Chromate conversion coating

Chromate is a group of surface engineering processes. While ( chromates ) are formed on metal surfaces by the action of chromic acid complex of chromic acid salts. In this process, the basic material is dissolved. The dissolved metal ions of the base material are incorporated into the chromate. The chromate so obtained are among the passivation layers, ie they are inorganic non-metallic protective layers. Industrially, the chromate is mainly applied to zinc and aluminum.

Application

The most common function of chromate is corrosion protection, but they can also be used to generate an adhesion substrate for subsequent layers, to prevent tarnishing ( in silver ), reducing the visibility of fingerprints or to change the appearance (gloss, color). Chromating can be applied to aluminum, magnesium, silver, cadmium and zinc.

Large industrial importance chromating of zinc. Very often steel parts are first galvanized by electroplating and then plated. If parts are zinc die cast to be passivated, then these parts are mostly previously galvanized by electroplating. Rare of die-cast zinc is plated directly.

The chromating of zinc is carried out by immersion in a solution of chromic acid and various additives. This forms without the application of an electrical voltage (normally ) a very thin layer chemical conversion ( conversion coating ). The layer thicknesses are between about 0.01 and 1.25 microns. When Chromate may cause a removal of the zinc surface.

Distinction of Chromatierungsarten

The Chromatierungsarten be distinguished according to their colors:

A yellow chromated workpiece can be easily confused with a workpiece made ​​of brass.

Popularly all types of chromate coatings are often referred to as chromating. Correct, however, would be the name passivation for all RoHS -compliant layers, ie chromium (VI ) - free layers. The term is here used chromate chromate only Cr (VI )-containing.

Chromating and RoHS

Due to new legal regulations, such as the EU ELV and the RoHS Directive are chromatings with carcinogenic chromium (VI ) - containing materials in many areas no longer allowed. This created a lot of interest of chromium ( VI)-free alternatives. Even with chromium ( III) passivation, different colors can be realized (mostly bluish transparent or black). To find an equivalent or even better replacement for the still widespread chromium ( VI )-containing yellow chromate, various approaches have been tested:

  • Reinforcement of the comparatively weaker chromium ( III )-containing chromates by subsequent sealing
  • Achieve the desired corrosion resistance by changing the upstream zinc coating of pure zinc in a zinc alloy layer ( ZnFe, ZnNi or ZnCo )
  • Conversion of the chromating on the thick film passivation
  • The use of treatment baths containing complex titanium or Zirkonfluoride forming a passivation layer made of titanium or zirconium

The development in this area is not yet complete.

Primer (paint) MIL-DTL-5541
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