smaller, more manageable sections for the purposes of ICCP protection.
Figure 3. Schematic of an object being protected by an anode using impressed current cathodic protection (ICCP) methods. Notice how an external DC power source is involved.
Limitations of Cathodic Protection
In large pipeline networks, there may be many crossings, parallelism and approaches near the pipeline’s CP system. DC interference may occur between pipelines, which accelerates corrosion. In order to overcome this problem, pipelines can be electrically coupled, either directly or through resistance.
For coated pipelines, cathodic disbondment may occur due to high CP levels where the applied coating quality is poor. Higher temperatures may also promote cathodic disbondment. High pH environments are also a concern in terms of stress-corrosion cracking.
Conclusion
Cathodic protection is a popular protection method for preventing corrosion in pipelines, offshore oil platforms and other steel structures. However, to be implemented effectively, it is crucial to understand the basic principles of bimetallic/galvanic corrosion. Choosing the right type of cathodic protection system depends on several factors, including cost-effectiveness and the size of the structure to be protected.
TEMPORARY CATHODIC PROTECTION INSTALLATION AND GOOD CONSTRUCTION PRACTICES:
Coating integrity: This is the first line of defense against corrosion is a healthy coating. A well-coated structure will often require only 1 percent or less of the current required to protect the same structure if bare. Particular attention should be paid to maintaining the condition of the coating on the structure and maintaining the structural continuity. If the coating on a structure is damaged cathodic protection requirements will increase dramatically. Properly selected and applied, and installation of the line without damaging the coating is vital to achieving protection at the low current levels desired. Often, coating application and prevention of damage during installation are more important than the materials used. During laying of pipelines attention should be paid to Open bonds and shorts to other structures, these are common causes of inadequate protection and the resulting interference can cause accelerated corrosion damage moreover sacrificial anode systems are the most susceptible to poor performance if the coating system does not meet original specifications.
Coating application: The use of yard-applied coatings is preferred over field-applied coatings since better surface preparation and application are normally achieved under the more controlled conditions at a stationary plant. Inspections should be performed immediately before and after unloading at the site, prior to placing the pipe in the ditch and after all joint and field patches have been made.
Inspections: Regular Inspections are necessary to ensure adherence to the agreed scope of work rather than fault-finding. Careful inspection during the entire construction process, both of the cathodic protection system and of the structures to be protected are vital to the successful application of cathodic protection. These inspections must be performed before backfilling. Once the structure is buried, identification and correction of any discrepancies is difficult and time-consuming and runs over budget and results in project delay. Holiday detectors should be used for all coating inspections in addition the inspector should also make a detailed visual inspection of the coatings and occasional measurements of the bond strength. Visual inspection should also include observation of surface preparation and coating, handling of the pipe, lowering of the pipe into the ditch, backfilling operations Any material, even the highest quality, when applied and handled carelessly will perform poorly, but a marginal quality material will perform well when carefully applied and installed.
Casing: The use of casings should be avoided wherever possible due to the difficulty of protecting the pipeline within the casing and difficulties in maintaining isolation between the casing and the pipeline. The only reason to use casing could be a requirement by law, code, or physical condition. Casings should be uncoated. The casing should be isolated from the pipeline with insulators and cradles, the annular space at the ends of the casing should be sealed to prevent the entry of moisture between the casing and pipe. The extra thickness of the coating on the pipeline for the section to be placed inside the casing may be required in order to prevent damage to the coating when the pipe is pulled into the casing. The annular space between the casing and the pipe must be kept dry until it is sealed. Casing-to-pipe resistance should be measured. A test station should be installed at the casing for future measurements.
Cable Connections: Electrical connections should be inspected visually before and after the insulation is applied. All electrical connections should be insulated.
Foreign pipeline crossings: Newly installed pipelines are commonly installed under existing lines. The owner of the foreign line should be contacted to obtain permission to install test leads and possibly to coat a short section of the foreign line. Since solutions to problems at foreign crossings require cooperative efforts, effective coordination is essential. Clearance of 2 feet between all lines at crossings is recommended. If 1-foot clearance cannot be obtained, the use of an insulating mat is required. Installation of test stations with provisions for bonding at all crossings is essential.
Insulating joints / Isolation Joints: This must be selected so that the materials are compatible with the service environment. Isolating couplings must be properly assembled and tested to ensure that they will be effective. When used on welded pipelines, short “spools” of pipe should be welded to each flange. The flange should then be assembled and the section welded into the pipeline. This will prevent mechanical damage to the insulating joint associated with misalignment. Flanges should be tested with a radio frequency type insulation checker after assembly to ensure that they have been properly assembled. The effectiveness of buried flanges must be verified by impressing a potential on one side of the flange and measuring the change in potential on the other side of the flange. If no potential change is noted, the isolating flange is effective. Test stations with provisions for future bonding should be installed at each buried insulating flange.
