Cathodic protection

In the application of passive cathodic protection, a galvanic anode, a piece of a more electrochemically "active" metal (more negative electrode potential), 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 (typically steel).

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 (10-20 years typically) 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 ICCP 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.[8] During the initial phase of high current, the potential of the steel surface is polarized (pushed) 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.[9][10][11]

In order for galvanic cathodic protection to work, the anode must possess a lower (that is, more negative) electrode potential than that of the cathode (the target structure to be protected). The table below shows a simplified galvanic series which is used to select the anode metal.[12] The anode must be chosen from a material that is lower on the list than the material to be protected.

Simple impressed current cathodic protection system. A source of DC electric current is used to help drive the protective electrochemical reaction.

In some cases, impressed current cathodic protection (ICCP) 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.[13][14]

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 (MMO), 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.[15] 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 optimised 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.[16] Analog or digital meters are often installed to show the operating voltage (DC and sometime AC) 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 have been used for over a decade and incorporate the coordination, monitoring and high restorative current flow of ICCP systems with the reactive, lower cost and easier to maintain galvanic anodes.

The system is made up of wired galvanic anodes in arrays typically 400mm apart which are then initially powered for a short period to restore the concrete and power ionic migration. The power supply is then taken away and the anodes simply attached to the steel as a galvanic system. More powered phases can be administered if needed. Like Galvanic systems corrosion rate monitoring from polarisation tests and half-cell potential mapping can be used to measure corrosion. Polarization is not the goal for the life of the system.

This technology is also used to protect water heaters. Indeed, the electrons sent by the imposed current anode (composed of titanium and covered with MMO) prevents 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 (formerly RP0388-2001) of the NACE National Association of Corrosion Engineers.[17]

An air cooled cathodic protection rectifier connected to a pipeline.

Cathodic protection markers over a gas pipeline in Leeds, West Yorkshire, England.

Hazardous product pipelines are routinely protected by a coating supplemented with cathodic protection. An impressed current cathodic protection system (ICCP) 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 anode groundbed).

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.[18]

Anodes can be installed in a groundbed consisting of a vertical hole backfilled with conductive coke (a material that improves the performance and life of the anodes) 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.[19]

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.

It is sometimes more economically viable to protect a pipeline using galvanic (sacrificial) anodes. This is often the case on smaller diameter pipelines of limited length.[20] 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.

The white patches visible on the ship's hull are zinc block sacrificial anodes.

Cathodic protection on ships is often implemented by galvanic anodes attached to the hull and ICCP for larger vessels. Since ships are regularly removed from the water for inspections and maintenance, it is a simple task to replace the galvanic anodes.[21]

Galvanic anodes are generally shaped to reduced drag in the water and fitted flush to the hull to also try to minimize drag.[22]

Smaller vessels, with non-metallic hulls, such as yachts, are equipped with galvanic anodes to protect areas such as outboard motors. As with all galvanic cathodic protection, this application relies on a solid electrical connection between the anode and the item to be protected.

For ICCP on ships, the anodes are usually constructed of a relatively inert material such as platinised titanium. A DC power supply is provided within the ship and the anodes mounted on the outside of the hull. The anode cables are introduced into the ship via a compression seal fitting and routed to the DC power source. The negative cable from the power supply is simply attached to the hull to complete the circuit. Ship ICCP anodes are flush-mounted, minimizing the effects of drag on the ship, and located a minimum 5 ft below the light load line[23] in an area to avoid mechanical damage. The current density required for protection is a function of velocity and considered when selecting the current capacity and location of anode placement on the hull.

Some ships may require specialist treatment, for example aluminium hulls with steel fixtures will create an electrochemical cell where the aluminium hull can act as a galvanic anode and corrosion is enhanced. In cases like this, aluminium or zinc galvanic anodes can be used to offset the potential difference between the aluminium hull and the steel fixture.[24] If the steel fixtures are large, several galvanic anodes may be required, or even a small ICCP system.

Marine cathodic protection covers many areas, jetties, harbors, offshore structures. The variety of different types of structure leads to a variety of systems to provide protection. Galvanic anodes are favored,[25] but ICCP can also often be used. Because of the wide variety of structure geometry, composition, and architecture, specialized firms are often required to engineer structure-specific cathodic protection systems. Sometimes marine structures require retroactive modification to be effectively protected [26]

The application to concrete reinforcement is slightly different in that the anodes and reference electrodes are usually embedded in the concrete at the time of construction when the concrete is being poured. The usual technique for concrete buildings, bridges and similar structures is to use ICCP,[27] but there are systems available that use the principle of galvanic cathodic protection as well,[28][29][30] although in the UK at least, the use of galvanic anodes for atmospherically exposed reinforced concrete structures is considered experimental.[31]

For ICCP, the principle is the same as any other ICCP system. However, in a typical atmospherically exposed concrete structure such as a bridge, there will be many more anodes distributed through the structure as opposed to an array of anodes as used on a pipeline. This makes for a more complicated system and usually an automatically controlled DC power source is used, possibly with an option for remote monitoring and operation.[32] For buried or submerged structures, the treatment is similar to that of any other buried or submerged structure.

Galvanic systems offer the advantage of being easier to retrofit and do not need any control systems as ICCP does.

