Cathodic protection
A sacrificial anode is a piece of base metal, for corrosion protection of the functional parts of other metals (especially iron, steel - and brass - even in reinforced concrete) will be used. It is thus targeted against contact corrosion.
The name follows from the fact that the anode is destroyed function caused over time. It is sacrificed to protect another metallic part from corrosion.
Principle
Come parts of iron or steel with water, moist air or another electrolyte in contact so engages, for example, the oxygen dissolved in water, the metal to (oxidation) → it forms a galvanic cell. In this case, the metal electrons can be withdrawn, and the positively charged ions go into the solution, the metal is corroded. To prevent this, the cathodic corrosion protection is used with sacrificial anodes or impressed current anodes. In order for a sacrificial anode can fulfill its purpose, it must be in the negative voltage range (eg magnesium) than the metal to be protected (eg, iron ).
Operation
The metal to be protected is conductively connected to the sacrificial anode connected. The result is a primary element, in which the metal to be protected acts as the cathode and the base metal as the anode. In this case, a current flows in the direction of the metal to be protected. Instead this is now the less noble metal anodes its electrons to the oxygen from, is oxidized and goes into solution. The water is in the local element of the electrolyte, which allows the transport of charged particles, thus closing the circuit. The sacrificial anode is consumed with time, and must be replaced, so that corrosion protection is maintained.
Applications
This is often used to protect ships' propellers, especially in ships sailing in aggressive salt water. As a sacrificial anode blocks are screwed rings of zinc in the required size and number around the screw on the ship's hull or riveted. Special brackets are often attached to already at the shipyard. Additional sacrificial anodes to protect the steel hull can be installed in smaller numbers.
Another common application is the corrosion protection for galvanized and enamelled boilers hot water tanks with machined steel base bodies. The zinc coating of the tank dissolves without sacrificial anode on the time in the water. Even with enamelled boilers sacrificial anodes are required because there is no method to this day, which can ensure a durable 100% error-free Innenemaillierung a boiler. In order to avoid a subsequent rusting of the hot water storage tank, a sacrificial anode is screwed magnesium on the inner vessel wall or imported and subsequently screwed sealing. For rod or chain anodes are used. The latter are used when the structural conditions ( height above the insertion ) for the insertion of a rod anode is insufficient. The solid solution in magnesium ions are classified as harmless to health, even if the water is used for food preparation. Sacrificial anodes should be inspected annually to 2 - annually for wear (decrease in the thickness of the material ) to replace them, if necessary, depending on the application and local conditions.
A maintenance- free variant are impressed current anodes connected to a DC power source and provide permanent protection against corrosion.
It is important for cost minimization, that the risk of corrosion parts, generally verschleissende parts can be easily replaced.
Sacrificial electrode
The concept of sacrificial electrode comes from the electroplating. Also in the galvanotherapy, namely the Stanger, iontophoresis and Fussbadelektrolyse the term is being used. Elements with a negative electric potential, among other things, as zinc, magnesium, aluminum, iron or copper - bronze, anodically dissolve very rapidly to, and are thus elsewhere no longer be used as an anode material, for example in medical technology. In this oxidation process already had the doctor FH Martens, 1803, pointed at the instigation of his Galvanoskopes. See in Remak, doctor, galvanotherapy. From the sacrificial anode Report and Martens knowledge from 1803 shows that electrodes made of zinc, magnesium, aluminum, iron or copper - bronze are quite unsuitable for medical use, just because they dissolve under the current supply (iontophoresis, Stanger, Fussbadelektrophorese ). In addition, the medical and therapeutic properties of electrode material, tin, aluminum, lead, copper or zinc release, described as very worrisome, because these metal ions on human skin cause toxic reactions. Scientific Background of the resolution is that these elements have very strong negative standard electrode potentials: Zinc ( -760 mV ), magnesium ( -2400 mV ), iron ( -440 mV ), aluminum ( -1670 mV), and bronze ( -440 mV),. The negative electrochemical potential indicates that these elements are more or less charged with electrons. These electrons can then easily release the elements under galvanic conditions to other elements that have a shortage of electrons, such as Gold or silver. With the transfer of electrons from, for example, Zinc anodes dissolves the anode.
Toxicology of chromium-nickel - titanium - platinum-palladium
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The toxicology of the anode material must be complied with where applicable. In occupational medicine is known that anode material consisting of chromium, nickel, manganese, platinum, titanium or palladium may be used only under certain protective conditions in the industrial sector, since these elements release harmful ions in the dielectric. Surgical steel (V2A - steel) is carcinogenic chromium - nickel ions in the form of free (anodic, hexavalent chromium, and cationic nickel -II) and must in medical technology, such as iontophoresis, Stanger or Fussbadelektrolyse, not be used. Platinum, titanium and palladium are elements that release highly allergenic ions and are therefore in the medical field as an anode material for implants and unsuitable ( Ordinance on Hazardous Substances, 2005). In the lymphocyte transformation test LTT there is a significant allergic reaction. In galvanotherapy and can therefore medical anode material made of gold or silver is used which is electropositive, namely gold with 1700 mV and 810 mV with silver record, electrons, or not only geringradig dissolves and no hazardous releases ions.
These elements are there as a sacrificial electrode suitable as consumables, where one aims the dissolution of the electrodes, namely the electric - arc welding ( MAG, TIG, tungsten inert gas) welder what everyone knows from the electric welding ago. Also in spot welding, a connection method used today in automobile, the anode material comes to the same function as in shipbuilding.