All About Cathodic Protection
Corrosion is a leading cause of premature failure in metallic structures. Operators can extend the service life of their facilities and equipment by installing cathodic protection systems and testing them regularly.
A wide range of civil and industrial applications use these systems to prevent corrosion for many years. They are typically installed during original construction, major expansions or upgrades.
This post covers the two types of cathodic protection systems, the types of structures protected and provides an example of CP for pipeline corrosion prevention. To get more in depth, view the training video below or check out our FAQs at the bottom of this page.Jump to Cathodic Protection FAQs
There are two types of cathodic protection systems: galvanic and impressed current.
Galvanic Cathodic Protection
In a galvanic system, the anodes connected to the protected structure have a natural potential that is more negative than the structure’s. When connected in a circuit, cathodic protection current flows from the anode (more negative) to the structure (less negative).
Galvanic anodes (also referred to as sacrificial anodes), when properly applied, can protect underground steel, marine, internal and industrial structures from corrosion. They do not require an outside power source to operate, and are therefore limited in their use. Where properly applied they can be designed to provide long life with ease of operation.
Galvanic anodes are available in a variety of configurations, including:
- Bare metal anodes including magnesium, zinc, aluminum and other alloys
- Packaged in backfill for underground use
- Made with external steel straps for mounting to structures
- Ribbon types
- Rod and special shapes
Impressed Current Cathodic Protection
In many applications, the potential difference between the galvanic/sacrificial anode and the steel structure is not enough to generate sufficient current for protection to occur. In these cases, a power supply (rectifier) can generate larger potential differences, enabling more current to flow to the structure being protected. This is referred to as an impressed current cathodic protection (ICCP) system.
MATCOR Cathodic Protection Systems
CP Systems protect infrastructure assets from corrosion. MATCOR typically designs systems to operate for 30 years or longer. These structures include:
- Above ground storage tanks
- Buried pipelines
- Reinforcing steel in concrete structures
- Heat exchangers
- Marine piles
- Sheet pile walls
- Other metallic structures
Cathodic Protection Design
CP engineers can design systems for maximum life and ease of replacement. MATCOR typically designs systems to operate for 30 years or longer.
To be the most effective and economical, CP systems must be designed properly. CP design is the scientific discipline involving:
- An understanding of the environmental conditions and the structure to be protected from corrosion
- Review of options for the structure or application
- Selection of the appropriate system
- Complete design including comprehensive specifications and drawings utilizing the latest engineering software
Design engineers possessing the right expertise and knowledge of the structure to be protected from corrosion should perform all phases of system design.
Pipeline Cathodic Protection Example
On an unprotected pipeline, potential variations occur naturally. Wherever you go from a minor positive to a minor negative, current flows and galvanic pipeline corrosion will occur. If you apply CP to that pipeline—for example MATCOR’s linear anode that runs parallel to the pipeline—current discharges off of the anode and onto the pipeline, preventing corrosion.
Pipeline without CP applied
CP applied to pipeline
Cathodic Protection FAQs
Cathodic Protection (CP) is an electro-chemical process that slows or stops corrosion currents by applying DC current to a metal. When applied properly, CP stops the corrosion reaction from occurring to protect the integrity of metallic structures.
Cathodic protection works by placing an anode or anodes (external devices) in an electrolyte to create a circuit. Current flows from the anode through the electrolyte to the surface of the structure. Corrosion moves to the anode to stop further corrosion of the structure.
The two basic types of cathodic protection systems are galvanic and impressed current.
An anode is one of the key components in a Cathodic Protection system. It is the component from which DC current will be discharge. It is the source of electrons in the CP system. It is the component that is more negative relative to the structure being protected.
The cathode is the structure being cathodically protected and is where current flows to after discharging from the anode. It is the component that is more positive relative to the structure being protected. As the cathode receives electrons, it becomes polarized, or more electrically negative.
An electrolyte, for cathodic protection purposes, is an environment around the cathode (structure being protected) that is electrically conductive enough to allow current to flow from the anode to the cathode. The anode and cathode must both be in this environment that allows cathodic protection current to flow from the anode to the cathode. In some cases, there might be multiple electrolyte layers or types through which the current might flow.
Several buried or submerged structures require or can benefit from the proper application of cathodic protection. This includes all oil and gas steel pipelines, steel and ductile iron water piping systems, the tank bottoms on large diameter above ground storage tanks, ductile iron fire hydrant risers, and HVAC transmission tower guide wire anchors are examples of structures that are commonly protected using CP. For marine structures, cathodic protection is commonly applied to steel pilings and sheet pile walls on a wide range marine near shore structures. Additionally, ships and other large vessels commonly use CP. These are some of the common CP applications but there are numerous others as well.
