Chromate

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Chromating refers to a group of surface technology processes . Complex chromic acid salts ( chromates ) are formed on metallic surfaces through the action of chromic acid . During this process, the base material is dissolved. The dissolved metal ions of the base material are built into the chromate layer. The chromate layers obtained in this way belong to the passivation layers , ie they are inorganic, non-metallic protective layers. On an industrial scale, chromating is mainly used on zinc and aluminum.

application

The most common function of chromate layers is corrosion protection , but they can also be used to create a primer for subsequent layers, as tarnish protection (for silver ), to reduce the visibility of fingerprints or to change the appearance (gloss, color). Chromating processes can be applied to aluminum , magnesium , silver, cadmium and zinc .

The chromating of zinc has achieved great technical importance. Very often steel parts are first electroplated galvanized and then plated. If parts made of die-cast zinc are to be chromated, then these parts are usually galvanized beforehand. Die-cast zinc is rarely chromated directly.

Chromating zinc is done by immersion in a solution of chromic acid and various additives. A very thin chemical conversion layer ( conversion layer) is formed without applying an electrical voltage (currentless ). The layer thicknesses are 0.01… 1.25 µm. When chromating, the zinc surface can be eroded.

Differentiation of types of chromating

The types of chromating are differentiated according to their colors:

colour description
transparent this chromating layer is inconspicuous and is usually not perceived visually. It is chromium (VI) -free, the corrosion protection is relatively weak.
blue This hue is only to be understood as a very light bluish color. The transition to transparent is often fluid. The metallic character of the surface is completely retained. This layer is available chromium (VI) -free or chromium (VI) -containing, the corrosion protection is relatively weak.
yellow This color cannot be compared with a post yellow, but rather with a tinted gold tone. The color tone is often a bit inconsistent and can turn reddish or greenish in places. This effect is called " iridescent ". The layer contains chromium (VI) . This process has found widespread use because of its excellent protection against corrosion and its economy.
Olive This color is very strong and rather resembles a not particularly beautiful paintwork. The metal character of the surface is largely lost. The layer contains chromium (VI), the corrosion protection is also very good.
black The color tone is less strong than with a lacquer finish, the metallic character of the surface is partially retained. For a long time, the coating was only available with chromium (VI), but due to demand, chromium (VI) -free chromating was also developed here, the corrosion protection is medium to good. Black chrome plating should not be confused with black chrome plating .

Popularly, all types of chromate layers are often referred to as chromating. However, the designation passivation would be correct for all RoHS-compliant layers, i.e. chromium (VI) -free layers. The term chromating is only used for chromate layers containing Cr (VI).

Chromating and RoHS

Due to new legal directives such as the EU end-of-life vehicle regulation , the RoHS directive and the REACh regulation , chromating with carcinogenic substances containing chromium (VI) is no longer permitted in many areas. This created a great deal of interest in chromium (VI) -free alternatives. Different shades of color can also be achieved with chrome (III) passivations (mostly bluish-transparent or black). In order to find an equivalent or even better substitute for the chromium (VI) -containing yellow chromating that is still widespread today, various methods were tried:

  • Reinforcement of the comparatively weaker chromium (III) -containing chromating by subsequent sealing
  • Achieving the desired corrosion resistance by converting the upstream galvanizing from pure zinc to a zinc alloy layer (ZnFe, ZnNi or ZnCo)
  • Changeover from chromating to thick-film passivation
  • Use of treatment baths that contain complex titanium or zirconium fluorides that form a passivation layer made of titanium or zirconium oxide

The development in this area is still ongoing.

See also

Individual evidence

  1. ↑ DIRECTIVE 2000/53 / EC OF THE EUROPEAN PARLIAMENT AND OF THE COUNCIL of September 18, 2000 on end-of-life vehicles.
  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.

literature

  • Hans-Jürgen Böhnstedt: Corrosion protection with aqueous paint systems. (= Contact & Study. Volume 530). Expert Verlag, Renningen 1997, ISBN 3-8169-1462-4 .
  • Jürgen Feßmann, Helmut Orth: Applied chemistry and environmental technology for engineers. Manual for study and business practice. 2nd Edition. ecomed Verlag, Landsberg 2002, ISBN 3-609-68352-X .
  • Judith Pietschmann: Industrial powder coating. Basics - Applications - Procedures. 3. Edition. Vieweg + Teubner Verlag, Wiesbaden 2010, ISBN 978-3-8348-0463-1 .
  • Klaus-Peter Müller: Textbook surface technology. Friedrich Vieweg & Sohn Verlagsgesellschaft, Wiesbaden 1996, ISBN 3-528-04953-7 .

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