REPLACING Hexavalent Chromium - plating

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

Alternatives

A group of commercially acceptable alternatives to hexavalent chromium products were patented in the 1990s [4], but until recently the finishing industry showed little interest in these. Many strategies to replace hexavalent chromium have been proposed (see Fig. 3), but today's interest is now focused upon passivation films obtained from trivalent chromium compounds. In most respects, trivalent chromium closely resembles the characteristics of hexavalent chromium and is a suitable alternative (see Fig. 4).

Many will be familiar with the clear, blue passivation films on electroplated zinc, which have been successfully obtained from products based upon trivalent chromium compounds since their commercial adoption in the 1970's. These have developed in reliability and performance in recent times, having found particular favor with alkaline non-cyanide zinc users. Their increased material and process control cost is balanced by their longevity and color consistency. Many end users have changed their specification requirements in favor of this.

The adoption of trivalent chromium systems would therefore be a logical step in the development of replacement technology. Iridescent colors can be achieved, but these are different from those obtained from hexavalent chromium. The changing attitude of end-users means that color shade is becoming less of an issue, which helps finishers justify installation of these products.

There exist some issues with trivalent chromium on zinc compared to hexavalent chromium. For the end-user, color is not identical to hexavalent chromium; corrosion resistance is reduced; and there is no "self-healing" benefit. For the finisher, the current crop of products in the market need to be operated at elevated application temperatures (30 to 60C), although they remain relatively easy to operate. Research and practical experience indicate that the most suitable medium for use of this technology will be zinc alloy deposits.

Corrosion test numbers are significantly better for trivalent chromium over zinc alloy than for hexavalent chromium over zinc[5]. High alloy zinc nickel deposits represent the highest level of performance even after deposit post forming and heating. This means that current end-user specification requirements can still be achieved with this technology change. In fact, both white/zinc and red/ferrous protection is improved.

The corrosion performance of chromate passivation films is frequently evaluated by the 5% neutral salt spray test. Although it is recognized as having limitations, it is to date the most accepted test method for performance testing. Many companies and organizations have issued their own specifications controlling test conditions, the most obvious change being the adoption of a heat test (typically 120C for 24 hours) prior to salt spray. However, ASTM [6] B117 remains globally the most accepted and recognized test standard. Despite the introduction of other performance tests, such as the Cyclic Corrosion Test and the Kesternich test DIN 50018, this neutral salt spray standard remains the universally accepted method for defining performance variations between coating systems.