Cathodic Protection Basics

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.

Let’s get into the topic…

Cathodic protection systems protect a wide range of metallic structures in various environments. Common applications are: steel pipelines that transport water or oil/fuel, steel storage tanks, and other assets from 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, which is the growth of crack formation in a corrosive environment. It can lead to unexpected and sudden failure of normally ductile metal alloys subjected to a tensile stress, especially at elevated temperature.

Let’s talk about the history of cathodic protection…

Cathodic protection was first described by Sir Humphrey Davy in a series of papers presented to the Royal Society in London in 1824. The first application was to HMS Samarang in 1824. Sacrificial anodes made from iron were attached to the copper sheath of the hull below the waterline, dramatically reducing 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.

Then in 1890, Thomas Edison experimented with impressed current cathodic protection on ships, 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 States successfully cathodic protection was applied to steel gas pipelines beginning in 1928 with wider implementation during the 1930's.

Now let’s talk about Galvanic, Impressed Current Cathodic Protection (or ICCP), and Hybrid Systems.

Starting with Galvanic, 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 metal surface where it is exposed to an electrolyte, an electrolyte is any substance that undergoes ionization when dissolved in water or ionizing solvents. This covers almost all soluble acids, bases and salts. At times, gasses like hydrogen chloride can also act in a similar way to electrolytes, given there is low pressure or high temperature. Electrolytes play a vital role in the corrosion process since their presence triggers a reaction between two dissimilar metals. 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 impressed current cathodic protection (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. 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 aluminum. 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 (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. The anode must be chosen from a material that is lower on the list than the material to be protected.

Impressed current cathodic protection (ICCP)...more detailed.

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.

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 ground beds 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 types 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 (DC and sometimes 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

Hybrid systems have been 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 400 millimeters (16 in) 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.

Now let’s talk about some real-world applications of Cathodic Protection.

Pipelines

Hazardous product pipelines are routinely protected by coatings and 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 ground bed).

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 ground bed consisting of a vertical hole back-filled 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 back-filled. The choice of ground bed 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.

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. 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.

Ship Hulls

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. Galvanic anodes are generally shaped to reduce drag in the water and fitted flush to the hull to also try to minimize drag.

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 platinized 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 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 aluminum hulls with steel fixtures will create an electrochemical cell where the aluminum hull can act as a galvanic anode and corrosion is enhanced. In cases like this, aluminum or zinc galvanic anodes can be used to offset the potential difference between the aluminum hull and the steel fixture. If the steel fixtures are large, several galvanic anodes may be required, or even a small ICCP system.

Marine Structures

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, 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.

Steel Rebar In Concrete

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, but there are systems available that use the principle of galvanic cathodic protection as well, although in the UK at least, the use of galvanic anodes for atmospherically exposed reinforced concrete structures is considered experimental.

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. 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 (or 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.

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. 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.

Internal Cathodic Protection

Vessels, pipelines and tanks (including ballast 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. ICCP and galvanic systems can be used. 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. Lead or antimony are often added to the molten zinc bath, and also other metals have been studied. 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 create 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 but sacrificial protection. 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.

I appreciate subscribers of the Coatings Talk INSIGHT Newsletter and ask if you would be so kind as to forward this edition to your colleagues and/or connections here on LinkedIn, who will find value in reading this information.

War of The Worlds Listening Party event link.

Disclaimer

This newsletter and the content and materials produced under the Coatings Talk banner does not represent the opinions or positions of 阿克苏诺贝尔 /International.

Make sure to check out and consider joining the Coatings Talk community LinkedIn Group page, just follow this link: https://www.dhirubhai.net/groups/13936465/