Additionally, ships and other large vessels commonly use CP. These are some of the common CP applications but there are numerous others as well. 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 mV 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 mV — this is commonly known as the mV shift criteria. The other criteria is the 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 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.
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. 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. 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.
There are two basic classes of anodes — there are those anodes that are electro-chemically reacting to generate the electrical current flow. 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.
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. Some of the common MMO anode shapes include wire, rods, tubes, strips, ribbon mesh strips and sheets, plates and discs.
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 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.
Other materials, including graphite, magnetite, lead, platinum-coated titanium and niobium, have also been used, though performance and cost have combined to reduce their use. These can be used in both seawater and saline mud, though in the latter their consumption rate is greater.
There is a tubular anode formed into a conductive ceramic of MMOs. This increases the surface area, reduces the electrical resistance to ground, and extends the anode life. If the cathodic protection system is well designed, installed, operated and maintained, both galvanic anode and impressed current cathodic protection can be equally effective. However, GACP is simpler and has proved to be more reliable offshore.
Onshore, ICCP systems are easier to access for maintenance and, once installed, their components are not subject to the challenges of offshore environments. If properly designed, ICCP can protect many kilometres of well-coated pipelines. ICCP is also advantageous for bare or poorly coated steel as it can deliver hundreds of amps of low voltage direct current, while a typical galvanic anode will seldom deliver more than 5 amps. Cathodic protection is used extensively to protect critical infrastructure from corrosion.
For example:. However, across all functions — from design through installation to testing and maintenance — cathodic protection is highly specialised. A key takeaway from the standards is that they make it clear that cathodic design must be undertaken by cathodic specialists with a documented, appropriate level of competence.
There are no degrees that can be gained in cathodic protection, and there are no postgraduate courses in cathodic protection engineering, either.
Instead, you find that cathodic protection specialists may hold a science or engineering degree or complete an apprenticeship before undertaking specific training and gaining experience and expertise in cathodic protection. The Institute of Corrosion offers courses in cathodic protection, providing the training required for levels 1 to 3 for cathodic protection data collectors, technicians and senior technicians.
This is recognised internationally as confirmation of experience, knowledge and task skills as defined in standard BS EN ISO ; it is valid internationally. For cathodic protection companies and for independent cathodic protection specialists, attainment of cathodic protection training and certification will ensure demonstration of competence, experience and expertise.
The most common example is the rusting of steel. Corrosion is an electrochemical process, normally occurring at the anode but not the cathode. The principle of cathodic protection is to connect an external anode to the metal to be protected and to pass a DC current between them so that the metal becomes cathodic and does not corrode. Using an external galvanic anode, where the DC current arises from the natural difference in potential between the metals of the anode eg Zn, Al or Mg and the pipe eg carbon steel.
The anode is electrically connected to the pipeline, causing a positive current to flow from the anode to the pipe so that the whole surface of the steel becomes more negatively charged, i. Using an external DC power source rectified AC to impress a current through an external anode usually inert onto the surface of the pipe, which becomes the cathode. Galvanic systems are easy to install, have low operating costs and minimal maintenance requirements, do not need an external power supply and rarely interfere with foreign structures.
However, they offer limited protection of large structures and are therefore used for quite localised CP applications. Impressed current systems are more frequently used to protect pipelines and underground storage tanks. Their high current output is capable of protecting large underground metal structures economically, is flexible to deal with varying conditions and less susceptible to soil resistivity.
However, they rely on the continuity of their AC power source and can interfere with other nearby buried structures. The level of CP current that is applied from impressed current systems is important. Too little current will lead to corrosion damage; excessive current can lead to disbanding of the coating and hydrogen embrittlement. For these reasons impressed current systems require regular monitoring.
Pipe-to-Soil Potential ON Potential - The potential of a pipeline at a given location is commonly referred to as the pipe-to-soil potential. It results from the corrosive electrolytic reaction between the buried pipe and its surrounding soil the electrolyte. It is actually measured between the pipeline and a reference electrode most commonly copper sulphate , placed in the soil directly over the pipeline.
It is also known as the ON potential because the measurement is made while the CP system is energised. Instant OFF Potential - When a pipe-to-soil measurement is made, the pipeline potential will appear to be more negative then its true potential, due to IR drop errors. The instant OFF measurement corrects for these errors; the CP current is briefly interrupted to produce a "true" pipe-to-soil potential, free from undesirable IR drop effects and before any appreciable depolarisation has occurred.
This is a truer measure of the level of protection afforded to the pipeline. If it is not possible to disconnect the CP momentarily then an alternative approach is the use of a corrosion coupon see below.
Coupon Current - Corrosion coupons connected to cathodically-protected structures can be used to monitor the effectiveness of the CP system. A coupon is a representative sample of the pipeline material, buried close to the pipe so that it is subjected to the same environment.
Connected to the pipeline via a test post, it simulates how the pipeline would react if there were a defect often referred to as a "holiday" in its coating.
It is especially useful when it is not possible to interrupt the CP system, since instant OFF potentials can conveniently be measured by interrupting the CP connection to the coupon. The surface area of the coupon allows the current density to be calculated.
However, they are only representative of the pipeline at that point — and for a short length either side. A direct connection is made to the pipeline and this trailing wire is unwound from a spool as the technician walks along its length.
As he goes, the TR current output is interrupted to enable the technician to take a pipe-to-soil OFF potential measurement at approximately 1m intervals. On pipelines with multiple TRs, all the outputs or at least those that influence the potential measurement at that point have to be interrupted synchronously.
Interruption cycle times vary but the selected "on" period is longer than the "off" period to limit depolarisation of the pipeline during the survey. DCVG is used for locating and sizing defects in the coating of the pipeline.
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