REPLACING Hexavalent Chromium - plating

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

Totally non-chromium passivations have been under development for some time, and there exists a number of 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 with the use of a combination of these in a multiple step process.

After zinc or zinc alloy deposition, the multi step process requires the application of a color, such as achieved from a non-chromium inorganic salt. A bonding layer to provide adhesion is required; an organometallic coupling agent could be used. The film is then dried prior to application of a corrosion inhibiting topcoat such as an acrylate or silicate, followed by a second dry operation (the wet-dry wet-dry method). At present, when applied over a zinc alloy this represents the most suitable non-chromium process. However, even over high alloy zinc nickel, it is not possible to offer the same level of white corrosion performance compared to that achieved from zinc and hexavalent chromium.

There is a growing acceptance that industry will adopt trivalent chromium as an intermediate solution to the replacement of hexavalent chromium. It offers the best available technology not requiring excessive cost (BATNEC), and when applied over a zinc alloy can achieve suitable or superior corrosion protection against zinc with hexavalent chromium passivation: Trivalent chromium systems over zinc alloy are the easiest to apply and the most enduring. Non-chromium is the ultimate goal, but presently compromises must be made for acceptance of this technology. The other most likely to succeed will be the multiple step wet-dry-wet-dry method.

References:

(1.) U.S. Department of Commerce, Agency of Toxic Substances and Disease Registry (Atlanta) PB93-182434 app. 230 pages.

(2.) European Union end-of-life vehicle directive 23 May 2000 8828/00 (Presse 179): Further details can be obtained from the Council site at http://ue.eu.int Newsroom.

(3.) T. Biestek and J. Weber, "Electrolytic and Chemical Conversion Coatings", Portcullis Press, UK (1976).

(4.) 1995 Bishop et al. 5,393,353, 5,393,354, 5,407,749, 5,415,702.

(5.) 5,7 C.V. Bishop & J.R. Kochilla Non-Hexavalent Chrome Passivation Technologies for Zinc and Zinc Alloys; AESF Sur/Fin 2000, June 2000.

(6.) American Society for Testing and Materials.

* Lubricious

* Easily applied

* Not flammable

* Stable for weeks or months

* Resilient (repairs itself)

* Re-hydrated after baking

* Coats in recesses

* Inexpensive equipment

* Prevents oxide formation

* Slow corrosion in prototypic tests (e.g. salt

spray roof top, etc.)

* Provides adhesion for organics (e.g. paint)

* Helps prevent corrosion of painted surfaces (e.g. creep) * Provides adhesion for organics (e.g. paint)

* Inexpensive

* Thin

* Provides color

* Conductive

* Flexible

* Easy to strip

* Single tank

* Durable

           mg/         Cr   Cr(III)  mg Cr(VI)/  [dm.sup.2]/  [ft.sup.2]
           [dm.sup.2]                [dm.sup.2]           2g      2g
Cd clear   1           30%  90%      0.27               7407     797
Zn Clear   1           30%  90%      0.27               7407     797
Al Clear   3           30%  80%      0.72               2778     299
Al Yellow  10          35%  70%      2.45                816      88
Zn Yellow  12          35%  70%      2.94                680      73
Cd Yellow  15          38%  70%      3.99                501      54
Mg Yellow  15          38%  70%      3.99                501      54
Zn Olive   23          42%  70%      6.752               296      32
Cd Olive   25          42%  70%      6.752               296      32