Cathodic Protection (CP) is one of the most effective methods for preventing most types of corrosion on a metal surface. In some cases, CP can even stop corrosion damage from occurring. Metals, especially ferrous metals, corrode in the presence of oxygen, water, and other impurities such as sulfur. Without CP, metals act as the anode and easily lose their electrons and thus, the metal becomes oxidized and corroded. CP simply supplies the metal with electrons from an external source, making it a cathode.
Oxidation — Loss of electrons
Reduction — Gain of electrons
Anode — Where oxidation reactions take place
Cathode — Where reduction reactions take place
The following are two helpful mnemonics to remember how electrons are transferred in oxidation-reduction (redox) reactions.
Galvanic cathodic protection involves protecting a metal surface of a piece of equipment using another metal that is more reactive. The latter metal, usually called the galvanic or sacrificial anode, has a less negative electrochemical potential compared to the metal component being protected. Therefore, the sacrificial anode undergoes oxidation rather than the operating equipment. This technique is illustrated in Figure 1 below for an offshore platform with a steel pipe submerged into seawater. The sacrificial anode is an aluminum anode in this example.
Sometimes, steels are galvanized rather than connected to galvanic anodes. Galvanized steels are steels that are coated with a protective zinc layer. The zinc layer acts to cathodically protect steel against corrosion in most underground and marine environments.
Figure 1. Offshore oil rig using a sacrificial anode.
ICCP is a more economical method of CP when underground pipelines are long or offshore equipment is too large to protect via one or few galvanic anodes. In ICCP, electrons are supplied to the cathodic structure using an external DC power source (also called a rectifier). The steel component is connected to the negative terminal of the power source and the impressed current anodes are connected to the positive terminal of the power source. For simplicity, Figure 2 shows one cathode and one anode connected by a rectifier. In application, multiple anodes are connected to the positive terminal of the power source.
Cathodic protection is routinely used to protect equipment operating in aggressive environments. The two most common applications of CP are for buried pipeline systems and vessels as well as offshore platforms. CP is not used to protect equipment in atmospheric conditions or protect components internally.
Once installed, CP should be monitored and maintained. Furthermore, inadequate CP designs may not maximize the amount of current reaching the protected item. CP designs should consider the environmental conditions and the component to be protected against corrosion. Another critical factor to monitor is stray currents that may interfere with the system. These interfering currents could be due to the environment or neighboring components (especially if new equipment is commissioned). Additionally, the anodes and rectifiers must be maintained in order for CP to be effective and reliable.
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May/June 2016 Inspectioneering Journal
By Gary Mulcahy at Astrodyne TDI
This is the second article of a two-part series published in Inspectioneering Journal, which is intended to provide a basis for understanding the differences between traditional tapped-transformer, fixed voltage type rectifiers, and High Frequency Switched Mode (HFSM) units, as well as highlight some opportunities for optimization provided by HFSM. |
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July/August 2015 Inspectioneering Journal
By Melissa Ramkissoon at Petrotrin
Aboveground Storage Tanks (AST) are essential to any successful oil and gas operation and must be properly managed to ensure operations function in a safe and reliable manner. In this 2-part series, I will identify some common failures related to ASTs in crude oil service and recommend strategies to prevent and/or mitigate such failures. |
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July/August 2015 Inspectioneering Journal
By Gary Mulcahy at Astrodyne TDI
This is the first article of a two-part series to be published in Inspectioneering Journal and will provide a basis for understanding the differences between traditional tapped-transformer, fixed voltage type rectifiers, and High Frequency Switched Mode (HFSM) units. |
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Blog
July 15, 2013 By John Reynolds at Intertek
This week’s post takes up right where last week’s post left off in our discussion on Corrosion Management and Control (CM&C) Management Systems. Here are the last two Corrosion Management and Control Management Systems. |
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November/December 2005 Inspectioneering Journal
A few years ago, TWI investigated a corrosion failure in a 30 inch crude oil pipeline that regrettably led to an explosion and fire, and the death of several operating personnel. The pipeline was designed to ASME B31.4 and the investigation found that corrosion resulted from the break-down of the external coating. The exposed area of pipe was too large for the cathodic protection system. Pitting corrosion initiated at several locations that coalesced over a large area to cause failure by rupture. The lost production from this failure was 300,000 bbl/d. The corrosion in this pipeline was not detected before failure. However, if corrosion is found in service pressure equipment, there are safe guidelines available for inspection engineers to assess the fitness-for-service (FFS) of corrosion damage. |
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May/June 2004 Inspectioneering Journal
By John Reynolds at Intertek
Soil corrosion (underground corrosion) is another one of those extensively researched and documented types of corrosion, since so many pipes and pipelines are buried and nearly all storage tanks rest on the soil. An entire industry/ technology is associated with preventing soil corrosion (cathodic protection). |
