Corrosion is the alteration of a metallic material and its characteristics, if a reaction takes place with other materials in its immediate surroundings. The most common, and very obvious example, is the appearance of rust (oxidation) caused when the surface is exposed to the oxygen in the air.
In the electro-chemical voltage table (see above), the values in the table depict the voltage difference of electro-chemical semi-conductors (electrodes) against the reference potential of the normal hydrogen electrode. When coming into contact with an electrolyte which contains the ions from the solution of the substance, electro-chemically active substances form an equilibrium potential, i.e. per time unit, the amount of ions is the same when giving off electrons into a solution as the amount is, when electrons are, once again absorbed. If an electrode is dipped into a 1-active solution of the associated ions, one speaks of the normal potential. The position of a metal inside of the voltage-range indicates the tendency of the metal, during the emission of electrons, to go into solution. The very negative alkali metals, as opposed to the normal hydrogen electrode, react fiercely, e.g., with water by developing lyes and reducing the water to hydrogen gas, i.e. alkali metals very easily emit electrons, whereas the precious metal, gold, remains in the earth's crust as an element, without reacting. Based on the electro-chemical voltage-range, it is easy, through the addition of the numerical values, to determine the tension (electromotive force) of a combination of semi-conductors.
The table shows the voltage difference of electro-chemical semi-conductors to hydrogen. The further the two materials are from each other, the more the electro-chemical corrosion increases, Thereby, the metal further down in the table is used up. If the acting material forms a connection to the surface of the metal, one then speaks of chemical corrosion. This newly created layer can, (e.g., with aluminium) protect, or cause the further destruction of the workpiece.
The corrosion protection can begin directly on the material, e.g., through alloying. A chrome content of more than 10%, e.g., has a protective effect with steel. Indeed, the workpiece can also be coated with a preserving substance. Should metallic preservers be used, they should be made of a less precious metal (see above). Whereas zinc can be used for steel, steel cannot be used for zinc, because in the end, only the tin would be left over. Non-metallic preservers must perform a sealing
function to keep moisture out, thus preventing the widespread coverage with rust. 01/12