Corrosive medium

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As corrosion medium - also engaging or corrosive known means - is called a liquid or gaseous substance, the solid so on coming in contact body, in particular metals , a corrosive effect exerts. A liquid corrosion medium can also be in the form of a metal melt. The corrosive effect can take place chemically, electrochemically or biochemically .

Liquid as a base

If the base of the corrosion medium is a molten metal, one speaks of a (purely) chemical corrosive effect. In most cases, however, water is the basis of the corrosion medium. This then exerts an electrochemical corrosive effect. With the ubiquitous gaseous oxygen, some metallic materials can react to form metal oxides without the participation of an electrolyte , but mostly water is involved. For this it is sufficient if this is only available in the form of the omnipresent humidity. In medicine, blood or saliva should be considered as a corrosion medium.

Additives

If it is a corrosive medium in the liquid state of aggregation is such. B. water, dissolved or added salts (e.g. chlorides , sulfates, nitrates , nitrites , ammonium salts ), acids (e.g. sulfuric acid ), alkalis (e.g. sodium hydroxide ) or gases that form ions in solution through hydration ( e.g. carbon dioxide , ammonia , sulfur dioxide , nitrogen oxides , hydrogen sulfide ) increase the corrosive effect. This is why seawater is the most common corrosion medium: Because of its content of dissolved salts, their anions have a highly corrosive effect on many metals and their alloys .

If the water contains corrosive microorganisms, one speaks of biochemical corrosion by the medium.

Number, composition and concentration

It goes without saying that in the case of the substances mentioned above, which may be contained in the water, not only the content as such is important, but also the number and composition of them and their concentration. Even a comparatively low concentration of one or more of the substances can certainly have an intensifying influence on the corrosive effect of the liquid corrosion medium. At higher concentrations, however, the corrosive effect will generally increase significantly. A corrosion medium in the strongly acidic range ( pH value 0 to 2), for example, will achieve a noticeably stronger corrosive effect than a corrosion medium in the weakly acidic range (pH value> 2 to <7). The situation is similar with basic (especially alkaline) corrosion media (pH value> 7 to 14) and those that contain dissolved salts and / or certain gases (see above).

Corrosive effect on metals

The corrosive effect of the corrosion medium is based on the property of metals that, when they come into contact with electrolytes, they tend to change into the thermodynamically favored ionic state. If, on the other hand, they come into contact with non-electrolytes (e.g. gases), they strive for a transition to thermodynamically more stable, lower-energy states - with the formation of compounds.

Effect of external influences

The corrosive effect is also influenced by the temperature and movement (static or flowing) of the corrosion medium. Furthermore, a static or dynamic load on the metallic material exposed to the corrosion medium has an influence on the corrosive effect. For example, tensile stress or superimposed low-frequency threshold tensile stress in interaction with a corrosion medium can promote the occurrence of stress corrosion cracking . Higher-frequency alternating loading of the metallic material, on the other hand, can lead to vibration cracking corrosion with the simultaneous action of a corrosive medium .

Protective measures

Measures to protect metals from the effects of corrosive media are

  • Protective layers of all kinds to keep corrosion media away from the metal;
  • Contact with less noble metals, which means that only these corrode at first, until none of them is uncorroded (see e.g. galvanizing , sacrificial anode );
  • Passivation , e.g. B. forms a natural protective layer on aluminum , but its thickness can also be increased (see anodizing );
  • Application of an artificial electrochemical potential (e.g. approx. 500 to 600 mV) to the metallic material exposed to the corrosion medium, possibly mechanically stressed (see above).

See also

literature

  • Corrosion Lexicon. 2 volumes, Mannesmannröhren-Werke, 1970.
  • Chemical Industry Fund: Corrosion / Corrosion Protection. Slide series and text booklet No. 8, Frankfurt am Main 1994.

Web links

Individual evidence

  1. ↑ a b J. Falbe, M. Regitz (Ed.): Römpp Chemie Lexikon. 9th edition. Stuttgart / New-York 1995, p. 2344.
  2. ↑ a b c d dtv-Brockhaus Lexikon. (developed according to the lexical documents of FA Brockhaus). Volume 10, Mannheim / Munich 1988, p. 112.
  3. ↑ Klaus Müller: Complaints from corrosive metals. In: Selecta the doctor's weekly magazine. No. 22, June 2, 1986, p. 1742.
  4. ↑ a b J. Falbe, M. Regitz (Ed.): Römpp Chemie Lexikon. 9th edition. Stuttgart / New-York 1995, p. 2345.