Cathodic Protection

August 08, 2016

Cathodic protection is a corrosion control technique that makes a metal surface the cathode of an electrochemical cell. This cathode is then connected to a sacrificial anode, which corrodes instead.

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An impressed current cathodic protection system is employed to protect the steel tubular piles for the Kirkwall Harbour Development, Hatston Pier extension. Via Corrocell.co.uk

Cathodic protection is a corrosion control technique that makes a metal surface the cathode of an electrochemical cell. This cathode is then connected to a material that is more easily corroded, known as the “sacrificial metal,” which acts as the anode of the cell. Because the anode is more susceptible to corrosion, it is corroded instead of the cathode. A prominent example of cathodic protection is galvanized steel, where a zinc coating acts as the sacrificial metal on steel parts to protect them from rust.

History of Cathodic Protection

Cathodic protection was first outlined by Sir Humphry Davy in 1824 in a presentation for the Royal Society in London. The discovery was quickly put to use on the HMS Samarang where iron sacrificial anodes were attached below the waterline to the copper hull. The result of this was that the corrosion rate of the copper was reduced significantly. However, an unintended side effect of this was that marine growth on the underside of the ship was increased. This was because when copper corrodes, it releases copper ions which serve to discourage fouling. The Royal Navy eventually chose not to use cathodic protection on its ships because too much marine growth affected the ships’ performance. More than 100 years later, in 1928, cathodic protection was implemented to reduce corrosion in steel gas pipelines and is still widely used today.

Risks of Hydrogen Production

Although cathodic protection is effective at reducing corrosion in vulnerable materials, this form of corrosion control does have some drawbacks. If cathodic protection is not properly implemented, atomic hydrogen can be produced. This hydrogen is then absorbed into the metal and causes hydrogen embrittlement in welds and materials with high hardness. Additionally, cathodic disbonding can be caused by the formation of hydrogen ions over the surface of the cathode, thereby causing the sacrificial anode, or protective coating, to separate from the cathode. Disbonding is increased when there is an increase in cathodic polarization, so cathodic protection systems must avoid excessive polarization.

Use with Coatings

Cathodic protection systems on steel pipelines can also lose effectiveness when they are used in conjunction with solid film backed dielectric coatings (polyethylene tapes, shrinkable pipeline sleeves, etc.) because these film backings have high electrical resistivities. Therefore, the electric current that protects the cathode cannot reach the vulnerable metal. Since the 1980s when it was discovered, much research has been conducted on the subject of cathodic shielding and it was determined to play a significant part in a 1999 spill from a Saskatchewan crude oil line.

While cathodic protection can significantly reduce the corrosion rate of materials susceptible to certain types of corrosion, it must be implemented carefully to avoid situations in which cathodic protection can be rendered ineffective. The system must always be analyzed to ensure that the vulnerable metal will be adequately protected by the sacrificial anode, and that the cathodic protection system will offer full efficiency.

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