Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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like a sealer but reinforces the trivalent chromium–based conversion coating. A µ-phosphate-bridged chromium(III) complex structure, bearing a similar con- stitution as that of the passivate layer, has been proposed. In the course of the development of this additional step of substituting hexa- valent with trivalent chromium, several efforts were necessary to adjust the for- mulation to achieve both the requirements for decorative appearance as well as those of corrosion protection. The objective was to develop a post-treatment process that acts as a second conversion coating and, therefore, can also easily be applied in normal plating equipment. This was successfully achieved with an elab- orate new additive system. This system governs the deposition process in the background without sig- nificantly contributing to the layer's composition. The corrosion-protection properties of surfaces with Tridur Finish 300 applied are found to be excellent but slightly lower than those of surfaces treated with film-building, polymer-based sealers. The tribological properties of the Tridur Finish 300-treated surfaces were essentially the same as those from hexavalent chromates. Although devel- oped with black passivates in mind, the new Tridur Finish 300 final finish process can be applied to any trivalent chromium–based conversion coating in both rack and barrel applications. While satisfying the high decorative demands issued when switching to triva- lent conversion coatings, the new process achieves the corrosion-protection demands of the automotive industry, even with respect to non-sealed black pas- sivated surfaces. NOTES 1. Wilhelm, E.J., US Patent 2,035,380. 2. Johnson, D.M., US Patent 2,559,878. 3. Directive 2000/53/EC of the European Parliament and of the Council of 18th of Sept. 2000, on end-of-live-vehicles. 4. Directive 2002/95/EG of the European Parliament and of the Council of 27th of Jan. 2003. 5. Directive 2002/96/EC of the European Parliament and of the Council of the 27th of Jan. 2003, on waste electrical and electronic equipment. 6. Lukaszewski, G.M., Redfern, J.P. Nature 1961;190:805–6. 7. Bard, A.J., Frankel, M, Stratmann, M. Encyclopedia of Electrochemistry. Vol. 4. Weinheim: Wiley-VCH, 2003. 8. Jelinek, T.W. Galvanisches Verzinken. Saulgau: Eugen G. Leuze Verlag, 1982. 9. Sonntag, B, Vogel, R. Galvanotechnik 2003;10:2408–13. 10. Redfern, J.P., Salmon, J.E. J Chem Soc 1961;291. 11. Springborg, J. Acta Chem Scand1992;46:906–8. 12. Haromy, T.P., Linck, C.F., Cleland, W.W., Sundaralingam, M. Acta Cryst 1990;C46:951–7. 298

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