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In surface technology, passivation is understood as the spontaneous emergence or targeted creation of a protective layer on a metallic material, which prevents or greatly slows down the corrosion of the base material.

Spontaneous passivation

If bare metal is exposed to air or another corrosive environment, then it depends on the chemical composition of the metal whether corrosion occurs. While z. B. gold and platinum corrode very slowly due to their property as precious metals , the less noble metals such as iron , zinc and aluminum have a tendency to corrode . Whether and how quickly corrosion occurs also depends on the possible formation of a passivation layer. The best example of such a passivation layer is the metal chromium : although chromium is less noble than iron in the chemical sense, it behaves almost like a noble metal when it comes to corrosion when it comes to air and water - we know this effect from chrome-plated bathroom fittings that have been shiny and shiny for decades stay. A very thin, invisible oxide layer [in chromium-nickel steels in the order of magnitude of 10 nm (approx. 50 atomic layers, with pure chromium 5 layers)] separates the metal from the atmosphere, so that further oxidation is only possible by diffusion through the oxide layer . The passivating layer hinders diffusion, so that further corrosion of the material is stopped.

Another example of this phenomenon is stainless steel : the chromium contained forms a chromium oxide layer from 10.5% by mass , which prevents further oxidation. If this oxide layer is damaged, bare metal comes into contact with the atmosphere and a new passivating layer is automatically formed. i.e. the layer is self-healing. Other technically important materials that form passive layers are aluminum , nickel , titanium , lead , zinc and silicon .

Under unfavorable conditions (halogen-containing medium, electrochemical potential , see also Pourbaix diagram ), materials with a passive layer can become susceptible to pitting corrosion .

The best-known example in which there is no spontaneous passivation is ordinary steel. The corrosion layer - the rust - consists of a rapidly growing layer of iron oxide, which does not slow down the further progress of corrosion .

The Pilling-Bedworth ratio can be used to calculate whether the oxide layer has a protective or non-protective character .

Passivation process

In the case of stainless steel with a chromium content of more than 12%, the natural passive layer can be significantly improved by using passivating acids such as nitric acid and citric acid . This process reduces the amount of free iron on the surface. The remaining chromium forms a passive layer through oxidation. Passivation with citric acid has decisive advantages over passivation with nitric acid. Citric acid is harmless in terms of occupational safety and also more effective to use. This also causes an elementary difference at the atomic level: although nitric acid attacks the iron content first, it also reduces the amount of the other alloy components. As a result, the thickness of the chromium oxide layer can only be represented to a limited extent.

With some metals, it makes sense not to leave the creation of a passivation layer to chance, but rather to technically generate the passivation layer using a defined process. One such example is aluminum, but in this case one does not speak of passivation, but of anodizing .

With aluminum, magnesium , silver , zinc and cadmium , the chromating process can be used to create a passivation layer which, in addition to improved corrosion protection, also serves as a primer for subsequent process steps, as tarnish protection (silver), as protection against fingerprints or to change the appearance (gloss , Hue).

The chromating of zinc layers has achieved great technical importance. The passivation layer created in this way can delay the corrosion of the zinc ( white rust ) for a very long time. The passivation layers can have the colors (weak) blue, yellow, black, olive or transparent, depending on the process. Depending on the process, the passivation layers can contain toxic chromium (VI) oxide . Chromium (VI) -free passivations have also been developed in recent years. Sometimes they do not achieve the same corrosion resistance as the chromium (VI) -containing processes.

The new legislation in the EU has banned passivation with chromium (VI) for use in automobile construction (cars <3.5 t) and in household appliances.

Modern replacement processes use chromium (III) or are completely chromium-free. For example, chromium-free processes use treatment solutions that contain complex zirconium or titanium fluorides. This then creates a passivation layer made of titanium or zirconium oxide.

The relatively new process of thick-film passivation of zinc layers combines the advantages of being chromium (VI) -free with good to very good corrosion resistance.

In the semiconductor industry, silicon is a frequently used material, which oxidizes quickly and thereby loses part of its desired positive electrical properties. Up to now, silicon nitride has often been used to passivate the surface. This process takes place in a vacuum chamber, which is heated to 400 ° C. This relatively high temperature means that not all otherwise suitable materials can be used for the passivation process and that the products are manufactured at relatively high cost. MIT scientists are currently developing a new process that will enable the passivation process at room temperature. A heating wire mesh, which is heated to approx. 300 ° C, is also positioned over the silicon in a vacuum chamber. The polymer material introduced inside the vacuum chamber evaporates in the vicinity of the heating wires and condenses on the surface of the silicon. The advantages of this vapor phase deposition process are said to be a drastically reduced energy consumption, the possibility of using a wide variety of materials for passivation and a significant reduction in production costs. Since the silicon itself is not heated to more than 20 ° C in this process, a significantly better overall efficiency should result in the manufacture of solar cells.

See also

Web links

Wiktionary: passivate  - explanations of meanings, word origins, synonyms, translations
Wiktionary: Passivation  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. Directive 2000/53 / EC of the European Parliament and of the Council of September 18, 2000 on end-of-life vehicles - Commission statement . In: Official Journal of the European Parliament and of the Council . L 269, September 21, 2000, p. 34-43 .
  2. Directive 2002/95 / EC of the European Parliament and of the Council of January 27, 2003 on the restriction of the use of certain hazardous substances in electrical and electronic equipment . In: Official Journal of the European Parliament and of the Council . L 037, January 13, 2003, p. 19-23 .
  3. ^ David L. Chandler, MIT News Office: A cooler way to protect silicon surfaces . February 13, 2013