Replacing Hexavalent Chromium in Passivations on Zinc Plated Parts

Chromate passivation systems contain- ing hexavalent chromium compounds are an extremely versatile group of aqueous chemistries that are extensively used in a diverse range of electroplating and metal treatment processes. They impart many beneficial and essential characteristics to metallic substrates and to deposits obtained from a number of techniques such as zinc electroplating.

Hexavalent based passivations (CrVI) exhibit a number of desirable characteristics (see Table I). 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 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.

The initial stage in the formation of a chromate is dissolution. Acidic attack from the aqueous solution results in a pH rise at the zinc surface, releasing both zinc and hydrogen. Following this is the reduction of hexavalent chromium to the trivalent state and the subsequent precipitation into the Cr-O-Cr olation polymer¹. (Hexavalent chromium passivations are based on a thick, gelatinous trivalent chromium layer known as the olation polymer.) Final adsorption into a topcoat of hexavalent chromate then takes place (see Table II). Anions (chromatic acid anions) in the olation polymer control color, thickness and the "self healing" ability of the film.

For the applicator, hexavalent chromium based passivations are a well-established, user friendly technology, offering an attractive cost-to-benefit ratio. They can be readily bulk applied, often through single-stage immersion, and are generally inexpensive to make-up and operate. Production control requirements are low, and they are easy to waste treat.

However, the need to replace chromates based upon hexavalent chromium is gathering momentum. Hexavalent chromium salts are classified as hazardous substances (toxic, sensitizing and carcinogenic); they are environmentally and toxicologically hazardous. Therefore, it would be beneficial to replace these products with "environmentally friendly," commercially acceptable alternatives.

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 do cause increased respiratory cancer among workers. This has produced regulatory control governing their use and disposal, thus enabling industry to continue the legal use of the technology and consequently limiting its desire to adopt suitable alternative strategies.

Not all forms of chromium present a danger to humans. According to some, there are forms of chromium that may be non-carcinogenic, such as dichromates of sodium and potassium. Trivalent chromium² is an essential nutrient that helps the body use sugar, protein and fat. An intake of 50 to 200 µg per day is recommended for adults; we should therefore appreciate that not all chromium is "bad" and guard against an ecological witch hunt.

The Automotive Market

The European Union's "end-of-life" vehicle directive³ 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
• July 1, 2002, for market vehicles from that date, and
• 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, according to the document.

Contained in the directive is an assigned concession of 2 gms of hexavalent chromium per vehicle, allowing the continued use of hexavalent in the interim since it is recognized that suitable replacement technology is still being developed and implemented.

One European sited auto manufacturer conducted a detailed vehicle tear down and established that less than 1 gm 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. Its results would imply that present automotive industry arrangements will meet the figure.

Messer's Biestek and Weber⁴ 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 with yellow hexavalent chromate has a film weight of 12 mg/dm2, of which 70% is identified as hexavalent chromate. This equates 2 gms per every 73 sq ft of vehicle (see Table III).

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 in acceptable use of hexavalent chromium passivation?

The impact of this directive has galvanized the efforts of global automotive companies that supply 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 hexavalent chromium completely from its vehicles. This means it plans to specify chromium-free passivations for new vehicle components from 2005. This, however, leaves the interim problem of hexavalent chromium replacement for 2003 compliance.

Alternatives

A group of commercially acceptable alternatives to hexavalent chromium products was patented in the 1990s⁵, but until recently the finishing industry showed little interest in them. Many strategies to replace hexavalent chromium have been proposed (see Table IV), 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 Table V).

Clear, blue passivation films on electroplated zinc deposits have been successfully obtained from products based upon trivalent chromium compounds since their commercial adoption in the late 1970s⁶. 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 zinc plating specification requirements in favor of this more environmentally aware technology. Until recently, only a small number of companies had expressed the desire to extend the replacement of hexavalent chromium to other passivation films such as iridescent colors, examples include the GM specification GME00252⁷. Many have imposed limits on the content of free hexavalent chromium contained within the conversion coating. Volvo specifies free hexavalent chromium must not exceed 0.3 µg/cm2 after testing⁸.

The adoption of trivalent chromium systems would therefore seem a logical step in the development of a suitable hexavalent chromium replacement strategy. Iridescent colors can be achieved from trivalent compounds, but these are visually different from those obtained from hexavalent chromium (see Table VI). The pressing need to adopt alternative technologies means that the attitude of end-users towards color shade is becoming less of an issue. This greatly assists the technology developer and finisher to justify commercialization of these products.

There exists some issues with trivalent chromium on zinc compared to hexavalent: Color is not identical; corrosion resistance is reduced; and there are no "self healing" benefits due to the absence of soluble hexavalent chromium compounds in the olation polymer⁹.

For the finisher, the newest products on 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.

Scanning Electron Microscopy (SEC) evaluation of an iridescent (yellow) trivalent chromium passivation build onto an electrodeposit of zinc nickel 12-15% alloy reveals two interesting factors. First, the familiar crack pattern for surface topography of hexavalent chromium is similar with the trivalent chromium alternative (see Table VII). Second, cross-sectioning using cryo-fracturing indicates a good build thickness for the passivation film (see Table VIII).

Corrosion test numbers are significantly better for trivalent chromium over zinc alloy (see Table IX) than for hexavalent chromium over zinc⁹. Electrodeposits of zinc nickel 12-15% alloy 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.

Corrosion Performance

The corrosion performance of chromate passivation films is frequently evaluated by the 5% neutral salt spray test. Although this test is recognized as having limitations, it is to date the most accepted performance test method. 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 hr) prior to salt spray. However, ASTM¹⁰ 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.

Non-Chromium Passivations

Totally nonchromium passivations⁹ have been under development for some time, and there exists a number of proposed alternatives. Organic films, inorganic salts, oxides and organometallics have all been proposed as suitable materials. All have problems when used as individual steps, so their individual adoption seems unlikely. The best chance seems to be a combination of these in a multiple step process.

After zinc or zinc alloy deposition, a suggested nonchromium process route using existing technologies would be a multi-step system that would initially require the application of a color, such as that achieved from a nonchromium inorganic salt. A bonding layer to provide adhesion would then be required, an organometallic coupling agent could be used for this. The film is then dried prior to application of a nonchromium containing topcoat such as an acrylate or silicate, which would be the corrosion inhibiting layer. A final stage would be the second drying operation. This suggested process sequence would be characterized as the wet-dry-wet-dry method.

Presently, when applied over a zinc alloy, this represents the most suitable nonchromium process available. However, even over zinc nickel 12-15% alloy, it is not possible to offer the same level of white corrosion performance compared to that achieved from zinc with hexavalent chromium (see Table X).

There is a growing realization that trivalent chromium as an intermediate stage is the most commercially acceptable alternative to hexavalent chromium. Its adoption into industry offers the best available technology not requiring excessive cost. When applied over a zinc alloy it can achieve suitable or superior corrosion protection against zinc with hexavalent passivation. Trivalent chromium systems over zinc alloys are the easiest to apply and the most enduring. Non-chromium would be an ideal goal, but presently compromises must be made for acceptance of this technology. The most likely to succeed will be the multiple step wet-dry-wet-dry method. Adoption of current totally non-chromium alternatives would seem to be nonviable in the current "cost down," commercially sensitive automotive and finishing environments.