REPLACING Hexavalent Chromium - plating

Automotive Finishing, Spring, 2001 by Paul C. Wynn, Craig V. Bishop

This is a Follow-up article to "Alternatives to Hexavalent Chromium and Chromium Plating"

that appeared in the Winter 2000 issue of AUTOMOTIVE FINISHING.

Passivation processes based upon hexavalent chromium are under review due to pending legislation changes. We examined the alternatives available that are most likely to succeed. Chromate passivation systems containing hexavalent chromium compounds are extensively used in electroplating and metal treatment processing. They impart many beneficial and essential characteristics to metallic substrates and to deposits obtained from a number of techniques such as mechanical and electrodeposition.

Hexavalent-chromium-based passivations exhibit a number of desirable characteristics (see Fig.l). They will passivate the surface of zinc and zinc alloy electrodeposits with a thin film that provides end-user benefits such as color, abrasion resistance and increased corrosion protection. When damaged, these hexavalent chromium based systems possess a unique "self-healing" property. This means that soluble hexavalent chromium compounds contained within the passivation films will repassivate any exposed areas. For the applicator, hexavalent chromium passivations are easy to use and can be bulk applied often by a single-stage immersion and are inexpensive to make-up and operate.

The need to replace hexavalent chromium based chromates is gathering momentum. Hexavalent chromium salts are classified as hazardous substances (toxic, sensitizing and carcinogenic); they are environmentally and toxicologically hazardous. It would therefore be desirable to replace these products with a suitable "environmentally friendly" commercially acceptable alternative.

For some time it has been widely accepted that sufficient evidence exists for the carcinogenicity of hexavalent chromium compounds in humans and animals. Certain forms of hexavalent chromium have been found to cause increased respiratory cancer among workers. This has produced regulatory control concerning their use and disposal, ensuring that industry can legally continue to use the technology. This has produced a lack of desire within industry to adopt suitable alternative strategies.

It is interesting to note that not all forms of chromium present a danger to humans. According to some, there are forms of hexavalent chromium that may be non-carcinogenic, such as dichromates of sodium and potassium. Trivalent chromium [1] is an essential nutrient that helps the body use sugar, protein and fat. An intake of 50-200 [micro]g per day is recommended for adults; we should therefore appreciate that not all chromium is "bad" and guard against a witch hunt.

The European Union's "end-of-life" vehicle directive[2] aims to reduce waste disposal through waste prevention from vehicles and ensure where practicable the re-use, recycling and recovery of end-of-life vehicles and their components. The Directive requires introduction of a certificate of destruction for end-of-life vehicles, allowing authorities to control the destiny of end-of-life vehicles, while the car manufacturer meets all or a significant part of the take-back costs.

Provisions apply from

[right arrow] July 1, 2002 for market vehicles from that date, and

[right arrow] January 1, 2007 for vehicles before July 1, 2002

A prohibition on the use of heavy metals such as hexavalent chromium in materials and components of vehicles will exist from January 1, 2003.

From 2000, a concession of 2 g of hexavalent chromium per vehicle has been available, allowing the continued use of hexavalent chromium since suitable replacement technology is still being implemented. One European auto manufacturer conducted a detailed vehicle tear down and established that less than 1 g of hexavalent chromium was present on its vehicles. This is believed to be the first vehicle manufacturer to conduct a physical analysis to assess the current situation.

Biestek and Weber [3] identified the chromium content of chromate coatings, and through their work it becomes possible to calculate the maximum permissible amount of hexavalent chromium components. For instance, electroplated zinc and yellow chromate has a film weight of 12 mg/sq dm, of which 70% is identified as hexavalent chromium. This equates to 2 gm per vehicle per 73 sq ft (see Fig. 2).

Many technological advances are often driven forward by the legislative and competitive nature of the automotive industry, but there exists tremendous interest in this topic in other industries. Does this finally point to the end of the acceptable use of hexavalent chromium passivation?

The impact of this directive has galvanized the efforts of global automotive companies that supply into the European economy. They are responding to these concerns and pending changes in legislation by evaluating alternatives to hexavalent chromium based passivation systems. One North American based automotive company has stated that it wants to remove the issue of hexavalent chromium completely from its vehicles. This means that presently it plans to specify chromium-free-based passivations for new vehicle components starting in 2005. This however leaves the interim problem of hexavalent chromium replacement for 2003 compliance.