Trivalent passivation of zinc deposits and zinc alloys
N.B.: The information contained in this sheet comes from reliable sources. Nevertheless, it is provided without any guarantee, express or implied, of its accuracy.
Principle:
Zinc is too sensitive to atmospheric corrosion to remain unaltered outdoors for long periods. It is customary to protect zinc by chemical conversion with chromium. By chemical conversion, a thin surface film of 1 to 2 μm of zinc is dissolved in an oxidizing acid medium and forms a protective zinc salt. Chromium is the most effective element and forms the set of chromium passivations of various colors: iridescent clear, blue, iridescent yellow, green or black. Hexavalent chromium (which generates chromations on zinc), now banned for the automotive industry and all parts with an electrical or electronic system, is mainly replaced by trivalent chromium (which generates passivations on zinc also called chromitations).
History of passivation
Chromium is very dangerous in its oxidized form at degree 6: CrVI or Cr6 but its trivalent variant is no longer dangerous. The protection of the employees at the work place imposed that the legislator prevent the toxicological risks during the handling of the parts at the time of manufacture but also in daily use. The European Directives on end-of-life vehicles (ELV or Directive 2000/53) and on the use of hazardous substances (RoHS or Directive 2002/95) have banned the use of hexavalent chromium in chromium conversion passivation. By targeting electrical and electronic parts, Directive 2002/95 has effectively affected many industries and few chromate applications are still permitted with CrVI.
Passivates do not have good friction properties and are very often used with organo-mineral finishes or reinforced finishes. The first reinforced finishes appeared in the standards of PSA (B 15 4100 and following) and Renault (series of 01-71-002) in the 90s. They concerned particularly the bolts and nuts for which these lubricating properties are particularly sought after to control the friction coefficient. The evolution of automotive standards coincides with the search for quality. In particular, the requirements for resistance to corrosion have increased in terms of resistance to salt spray NF 41 002 from 200h before red rust in 1985 to 400h in 1992, then 600h in 2000 to reach 720h in 2002. This evolution is comparable to what happened in Western Europe during the same period when practically all car manufacturers chose the same policy of quality research.Processes
CrVI chromatation:
Zinc chromating was widespread and easily applied after electrolytic zinc by forming a thick complex layer up to 4 μm for black layers on zinc-iron. Zinc immersed in the acid chromating solution is dissolved on the surface up to 2 μm and redeposited as a complex insoluble salt according to the following reactions:
Oxidation of zinc by chromic acid:
Reduction of part of the CrVI to CrIII :
Formation of a complex salt :
Chromating forms a water-rich and rather soft layer. It is a gel that does not withstand temperature increases and loses its protective capacity above 120°C. The solution is to apply organo-mineral finishing layers whose silica guarantees the maintenance of hydration and thus of anti-corrosion properties.
CrIII passivation:
The oxidation of zinc takes place in an acidic environment, usually in the presence of oxidizing nitrates:
Monitoring the formation of chromium salts:
The passivation composed of hydroxides dehydrates easily during drying and becomes very hard. Its thickness reaches 200 nm or 400 nm for passivations reinforced with silica.
The colors of these passivation layers, which are thinner than chromate, are lighter and vary from iridescent to light yellow. The black color is obtained thanks to the presence of iron, nickel or cobalt oxides. Its color is reinforced by an organo-mineral finish, which is essential for this type of deposit because black passivation alone does not guarantee good corrosion resistance. The organo-mineral finish can be replaced by an additional purely mineral finish based on chromium phosphate, also called post-dip.
Applications
Chromating or passivating is usually done in a dipping process, integrated in a galvanizing line.
In the dipping process, the parts undergo wet treatments:
At the fastener, the parts suspended on a fixture undergo all the galvanizing treatments from preparation to drying before unloading. A FOM or Organic-Mineral Finish of a film-forming nature with resins or varnishes can be applied before drying for better protection and resistance to friction.
In bulk, the zinc-plated parts in barrels for electrolysis are transferred into centrifuge baskets, usually made of steel. These baskets are used for passivation and immersion in the hardened finish. Variable speed centrifuges can be tilted for better part rotation and uniform processing. FOMs are tailored with less film-forming material and a viscosity adjusted to the centrifuging conditions. The CrVI or CrIII based conversion must be protected from shocks by a particularly slow rotation thus avoiding mechanical shocks that can cause damage to the conversion layers.
Implementation
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