Cathodic protection

Cathodic protection is a technique used to control the corrosion of a metal surface by making it the cathode of an electrochemical cell. A simple method of protection connects the metal to be protected to a more easily corroded "sacrificial metal" to act as the anode. The sacrificial metal then corrodes instead of the protected metal. For structures such as long pipelines, where passive galvanic cathodic protection is not adequate, an external DC electrical power source is used to provide sufficient current.
Cathodic protection systems protect a wide range of metallic structures in various environments. Common applications are: steel water or fuel pipelines and steel storage tanks such as home water heaters; steel pier piles; ship and boat hulls; offshore oil platforms and onshore oil well casings; offshore wind farm foundations and metal reinforcement bars in concrete buildings and structures. Another common application is in galvanized steel, in which a sacrificial coating of zinc on steel parts protects them from rust.
Cathodic protection can, in some cases, prevent stress corrosion cracking.

History

Cathodic protection was first described by Sir Humphry Davy in a series of papers presented to the Royal Society in London in 1824. The first application was to in 1824. Sacrificial anodes made from iron attached to the copper sheath of the hull below the waterline dramatically reduced the corrosion rate of the copper. However, a side effect of cathodic protection was the increase in marine growth. Usually, copper when corroding releases copper ions which have an anti-fouling effect. Since excess marine growth affected the performance of the ship, the Royal Navy decided that it was better to allow the copper to corrode and have the benefit of reduced marine growth, so cathodic protection was not used further.
Davy was assisted in his experiments by his pupil Michael Faraday, who continued his research after Davy's death. In 1834, Faraday discovered the quantitative connection between corrosion weight loss and electric current and thus laid the foundation for the future application of cathodic protection.
Thomas Edison experimented with impressed current cathodic protection on ships in 1890, but was unsuccessful due to the lack of a suitable current source and anode materials. It would be 100 years after Davy's experiment before cathodic protection was used widely on oil pipelines in the United Statescathodic protection was applied to steel gas pipelines beginning in 1928 and more widely in the 1930s.

Types

Galvanic

In the application of passive cathodic protection, a galvanic anode, a piece of a more electrochemically "active" metal, is attached to the vulnerable metal surface where it is exposed to an electrolyte. Galvanic anodes are selected because they have a more "active" voltage than the metal of the target structure.
Concrete has a pH around 13. In this environment the steel reinforcement has a passive protective layer and remains largely stable. Galvanic systems are "constant potential" systems that aim to restore the concrete's natural protective environment by providing a high initial current to restore passivity. It then reverts to a lower sacrificial current, while harmful negative chloride ions migrate away from the steel and towards the positive anode. The anodes remain reactive through their lifetime, increasing current when the resistivity decreases due to corrosion hazards such as rainfall, temperature increases, or flooding. The reactive nature of these anodes makes them an efficient choice.
Unlike impressed current cathodic protection systems, steel constant polarization is not the goal, rather the restoration of the environment. Polarization of the target structure is caused by the electron flow from the anode to the cathode, so the two metals must have a good electrically conductive contact. The driving force for the cathodic protection current is the difference in electrode potential between the anode and the cathode. During the initial phase of high current, the potential of the steel surface is polarized more negative protecting the steel which hydroxide ion generation at the steel surface and ionic migration restore the concrete environment.
Over time the galvanic anode continues to corrode, consuming the anode material until eventually it must be replaced.
Galvanic or sacrificial anodes are made in various shapes and sizes using alloys of zinc, magnesium, and aluminium. ASTM International publishes standards on the composition and manufacturing of galvanic anodes.
In order for galvanic cathodic protection to work, the anode must possess a lower electrode potential than that of the cathode. The table below shows a simplified galvanic series which 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.

