corrosion

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Rusting railroad tracks

Corrosion (from the Latin corrodere , `` decompose '', `` erfessen '', `` gnaw '') is, from a technical point of view, the reaction of a material with its environment, which causes a measurable change in the material. Corrosion can impair the function of a component or system. Corrosion caused by living things is called bio- corrosion .

In geology , corrosion is the decomposition of rocks due to the action of water. Such weathering processes also play a role in buildings. In medicine , corrosion refers to the decomposition of tissues . In the case of metals , chemical corrosion is particularly important (DIN EN ISO 8044; formerly DIN 50900). Probably the best-known type of chemical corrosion in metals is rusting , i.e. the oxidation of iron.

chemistry

In chemistry, corrosion refers to the chemical reaction or an electrochemical reaction of a mostly metallic material with substances from its environment in a corrosion element , whereby a measurable change occurs in the material, usually a change in color and often a negative change in other surface properties. In certain cases there is an increase in the mass of the metal.

The main types of chemical corrosion are:

The most important electrochemical corrosion is bimetal corrosion when different metals come into contact.

Corrosion (examples)

  • Unpolished table silver with silver sulfide coating

  • Corrosion of copper

  • A heavily rusted girder of a riveted steel bridge

  • Rusted lock on a container

technology

definition

The term corrosion is defined in DIN EN ISO 8044 (formerly DIN 50900) as follows: "Corrosion is the reaction of a metallic material with its environment, which causes a measurable change in the material ( corrosion phenomenon ) and to impair the function of a component or of an entire system ( corrosion damage ). In most cases this reaction is electrochemical in nature, but in some cases it can also be chemical or metal-physical in nature. "

Corrosion rate

The corrosion rate or removal rate indicates the speed of the material change or the material removal. It is given in millimeters per year (mm / a), in the Anglo-Saxon area in mils / year (thousandths of an inch per year). Under standardized conditions, it characterizes the susceptibility to corrosion of metallic materials. The speed of the corrosion reaction depends on the concentrations of the substances involved (e.g. oxygen, water, chlorine), the pH value , the temperature and other parameters. Different forms of corrosion also occur depending on the conditions. The corrosion rate is therefore highly dependent on the location and can fluctuate in the range of two orders of magnitude. Therefore, the following figures should only be seen as a rough guide. For stainless steels, the rate is less than 0.001 mm / year, for aluminum in the single-digit thousandths and for low-alloy steels in the range of several tenths of a millimeter per year.

Corrosive agents

Are the substances that surround the component act on the material and cause corrosion, e.g. B. the indoor air, open air atmosphere with or without industrial pollution, marine atmosphere, fresh and salt water, soil or chemicals.

Types of corrosion

Initiation of pitting corrosion
High temperature sulfur corrosion on a 12 CrMo 19 5 PI nozzle

Types of corrosion are differentiated according to material, cause and appearance. The standard defines 37 different types of corrosion.

In technology, in addition to material-based corrosion, other types of corrosion are differentiated based on the location of their occurrence:

Other types of corrosion:

Corrosion protection

Passive corrosion protection prevents the material to be protected from coming into contact with the corrosion medium by shielding it, while active protection does not require complete separation of material and corrosive medium.

Active protection

Passive protection

Hot-dip galvanized screwed steel construction
  • Hot-dip galvanizing
  • Protective coating or protective coating with appropriate pretreatment or using adhesion promoters
  • Other organic transport protection: greases, waxes, fluorinated polyurethane (FPU)
  • Inorganic non-metallic protection: passivation , oxides, glass , especially borosilicate glass
  • Inorganic metallic protection: hematite (iron mica), zinc , aluminum
  • Constructive measures: continuous welding etc.
  • Do not install different metals in direct contact, e.g. B. no chrome or aluminum trim parts directly on steel; Mount the license plates with a plastic layer
  • Provide water drains in endangered areas (e.g. in car doors)
  • Cavity sealing
  • Relays with inert gas filling, reed relays
  • Electrolytic gold plating , e.g. B. in dental technology
  • Air filtering and overpressure ventilation of housings and control cabinets, e.g. B. by chemisorptive filters to eliminate corrosive gases such as H 2 S

Note: Hot-dip galvanizing creates both passive and active corrosion protection.

Buildings: Corrosion of Rock

In geology, corrosion is the decomposing chemical weathering of rocks by various agents dissolved in water (see also →  mixed corrosion ). Corrosion of this kind also occurs in buildings, often referred to there as stone corrosion . As the word weathering suggests, the weather, or rather the regional climate , plays a major role. Corrosion on stone components can be reduced by constructive measures, e.g. B. by

  • Use of solid materials,
  • Drip edges ,
  • protruding roof edges to protect the facades,
  • Maintenance,
  • Avoidance of vegetation,
  • Prevention of flood and groundwater damage,
  • Avoidance of tree planting above sewers.

Medicine: corrosion of tissue

In medicine , “corrosion” means the destruction of animal tissue caused by inflammation or caustic agents.

On the other hand, the corrosion of metals in implantology and dentistry also plays a role. The varied clinical appearance is known as metallosis and is usually symptom-free in the beginning.

See also

literature

  • Helmut Kaesche: The Corrosion of Metals - Physico-Chemical Principles and Current Problems . Springer-Verlag , Berlin, Heidelberg, New York 1979, ISBN 3-540-08881-4 .
  • Fund of the Chemical Industry: Corrosion / Corrosion Protection, film series and text booklet No. 8, Frankfurt am Main 1994

Web links

Commons : Corrosion  - Collection of pictures, videos and audio files
Wiktionary: Corrosion  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. ^ Corrosion Rate Website of the American Galvanizers Association. Retrieved January 2, 2016.
  2. ^ Metals for Corrosion Resistance: Part II - Catherine Houska Website of the Nickel Institute. Retrieved January 2, 2016.
  3. ↑ Klaus Müller: Four years of experience with EF-NF-NPM alloys . In: Zahnärztliche Praxis , No. 4, pp. 130-132, 36th year, April 12, 1984.
  4. ↑ Klaus Müller: Kleines Handbuch der Oralen Implantologie , chap. 16: On the clinical relevance of metallosis , pp. 137-143, ISBN 3-9800176-2-1 .