When cathodic protection current flows from the anode to the structure being protected (the cathode in the circuit) that structure’s electrical potential will shift more electrically negative – this is measured in mV typically. This shift is in potential is called polarization. The amount of polarization is a measure of the effectiveness of the cathodic protection current and once the polarization is sufficient, the structure is deemed cathodically protected. The time it takes to fully polarize a structure can vary depending on the structure and its environment but in some cases a structure can take weeks to fully polarize.
When the cathodic protection current stops flowing from the anode to the structure being cathodically protected, the polarized structure will begin to depolarize. The rate of depolarization can vary depending on the structure and its environment.
There are two basic criteria per NACE International standards that can be used to confirm that the structure is considered cathodically protected. The first criteria is 100mV of polarization – this is a pretty simple criteria to apply in that you measure the potential of the structure without any CP being applied (native potential) and then after applying cathodic protection for a sufficient period of time for polarization, measure the potential again and if the potential difference is greater than 100 mV – this is commonly known as the 100 mV shift criteria. The other criteria is the 850 mV Off potential criteria. In this case, it is not necessary that there be a native potential to use as a baseline – this criteria simply requires that the potential of the structure be more negative than -850 mV after accounting for all current sources (by turning them off for an instant).
Instant off refers to the process of taking measurements at the instant that the power is turned off on an impressed current CP system. When there are multiple current sources, they all need to be turned off simultaneously using interrupters that are synchronized. The purpose of turning all the current sources off is to eliminate the IR drops in the circuit. As current (I) flows through cabling there is a resistance (R) that the current must overcome – this is know as voltage drop because V=I x R. When attempting to measure the level of polarization, it is important to eliminate the IR drops in the circuit that are the result of current flow creating these IR drops. By instantaneously turning the current off these IR drop readings are immediately reduced to zero because the current (I) is now zero. This means that the polarization being measured immediately after the current is turned off is the true polarization current. Timing is critical because with the current turned off the structure will immediately depolarize and the polarization potential will begin to decay. The goal of instant off polarization readings is to catch the polarization level as the power is turned off and before the depolarization process begins.
Anodes can be broken down into two basic anode types – galvanic anodes (frequently referred to as sacrificial anodes) and impressed current anodes. The galvanic series anodes use the natural voltage differential between the anode and the structure to drive current off the anode and to the structure. The impressed current anodes use an external power supply to drive current off the anode and to the structure.
Galvanic anodes are basically metal castings that do not utilize an external power supply to drive current. They rely on the natural potential differences between the two metals to drive the cathodic protection current. There are three primary types of galvanic anodes. Magnesium which is the most active of the galvanic anodes and is used primarily in soil applications. Zinc which is les active a metal and is commonly used in low resistivity soils and brackish water. Zinc is also the primary metal in galvanized applications. And finally, Aluminum which is used primarily in seawater applications. Note that galvanic anodes are often call sacrificial anodes because they are consumed during the CP reaction – this is also true of many impressed current anodes as well. The term sacrificial implies no power supply and the use of anodes these anodes that are more active than the structure being protected.
There are two major advantages of galvanic anode systems. They do not require a power supply – in many applications the cost of providing power and installing a power supply can be quite significant. They require virtually no regular maintenance because the power supply has been eliminated. In the right applications, these two benefits make these anode systems cost effective.
Limited power, with galvanic anode systems the driving force between the anode and the structure is limited to around 1V maximum and frequently much less than 1V driving force. Larger structures often require more current than what can economically be provided by galvanic anodes.
Limited life, galvanic anodes consume at relatively large rates in terms of several kg/amp year. This significantly limits the anode life in some applications.
Limited control, galvanic anodes have no power supply whose output can be adjusted by varying the power being applied to the anode – with galvanic anode systems, they operate solely based on the system resistance relying on the voltage differential between the anode and structure.
Impressed current anodes are intended to discharge current when being powered by an external DC power source. Typically, this external source is a transformer/rectifier that converts AC power to DC power. With enough external power supply units, impressed current anode systems can discharge enough current to protect virtually any structure regardless of size or coating status. Because the anodes are not chosen based on their activity level, they can instead be chosen based on their current discharge characteristics – how much current can they handle. The three most common impressed current anodes are Graphite, High Silicon Cast Iron, and Electro Catalytic type anodes.
