Cathodic protection (CP) is a method used to control the corrosion on a metal surface by making it work as the cathode of an electrochemical cell. This is achieved by placing the metal which is to be protected in contact with another, less noble, metal to act as the anode of the electrochemical cell. This less noble metal will then be sacrificed in favour of the structure being protected. CP systems are most commonly used to protect steel water or fuel pipelines and storage tanks, steel pier piles, ships, offshore oil platforms, onshore oil well casings and reinforcing steel in concrete. Stainless steel and aluminium structures can also be protected.
History
CP was first introduced by the Englishman Sir Humphry Davy after a series of papers presented to the Royal Society in London in 1824. HMS Samarang was the first structure to have CP applied. Iron sacrificial anodes were attached to the layer of copper on the hull below the waterline which significantly reduced the copper's rate of corrosion, however it was noticed that a side effect of the CP was an increase in marine growth on the hull. This is attributed to the fact that as copper corrodes, it releases copper ions which have an anti-fouling effect so by reducing the corrosion of the copper the anti fouling properties were also reduced. Since the excess marine growth affected the performance of the ship CP was not used further. Instead, the Royal Navy decided that it was better to allow the copper to corrode and have the benefit of reduced marine growth.
CP became a more common practice from the 1940s onwards as buried steel and iron pipelines for water and oil transportation had systems installed.
Galvanic CP
In the modern day, sacrificial or galvanic anodes are made in various shapes using zinc, magnesium and aluminum alloys. For CP, the electrochemical potential, current capacity, and consumption rate of these alloys are far superior to iron. Galvanic anodes are designed and chosen to have a more "active" voltage than the metal of the structure.
Impressed Current CP
Sacrificial anodes cannot deliver enough current to provide complete protection for larger structures, so Impressed Current Cathodic Protection (ICCP) systems are used. In ICCP systems, anodes are connected to a DC power source known as a Transformer Rectifier.
Since the driving voltage is provided by the DC source there is no need for the anode to be less noble than the structure to be protected.
The anodes for ICCP systems are usually tubular and solid rod shapes or continuous ribbons of various specialised materials including high silicon Iron, graphite, platinised titanium and mixed metal oxide (MMO) coated titanium.