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Guest column: Dealing with hexavalent chromium compounds in electroplating
In electroplating, hexavalent chromium compounds form the basis for various processes. These processes are functional chrome plating (hard chrome plating), chrome plating for decorative applications with functional aspects (bright chrome plating or decorative chrome plating), etching in the pretreatment of plastics and chromate conversion coating on zinc layers or aluminium. In chrome plating and plastic etching, the final product leaving the electroplating process does not contain any hexavalent chromium compounds. Hexavalent chromium is only contained in the layers of conventional chromate conversion coating. In the other processes, chromium layers are reduced to metal. Metallic chromium (Cr 0) is considered non-hazardous.
Dr Martin Metzner.
The plating industry, the chemical process industry and research institutes have been looking for alternatives for more than 15 years to the current methods used in hard chrome and bright chrome plating. They are looking for alternatives to both the chromium layer itself and to replace hexavalent by trivalent chromium electrolytes.
The search for alternatives was triggered already long before REACH, because the oxidative effect of hexavalent electrolytes may harm the material of the plating plant. In addition, the legal requirements for the treatment of wastewater, rinsing processes and exhaust air are far stricter than for other electrolytes.
However, hexavalent electrolytes are still widely used in the coating industry because industry demands properties that cannot be met by other substances.
The reason why hard chrome plating is used is not only that it produces a hard layer. Studies carried out by industry and research institutions show that the required properties are not only hardness but also sandability, anti-adhesive behavior, corrosion resistance, reflectivity, brightness, suitability for food applications and chemical resistance. The industry demands property profiles that differ for each application. Similarly, in the field of decorative or bright chrome plating, brightness is required, as well as abrasion resistance, suitability for food applications, chemical resistance and colour stability during use.
Industry has devoted a lot of time and effort to finding alternative methods, for example, in the fields of nickel plating, alloy plating, hardening, thermal spraying, deposition from trivalent chromium electrolytes, lacquer coating and ceramic layers. The conclusion is that there is currently no marketable alternative available to most of the applications of hexavalent chrome plating. The required property profiles have not been achieved by other methods. This is particularly evident in sanitary fittings, where other surfaces such as lacquer-coated plating, PVD plating and stainless steel look are also used. However, 99% of sanitary fittings sold are chromium-plated - because the market demands it.
It looks as we will not be able to do without hexavalent electrolytes in the future. Decades of dealing with these substances, however, have led to state-of-the-art plant technology that is continuously improved. From the perspective of plant engineering, rinsing technology, handling, recirculation and clean production, this technology allows workers and the environment to be protected from hexavalent chromium compounds so that these remain where they belong - in the electroplating plant.
However, research will continue to try to find safer alternatives in the longer term which would be suitable for applications where hexavalent chromium is involved. However, there will still be applications where the property profiles can only be achieved by plating with hexavalent electrolytes. Aspects of material science and plating mechanisms will build up the borderlines for the alternatives.
Dr Martin Metzner
Head of department for electroplating at the Fraunhofer Institute for Manufacturing Engineering and Automation in Stuttgart
Fraunhofer, founded in 1949, is Europe's largest application-oriented research organisation. Its research drives economic development and is geared to people's needs: health, security, communication, energy and the environment. The Fraunhofer-Gesellschaft maintains 66 institutes and independent research units, and employs more than 22 000 staff.
Chromium compounds – what are they?
Chromium occurs in the environment primarily in two main states: trivalent chromium (Cr III) and hexavalent chromium (Cr VI). Exposure to humans and the environment may occur from natural or industrial sources of chromium. Chromium III is considered to be much less toxic than chromium VI, and is an essential element for humans. For example, the German Nutrition Society recommends a daily intake of chromium of 30-100 μg for adults. The human body can detoxify some amounts of chromium VI to chromium III.
Chromium compounds, in either the chromium III or chromium VI forms, are used for chrome plating, the manufacture of dyes and pigments, leather and wood preservation and the treatment of cooling tower water. Smaller amounts are used in drilling muds, textiles and in toners for copying machines.
Hexavalent chromium is known to cause cancer. It targets the respiratory system, kidneys, liver, skin and eyes.
One of the most commonly used form of hexavalent chromium is chromium trioxide. Chromium trioxide has various toxicological properties, such as acute and chronic toxicity, corrosivity as well as skin and respiratory sensitisation. It is also a reproductive toxicant, a germ cell toxicant and in particular, a carcinogenic substance.
Chromium trioxide is mainly used in metal finishing/surface treatment and as catalysts. Low volumes are also used by professionals as a laboratory agent.
Chromium III, although much less toxic than chromium VI, is shown to have moderate toxicity from oral exposure in acute animal tests.
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