For pipelines constructed from pre-stressed concrete cylinder pipe (PCCP), the techniques used for cathodic protection are generally as for steel pipelines except that the applied potential must be limited to prevent damage to the prestressing wire.[33]

The steel wire in a PCCP pipeline is stressed to the point that any corrosion of the wire can result in failure. An additional problem is that any excessive hydrogen ions as a result of an excessively negative potential can cause hydrogen embrittlement of the wire, also resulting in failure. The failure of too many wires will result in catastrophic failure of the PCCP.[34] To implement ICCP therefore requires very careful control to ensure satisfactory protection. A simpler option is to use galvanic anodes, which are self-limiting and need no control.[35]

Vessels, pipelines and tanks which are used to store or transport liquids can also be protected from corrosion on their internal surfaces by the use of cathodic protection.[36] ICCP and galvanic systems can be used.[37] A common application of internal cathodic protection is water storage tanks and power plant shell and tube heat exchangers.

Galvanizing generally refers to hot-dip galvanizing which is a way of coating steel with a layer of metallic zinc or tin. Galvanized coatings are quite durable in most environments because they combine the barrier properties of a coating with some of the benefits of cathodic protection. If the zinc coating is scratched or otherwise locally damaged and steel is exposed, the surrounding areas of zinc coating form a galvanic cell with the exposed steel and protect it from corrosion. This is a form of localized cathodic protection - the zinc acts as a sacrificial anode.

Galvanizing, while using the electrochemical principle of cathodic protection, is not actually cathodic protection. Cathodic protection requires the anode to be separate from the metal surface to be protected, with an ionic connection through the electrolyte and an electron connection through a connecting cable, bolt or similar. This means that any area of the protected structure within the electrolyte can be protected, whereas in the case of galvanizing, only areas very close to the zinc are protected. Hence, a larger area of bare steel would only be protected around the edges.

Several companies market electronic devices claiming to mitigate corrosion for automobiles and trucks.[38] Corrosion control professionals find they do not work.[39] There is no peer reviewed scientific testing and validation supporting the use of the devices. In 1996 the FTC ordered David McCready, a person that sold devices claiming to protect cars from corrosion, to pay restitution and banned the names "Rust Buster" and "Rust Evader."[40]

A side effect of improperly applied cathodic protection is the production of atomic hydrogen,[43] leading to its absorption in the protected metal and subsequent hydrogen embrittlement of welds and materials with high hardness. Under normal conditions, the atomic hydrogen will combine at the metal surface to create hydrogen gas, which cannot penetrate the metal. Hydrogen atoms, however, are small enough to pass through the crystalline steel structure, and lead in some cases to hydrogen embrittlement.

This is a process of disbondment of protective coatings from the protected structure (cathode) due to the formation of hydrogen ions over the surface of the protected material (cathode).[44] Disbonding can be exacerbated by an increase in alkali ions and an increase in cathodic polarization.[45] The degree of disbonding is also reliant on the type of coating, with some coatings affected more than others.[46] Cathodic protection systems should be operated so that the structure does not become excessively polarized,[47] since this also promotes disbonding due to excessively negative potentials. Cathodic disbonding occurs rapidly in pipelines that contain hot fluids because the process is accelerated by heat flow.

Effectiveness of cathodic protection (CP) systems on steel pipelines can be impaired by the use of solid film backed dielectric coatings such as polyethylene tapes, shrinkable pipeline sleeves, and factory applied single or multiple solid film coatings. This phenomenon occurs because of the high electrical resistivity of these film backings.[48] Protective electric current from the cathodic protection system is blocked or shielded from reaching the underlying metal by the highly resistive film backing. Cathodic shielding was first defined in the 1980s as being a problem, and technical papers on the subject have been regularly published since then.

A 1999 report[49] concerning a 20,600 bbl (3,280 m³) spill from a Saskatchewan crude oil line contains an excellent definition of the cathodic shielding problem:

"The triple situation of disbondment of the (corrosion) coating, the dielectric nature of the coating and the unique electrochemical environment established under the exterior coating, which acts as a shield to the electrical CP current, is referred to as CP shielding. The combination of tenting and disbondment permits a corrosive environment around the outside of the pipe to enter into the void between the exterior coating and the pipe surface. With the development of this CP shielding phenomenon, impressed current from the CP system cannot access exposed metal under the exterior coating to protect the pipe surface from the consequences of an aggressive corrosive environment. The CP shielding phenomenon induces changes in the potential gradient of the CP system across the exterior coating, which are further pronounced in areas of insufficient or sub-standard CP current emanating from the pipeline's CP system. This produces an area on the pipeline of insufficient CP defense against metal loss aggravated by an exterior corrosive environment."

Cathodic shielding is referenced in a number of the standards listed below. Newly issued USDOT regulation Title 49 CFR 192.112, in the section for Additional design requirements for steel pipe using alternative maximum allowable operating pressure requires that "The pipe must be protected against external corrosion by a non-shielding coating" (see coatings section on standard). Also, the NACE SP0169:2007 standard defines shielding in section 2, cautions against the use of materials that create electrical shielding in section 4.2.3, cautions against use of external coatings that create electrical shielding in section 5.1.2.3, and instructs readers to take 'appropriate action' when the effects of electrical shielding of cathodic protection current are detected on an operating pipeline in section 10.9.