There are two basic classes of anodes – there are those anodes that are electro-chemically reacting to generate the electrical current flow. This group includes Magnesium, Zinc and Aluminum as well as Graphite and High Silicon Cast Iron impressed current anodes. These anodes consume at a defined rate based on the current being generated and their consumption rate can be defined in terms of kgs of mass consumed for every so many amp-years of operation. There is always a consideration for utilization of the anode – you can never fully consume 100% of the anode mass – at some point the anode degradation impacts the anodes ability to perform. So, it is quite possible for these electro-chemically reactive anodes to calculate the expected anode life.
There is a second class of anodes – those anodes that are electro-catalytic and are not a reactant but promote electro-chemical reactions. These catalytic type anodes are either Platinum based or MMO type. MMO is short for mixed metal oxide and this is a coating that consists of an Iridium (or Ruthenium) metal oxides and other components. Because these anodes are catalytic, they do not consume in the same manner as the electro-chemically reactive anodes. There is no measurable loss of mass with MMO anodes as they are not directly reacting with the electrolyte. However, these catalytic anodes do have their own definable anode life also based on amp-years of operation.
MMO is a coating consisting a of a mix of rare earth metal oxides with either Iridium or Ruthenium as the active catalyst. Iridium is suitable for all CP environments while Ruthenium based anodes are suitable only for seawater applications. The exact mixture used in the coating can vary from manufacturer, but the key is that the manufacturer has a proven recipe and that its performance characteristics, including anode life, can be predictably calculated based on accelerated life testing programs. These MMO anode coatings are applied to a Grade I or Grade II commercially pure titanium substrate. Some of the common MMO anode shapes include wire, rods, tubes, strips, ribbon mesh strips and sheets, plates and discs.
A Rectifier is simply a Power Supply that converts AC power to DC power. For most impressed current cathodic protection systems, a rectifier is an integral component in the system design. Rectifiers are available in a wide range of enclosure types depending on the environment and hazardous area classification of the location. The rectifier is sized based on a maximum DC Power Rating – for example 50V x 50 Amps would imply that the rectifier is capable of 2500 Watts of power.
It is critically important that the polarity of the DC rectifier output be properly installed prior to energizing the rectifier or power supply. The DC positive must always be connected to the anode system while the DC negative is always connected to the structure lead(s) connected to the structure. To repeat, the anode must always be connected to the positive – structure to the negative. If the anode and structure leads are tied to the opposite polarity – current will be driven off the structure and towards the anode system. This could be catastrophic as this would cause accelerated corrosion of the structure – for steel this would be at the rate of 20 lbs/amp year.
Test stations are another key component in a cathodic protection sytem design. These test stations are typically installed at strategic locations to be able to provide access for testing. Test Stations is a generic name and can range from a simple lead from the pipe or buried structure to the test station to allow for easy electrical connection, to the very complex with corrosion rate probes, AC and DC coupons and remote data collection and monitoring equipment.
In the cathodic protection industry, the anodes are frequently buried or located in harsh service environments. To protect the integrity of the anode cabling system, the industry uses a “direct burial” cabling system. The most common in the US is a high molecular weight polyethylene cable or HMWPE. This cable insulation is generally 110 mils or more thick and is extremely robust and difficult to damage with even the harshest of handling. For some highly chlorinated environments, it is common to use a dual insulation with an inner sheath of a fluoropolymer. The common types used are PVDV (Kynar) and ECTFE (Halar) and they have very similar chemical resistance characteristics.
Dual insulated direct burial cable has an inner layer of chemically resistant fluoropolymer (Kynar or Halar) to provide additional chemical resistance in highly chlorinated environments. If salts are present, these salts can result in the formation of chlorine gas which reacts with water to create hydrochloric acid. This can be very damaging to standard cabling and the added chemical protection of dual insulated cables are highly recommended in areas where high current densities occur in a chloride rich environment with minimal gas or electrolyte mobility. Deep anode ground beds, salt laden soils, marshy areas can all create problems for standard cable necessitating a more chemically inert cable insulation.
For impressed current cathodic protection systems, it is critically important that there is no knicks, cuts or cracks in the cabling or any cable connections/joints. This is especially critical for the anode cabling tied to the positive side of the power supply. Should any part of the anode cabling system be compromised, and the copper conductor has an electrical path back to the environment, then the copper becomes an unintended anode and will begin to very quickly consume leading to an open circuit and an inoperable CP system. Thus, it is very critical on the anode side that every splice or connection is completely water tight and that all of the cable insulation is in good condition.
RMU is short for Remote Monitoring Unit. In cathodic protection remote monitoring, RMUs are commonly used to monitor, and in most cases control the performance of rectifiers in impressed current cathodic protection systems. RMUs can also be applied to test stations, critical bonds and other monitoring applications. A variety of technologies are available including broadband, cellular, and satellite communications to enable system monitoring and control.
