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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Cathodic protection | 1/5 | https://en.wikipedia.org/wiki/Cathodic_protection | reference | science, encyclopedia | 2026-05-05T10:46:35.089302+00:00 | kb-cron |
Cathodic protection (CP; ) 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 HMS Samarang 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 States—cathodic 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 (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 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 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 (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) ===