How does cathodic protection prevent corrosion

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—galvanic and impressed current—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. 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. 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. CP Systems protect infrastructure assets from corrosion. These structures include:. CP engineers can design systems for maximum life and ease of replacement. To be the most effective and economical, CP systems must be designed properly.

CP design is the scientific discipline involving:. Design engineers possessing the right expertise and knowledge of the structure to be protected from corrosion should perform all phases of system design. 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.

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. 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. 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. Go to cart. My account My orders My account Log in. Back Close menu. Go to front page Products All products. Anode shield. Control panels. Reference cells. Sacrificial anodes. Brackets and clamps. Shaft grounding. Floating offshore units.

Offshore subsea units. Sea cables. Storage tanks. Wind turbines. ICCP for hull. Sacrificial anodes for hull and tanks. Ultrasonic antifouling. Sacrificial anodes for hull. Sacrificial anodes for tanks. ICCP for external area of foundations.

Offshore mobile units. Subsea structures. Aluminium anodes. Zinc anodes. High temp anodes. 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. Measurement of the voltage gradient at the surface above the pipeline enables even small flaws to be detected and positioned accurately.

Both CIPS and DCVG techniques are increasingly used — but can be time-consuming to set up in the field because of the requirement to synchronise transformer rectifier outputs. Rectifiers can be configured to synchronise and interrupt their output simply by sending a message from a cellphone. Used in conjunction with specific MERLIN Transformer Rectifier Monitors , it enables interruption switching of the current output at a rectifier or solar station to be controlled remotely.

The Interrupter fails safe and switches the rectifier output loads encountered under the temperature, environmental and electrical conditions experienced in a rectifier cabinet. The solid state circuitry overcomes the limitations of electromechanical relays. All pipeline operators use CP extensively on their transmission pipelines. The big advantage of CP over other forms of corrosion treatment is that it is applied very simply by maintaining a DC circuit and its effectiveness can be monitored continuously.

Because of the importance of CP in protecting the pipe, operators are required to take and report regular measurements of CP data, both of the levels of protection applied to the pipe at source and the in situ levels measured along the pipe itself. The frequency of measurements at the various points is generally in compliance with NACE guidelines. Pipeline operators are responsible for providing their national regulatory body with evidence that their monitoring is adequate to demonstrate effective management of their CP systems.

The data is gathered in the field by technicians. However, the cost of this activity is significant and there are other disadvantages of manual data collection. It enables operators to:. Because it inhibits corrosion, CP allows the use of thinner metal thicknesses and can therefore be extremely cost-effective over the operating life of an underground asset. When designing a new CP system, a survey is usually made and an economic justification of the project produced.

This takes account of:. We work across the globe with CP design consultants and companies to assist with their CP monitoring requirements. Contact us for more information on how Abriox remote monitoring systems can improve your network management.

Just What is Cathodic Protection? Previous Next. View Larger Image. Water and fuel pipelines. Ships and boats. Offshore oil platforms.