CIS or close interval survey, internationally more commonly referred to as CIPS (close interval potential survey), is a common means of validating the proper performance of a cathodic protection systems along long length pipelines or within stations/plant piping networks. The survey consists of taking potential readings as the crew walks over the center of the buried pipeline. These readings are usually taken while all the influencing current sources are being cycled on and off at a regular interval. Thus, readings are taken capturing the potential between the pipe and a reference electrode. Both the current on and current off cycle readings are captured. This process repeats over the entire length of the pipeline. The on/off data is then analyzed to confirm the CP system is working properly and achieving the required system polarization.
An interrupter is a sophisticated switch that can be used to interrupt the operation of a rectifier. The interrupters being used today are automatically synchronized from a satellite signal allowing for numerous interrupters to all be synchronized to the same timing so that the OFF data collected is accurate. Many new rectifiers for pipelines are being supplied with built in interrupters that can be energized remotely for surveys and for CP system testing.
Sometimes referred to as deep anode well or deep anode ground bed, a deep anode system is often an effective means of injecting a large amount of current into the earth from a single location with a very small surface footprint. Conventional drilling equipment is used to drill a hole approximately 200-400 feet deep and lower one or more anodes into the hole before backfilling the hole. The anodes are located sufficiently far enough from the surface to be able to consider them electrically remote from the structure and thus able to project current into a congested underground environment or distribute current for miles in each direction for isolated pipelines.
During the cathodic protection electrochemical reaction, gas is generated as part of the reaction process that also liberates electrons to allow current to distribute through the electrolyte. In most environments that gas is able to diffuse or discharge somewhere; however, in those rare cases where the gas being generated cannot migrate away from the anode surface the gas can actually block the flow of electrons and choke the cathodic protection reaction. This is more common within deep anode systems where a hole is bored from the surface down into the earth and the surrounding environment around the bore hole may not be very permeable thus trapping gases. Most deep anode systems use a vent pipe to allow gasses to vent away to prevent gas blockage.
Vent pipes are small diameter piping that has drilled holes or cut slots that allow gases generated at the anode during the cathodic protection process to vent away from the anode. This can help reduce the buildup of gasses around the anode or the concentration of low pH hydrochloric acid that can form when excess chlorine gas is available and not vented away. This low pH environment can attack HMWPE cable insulation and result in premature failure of the cabling.
Almost all buried anodes have some form of backfill material either built into the anode packaging or supplied externally for installation. For impressed current anodes, a coke backfill is generally used. The primary role of the coke backfill is to provide a uniform low resistance environment into which the anode can easily discharge current. This helps to reduce any issues with poor earth contact of the buried anode while also increasing the anode’s effective size reducing the anode backfill to earth resistance.
Carbon itself can act as an impressed current anode and when another impressed current anode is installed in a coke backfill, some of the coke backfill will act as an extension of the impressed current anode and to the extent that carbon is consumed then the impressed current anode consumption is likely reduced. How fast the coke backfill consumes and how much of a positive impact that has on the actual anode life is very much site specific with variables such as coke column quality, coke particle compaction, moisture level, and particle shape. Basically, there are two conduction modes for electrons – there is electronic conduction where the electrons flow from the anode through the coke as an extension of the actual anode the electrochemical anodic reaction occurs from the carbon to the environment. This results in the carbon being a reactant and there is ionic transfer where the current is generated at the anode and then flows along a moisture path on the outside of the coke particles thus not involving the carbon as a primary reactant and thus no consumption. Bottom line is that it is difficult to know how an individual installation will operate or at what rate the backfill will consume.
You can always contact MATCOR of course, however ScienceDirect topics include may books and peer reviewed journals on the subject of CP.
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MATCOR Cathodic Protection Systems & Services
MATCOR offers a range of solutions that protect infrastructure assets including:
- Gas pipelines
- Above ground storage tanks
- Marine structures such as docks and piers
- Plant piping
- And more…
MATCOR cathodic protection systems include:
- Impressed current linear anodes
- Deep anode systems
- Tank anode systems
- Marine and water anode systems
- Ground bed anodes
- Internal probe anodes
Additional CP Systems Components
- Cathodic protection rectifiers
- Cathodic protection reference electrodes
- Junction boxes
- Splice kits
- Cathodic protection test stations
Browse all MATCOR CP Solutions
To get in touch with our team of cathodic protection experts for more information, to ask a question or get a quote, please click below. We will respond by phone or email within 24 hours. For immediate assistance, please call +1-215-348-2974.Contact a Corrosion Expert