Bonding: Bonds between structure sections and to foreign structures should be made and each bond should be brought into a test station to determine bond effectiveness and to install resistive bonds if required.
Test stations: These are required for the initial test and adjustment of the cathodic protection systems, and for future inspection and maintenance. Attachment of “spare” test leads to buried structures is recommended as excavation to reconnect test leads is expensive. All test station leads should be either color-coded or labeled. The connections to the structure should be inspected prior to the burial of the structure. Whenever the structure will be located under a paved area, or whenever paving is installed over a protected structure, soil contact test stations as shown in should be installed.
Sacrificial anode installation: Sacrificial anodes should always be installed at least 3 feet below grade whenever possible. The top of the anode should be at least as deep as the structure to be protected. Horizontal sacrificial anode installations should be used only if obstructions such as rock outcrops preclude vertical installations. The usual practice is to install anodes vertically in augered holes. Anodes should be located on alternating sides of the pipe when possible to reduce interference and allow for more even current distribution. Any impermeable wrapping should be removed from packaged anodes prior to placing them in the holes. The cloth bag used with packaged anodes should be carefully handled as loss of backfill will result in reduced anode output. The anodes should be lowered into the holes either by hand, or by the use of a line attached to either the anode, if bare, or the top of the bag of backfill. The anode lead cable should not be used to lower the anode into the hole as the anode-to-cable connection is easily damaged. Sufficient slack should be left in the anode cable to prevent strain on the cable. All connections should be properly made and inspected before the installation is buried. If packaged anodes are not used and special backfill is required, it should be poured into the holes as the anodes are installed. Anode holes should be backfilled with fine soil free of stones or other debris. Sand should not be used. The backfill should be placed in 6-inch lifts and each lift tamped into place to eliminate voids.
Initial potential survey: After the cathodic protection system is installed, it must be checked out to determine if protective potentials have been achieved without interference to other structures in the vicinity. The structure-to-electrolyte potential measurement is taken with respect to a saturated copper-copper sulfate reference electrode and these readings form the basis in determining if proper levels of protection have been achieved. The negative potential of at least 850 mV or a minimum of 100 mV cathodic polarization is recommended for evaluation of the effectiveness of cathodic protection systems on steel structures.
Problems with interference can be identified during the initial system checkout and should be corrected as soon as possible to prevent premature failure, take readings at each test station/test point. Due to polarization effects, these potentials may change substantially within the first year of installation and should be checked monthly for the first 3 months, then quarterly for the remainder of the first year. Low potential readings may be due to inadequate protective current flow, coating damage, or interference. Sacrificial anode output currents should be measured at each potential current test station since anode current measurements can be used to determine the cause of low potential readings and to better estimate sacrificial anode consumption rates. Anode currents should also be read monthly for the first 3 months and then quarterly for the duration of the first year. Interference is normally detected by analysis of structure-to-electrolyte potentials made during initial system checkout. Unusually high or low potential readings are found over points where the current is either entering or leaving the structure. If potentials are vastly uneven, overprotection or under protection in some areas in order to achieve proper potentials in other areas may result. In this case, if interference is not present, the output of anodes at the areas under protection will have to be increased or anodes added. The output of anodes in the areas of over-protection will have to be reduced. If the anodes are connected through resistors, resistor adjustment may be adequate. Protective output can also be increased or decreased locally by installing additional anodes or disconnecting anodes as necessary.
Over protection: In the sacrificial anode, cathodic protection systems are rare as the open circuit (maximum) potential of most anode materials is normally less than that potential considered to be excessive.
Under protection: This is common in sacrificial anode cathodic protection systems. Low anode current output can be the result of high soil resistivity or high circuit resistance. If high soil resistivity is suspected, wet down the soil over the anodes and re-measure the current output after a few days. If the current increases to give adequate protection, then high soil resistivity problems have been confirmed. If lowering the soil resistivity in the vicinity of the anodes does not result in increased anode output, then high circuit resistance may be the problem and all leads and connections should be checked. If all leads and connections are all right and the output is still too low, then more anodes are required to provide additional current. Inadequate potential at designed current levels. If a situation is encountered where anode current output is within design limits and adequate protection is not obtained, the current required for protection may be more than originally anticipated. This may be due to interference, unusually corrosive soil conditions, a poorly performing coating on the structure, or a shorted dielectric fitting. If any interference has been corrected and inadequate protection is still encountered, either more anodes will have to be installed.