Sacrificial anode

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An almost used sacrificial anode on a ship's hull
A number of sacrificial anodes on the former rescue cruiser H.-J. Kratschke

A sacrificial anode is an electrode made from a piece of base metal that is used on devices and vehicles to protect functional parts made of other metals (especially iron , steel - also in reinforced concrete - and brass ) against contact corrosion . The less noble material of the sacrificial anode itself is destroyed, ie “sacrificed”. In this way, the function of corrosion-prone and elaborately manufactured functional parts is guaranteed for longer and costly new procurement of these is postponed or avoided entirely.

principle

If components made of iron or steel come into contact with water , moist air or another electrolyte , the oxygen dissolved in the water, for example, attacks the metal ( oxidation ). A galvanic cell is formed . Electrons are withdrawn from the metal and the positively charged ions pass into the solution , and the metal corrodes . To prevent this, cathodic corrosion protection with sacrificial anodes or impressed current anodes is used. In order for a sacrificial anode to fulfill its purpose, it must be more negative (for example magnesium ) than the metal to be protected in the electrochemical series .

Mode of action

The metal to be protected is conductively connected to the sacrificial anode. A primary element is created in which the metal to be protected acts as the cathode and the less noble metal as the anode . A current flows in the direction of the metal to be protected. Instead of this, the less noble sacrificial anode metal gives its electrons to the oxygen, is oxidized and goes into solution. In this local element, the water is the electrolyte, which enables the charged particles to be transported and thus closes the electrical circuit. The sacrificial anode will be used up over time and must be replaced.

Applications

Sacrificial anodes are often used to protect ship propellers , which are often made of brass, and in the hull area of ​​the rudders. This happens in particular on ships that sail in electrochemically aggressive salt water . As a sacrificial anode, blocks made of zinc , formerly cadmium , are screwed or riveted in the required size and number around the screw on the ship's hull, in some cases also on large rudder blades . For this purpose, anode blocks are usually welded on at the shipyard using cast steel strips. Additional sacrificial anodes to protect the steel hull are usually attached in smaller numbers.

Another common application is the protection against corrosion in galvanized boilers and enamelled hot water storage tanks with basic bodies made of steel. Without a sacrificial anode, the zinc coating on the boiler will dissolve in water over time. Sacrificial anodes are also required for enamelled boilers, as there is still no method that can guarantee a permanently faultless internal enameling of a boiler. In order to avoid subsequent rusting of the hot water storage tank, a sacrificial anode made of magnesium is screwed or inserted onto the inner wall of the tank and then screwed in to seal it. Rod or chain anodes are used for this. The latter are used when the structural conditions (structural height above the insertion opening) are not sufficient for inserting a rod anode. The dissolved magnesium ions are harmless to health, even if the water is used to prepare food. Depending on the application and local conditions, sacrificial anodes should be checked for wear (decrease in material thickness) every one to two years and replaced if necessary.

Sacrificial anodes are also used in larger tank farms, underground pipelines and oil rigs to prevent corrosion from the damp soil.

An alternative are impressed current anodes , which however require a direct current source.

  • Sacrificial anodes

  • New zinc sacrificial anode

  • Link of an unused chain anode made of magnesium for a hot water storage tank

  • Used magnesium anode from a hot water tank

literature

  • Walter von Baeckmann, Wilhelm Schwenk (ed.): Handbook of cathodic corrosion protection. Theory and practice of electrochemical protection processes. 4th, completely revised edition. Wiley-VCH, Weinheim et al. 1999, ISBN 3-527-29586-0 .

Web links

Wiktionary: Sacrificial anode  - explanations of meanings, word origins, synonyms, translations