Metal	Potential with respect to a Cu:CuSO₄ reference electrode in neutral pH environment
Carbon, Graphite, Coke	+0.3
Platinum	0 to −0.1
Mill scale on Steel	−0.2
High Silicon Cast Iron	−0.2
Copper, brass, bronze	−0.2
Mild steel in concrete	−0.2
Lead	−0.5
Cast iron	−0.5
Mild steel	−0.2 to −0.5
Mild steel	−0.5 to −0.8
Commercially pure aluminium	−0.8
Aluminium alloy	−1.05
Zinc	−1.1
Magnesium alloy	−1.6
Commercially Pure Magnesium	−1.75

Impressed current cathodic protection (ICCP)

In some cases, impressed current cathodic protection systems are used. These consist of anodes connected to a DC power source, often a transformer-rectifier connected to AC power. In the absence of an AC supply, alternative power sources may be used, such as solar panels, wind power or gas powered thermoelectric generators.
Anodes for ICCP systems are available in a variety of shapes and sizes. Common anodes are tubular and solid rod shapes or continuous ribbons of various materials. These include high silicon, cast iron, graphite, mixed metal oxide, platinum and niobium coated wire and other materials.
For pipelines, anodes are arranged in groundbeds either distributed or in a deep vertical hole depending on several design and field condition factors including current distribution requirements.
Cathodic protection transformer-rectifier units are often custom manufactured and equipped with a variety of features, including remote monitoring and control, integral current interrupters and various type of electrical enclosures. The output DC negative terminal is connected to the structure to be protected by the cathodic protection system. The rectifier output DC positive cable is connected to the anodes. The AC power cable is connected to the rectifier input terminals.
The output of the ICCP system should be optimized to provide enough current to provide protection to the target structure. Some cathodic protection transformer-rectifier units are designed with taps on the transformer windings and jumper terminals to select the voltage output of the ICCP system. Cathodic protection transformer-rectifier units for water tanks and used in other applications are made with solid state circuits to automatically adjust the operating voltage to maintain the optimum current output or structure-to-electrolyte potential. Analog or digital meters are often installed to show the operating voltages and current output. For shore structures and other large complex target structures, ICCP systems are often designed with multiple independent zones of anodes with separate cathodic protection transformer-rectifier circuits.

Hybrid systems

Hybrid systems use a combination of the aforementioned systems to achieve some of the benefits of both, utilizing the restorative capabilities of ICCP systems but maintaining the reactive, lower cost, and easier-to-maintain nature of a galvanic anode.
The system is made up of wired galvanic anodes in arrays typically apart, which are then initially powered for a short period to restore the concrete and to power ionic migration. The power supply is then taken away and the anodes are simply attached to the steel as a galvanic system. More powered phases can be administered if needed. Like galvanic systems, corrosion rate monitoring from polarization tests and half-cell potential mapping can be used to measure corrosion. Polarization is not the goal for the life of the system.

Applications

Water heaters

This technology is also used to protect water heaters. The electrons sent by the imposed current anode prevent the inside of the tank from rusting.
In order to be recognized as effective, these anodes must comply with certain standards: A cathodic protection system is considered efficient when its potential reaches or exceeds the limits established by the cathodic protection criteria. The cathode protection criteria used come from the standard NACE SP0388-2007 of the NACE National Association of Corrosion Engineers.

Pipelines

Hazardous product pipelines are routinely protected by a coating supplemented with cathodic protection. An impressed current cathodic protection system for a pipeline consists of a DC power source, often an AC powered transformer rectifier and an anode, or array of anodes buried in the ground.
The DC power source would typically have a DC output of up to 50 amperes and 50 volts, but this depends on several factors, such as the size of the pipeline and coating quality. The positive DC output terminal would be connected via cables to the anode array, while another cable would connect the negative terminal of the rectifier to the pipeline, preferably through junction boxes to allow measurements to be taken.
Anodes can be installed in a groundbed consisting of a vertical hole backfilled with conductive coke or laid in a prepared trench, surrounded by conductive coke and backfilled. The choice of groundbed type and size depends on the application, location and soil resistivity.
The DC cathodic protection current is then adjusted to the optimum level after conducting various tests including measurements of pipe-to-soil potentials or electrode potential.
When designing the system, and periodically after installation, engineers will conduct a baseline measurement campaign, during which all existing active protection systems in the area will be switched off and the native or natural pipe-to-electrode potential is recorded. This measurement will provide clues about how the pipeline can be sectioned into electrical systems, how many anodes and rectifiers will be needed and where they should be placed.
It is sometimes more economically viable to protect a pipeline using galvanic anodes. This is often the case on smaller diameter pipelines of limited length. Galvanic anodes rely on the galvanic series potentials of the metals to drive cathodic protection current from the anode to the structure being protected.
Water pipelines of various pipe materials are also provided with cathodic protection where owners determine the cost is reasonable for the expected pipeline service life extension attributed to the application of cathodic protection.