Anodize

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In surface technology , anodizing , also known as anodizing or anodizing , describes an electrolytic process for the production or reinforcement of oxidic layers on metals . Anodizing is a special case of anodic oxidation in which a firmly adhering oxide is formed. The anodization serves in particular to protect metals against corrosion . The most important anodizing process is the anodizing process of anodizing aluminum .

Procedure

Schematic representation of the anodization using the example of a tantalum electrode

During anodization , the metal is immersed in a suitable aqueous solution (for example of sulfuric , oxalic or chromic acid ) and electrolysis is carried out with the metal serving as the positive pole . The electric current forms an oxide layer on the anode surface , while water is decomposed on the cathode and it is reduced to hydrogen .

Direct voltage is generally used for electrolysis, often with a voltage of 10 to 25 volts and a current density of 50 to 250 A / m 2 . For thicker layers, e.g. B. in sulfuric acid, up to 120 V can be used. The process with direct voltage and sulfuric acid is called the GS process, if oxalic acid is used in addition to sulfuric acid, the GSX process. Processes with only oxalic acid are referred to as GX or WGX processes and with chromic acid as GC processes or Bengough-Stuart processes. In addition, there are processes that use borax (50–500 volts), boric acid (230–250 volts) or citric acid for the production of electrolytic capacitors .

Areas of application

For this purpose, oxide layers are used which, in the case of aluminum, can be between 0.5 and 150 micrometers thick. 5–25 micrometer thick layers are common for corrosion protection. The layer thicknesses for decorative purposes can be up to 500 µm. The layer produced in this way serves primarily as a protective layer for metals against corrosion and abrasion. It is microporous and therefore only achieves its optimum resistance through a subsequent treatment - compression - which causes the pores to close. Layer thicknesses of up to 80 µm are common for magnesium. Oxide layers are also used as electrical insulation (dielectric) in electrolytic capacitors ( tantalum , niobium , aluminum ). For this purpose, the oxide layers must be thin enough, because only a small layer thickness (less than 1 μm) enables the desired high capacities, which are one of the main advantages of electrolytic capacitors.

Even if anodizing is possible with various metals, it has only achieved greater technical importance for light metals, especially aluminum and its alloys. There the process is also called anodizing process . "Anodized" aluminum is used extensively in architecture (house facades, doors, etc.) and in vehicle construction.

Another advantage of the process is that only the top metal layers (up to 40 μm thick) are affected by the conversion. Some metals (e.g. aluminum, titanium ) initially tend to become microporous and can therefore be easily colored with organic dyes. After that, they still have to undergo additional densification.

In semiconductor technology, the process is used to manufacture gate oxides.

Historical

Experiments with aluminum anodes were reported as early as 1853: If an aluminum electrode immersed in sulfuric acid is positively polarized by an external voltage, a “fairly vigorous evolution of gas” occurs first, and oxygen is produced. However, this gas development quickly subsides. A passivity of the aluminum can also be obtained "when immersed in nitric acid of any degree of concentration". In 1923 a patent was filed for the formation of a corrosion-resistant layer on aluminum and aluminum alloys, which worked with a chromate solution and a voltage increasing to 50 volts. The process was used to protect parts made of duralumin from seaplanes against corrosion .

In the past, the anodizing effect of some metals was also used for rectification. One design consisted of a platinum electrode and a niobium electrode, which are immersed in dilute sulfuric acid. As soon as the niobium sheet becomes the anode, the current flow ceases, since non-conductive niobium oxide forms, which is reduced back to the niobium metal when the polarization is reversed, which enables current to flow again.

variants

A variant is the anodic oxidation with spark discharge (English anodic spark oxidation , ANOF). In this process, oxidation is not carried out with direct current, but with a sawtooth-like voltage ramp rising from zero until a spark from the electrolyte jumps over to the material to be treated. This spark locally melts the material such as titanium, magnesium or aluminum and forms a hard oxide due to the high temperatures of the discharge. In the case of aluminum, alpha aluminum oxide (corundum) can be deposited without thermal treatment of the component. These layers are particularly suitable as chemically highly resistant wear protection layers.

In another method, plasma anodization, a semiconductor vapor-deposited with aluminum is exposed to a direct voltage glow discharge in an oxygen atmosphere at around 3 • 10 −3 Torr (0.4 Pascal).

Individual evidence

  1. ↑ a b Kirsten Bobzin: Surface technology for mechanical engineering . John Wiley & Sons, 2013, ISBN 978-3-527-68149-5 ( limited preview in Google Book Search).
  2. ↑ a b c Horst Briehl: Chemistry of materials . Springer-Verlag, 2014, ISBN 978-3-658-06225-5 , pp. 121 ( limited preview in Google Book search).
  3. ↑ a b Andreas Kalweit, Christof Paul, Sascha Peters, Reiner Wallbaum: Handbook for Technical Product Design, Material and Manufacturing, decision-making bases for designers and engineers . Springer Science & Business Media, 2006, ISBN 3-540-21416-X , p. 532 ( limited preview in Google Book search).
  4. ↑ a b Ulrike Kuhlmann: Steel Construction Calendar 2016 Eurocode 3 - Basic Standard, Materials and Sustainability . John Wiley & Sons, 2016, ISBN 978-3-433-60630-8 , pp. 299 ( limited preview in Google Book search).
  5. ↑ a b Eike Becker: Technologies for organic field effect transistors in display technology . Cuvillier Verlag, 2006, ISBN 3-86727-044-9 , pp. 41 ( limited preview in Google Book search).
  6. ^ Hansgeorg Hofmann, Jürgen Spindler: Process in coating and surface technology . Carl Hanser Verlag GmbH Co KG, 2014, ISBN 978-3-446-44183-5 , p. 198 ( limited preview in Google Book Search).
  7. ↑ a b c H. Buff: About the electrical behavior of aluminum . In: Justus Liebig's Annals of Chemistry . tape 102 , no. 3 , 1857, pp. 102 , doi : 10.1002 / jlac.18571020302 .
  8. ↑ Patent GB223994A : Improved process of protecting surfaces of aluminum or aluminum alloys. Registered August 2, 1923 , published November 3, 1924 , inventors: Guy Dunstan Bengough, John Mcarthur Stuart. ‌
  9. ↑ Richard Wilhelm Heinrich Abegg, Friedrich Auerbach, Ivan Koppel: Handbuch der inorganic Chemie, Volume 3, Edition 3 . S. Hirzel, 1907, p. 811 ( limited preview in Google Book search).
  10. ↑ Hans-Ludwig Graf, Alexander Hemprich, Wolfram Knöfler: Development of the technology of "anodic oxidation with spark discharge (ANOF)" for conditioning implant surfaces . In: Implantology . No. 3 , 2004, ISSN  0943-9692 , p. 257-269 .
  11. ^ P. Kurz, W. Krysmann, HG Schneider: Application Fields of ANOF Layers and Composites . In: Crystal Research and Technology . tape 21 , no. 12 , 1986, pp. 1603–1609 , doi : 10.1002 / crat.2170211224 (overview of the ANOF method).
  12. ↑ Wolfgang Harth: Semiconductor technology . Springer-Verlag, 2013, ISBN 978-3-322-94051-3 , pp. 91 ( limited preview in Google Book search).