Metal Finishing Guide Book

2013

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of the new chromium(III)-based post-dip generation could be achieved, circumnavigating the bluffs of trivalent chromium. Atotech has put quite a bit of effort into solving this problem. The outcome is Tridur Finish 300, a trivalent chromium–based post-dip solution, dedicated to passivated zinc, zinc–iron, and zinc–nickel alloys. FORMATION MECHANISM, STRUCTURE, AND PROPERTIES OF CONVERSION LAYERS Chromate conversion coatings. Chromate conversion coatings (CCCs) in general develop on zinc surfaces in acidic chromium(VI)-containing solutions by reduction of chromium(VI) to chromium(III). In the course of this reaction, oxonium ions are consumed: Cr2O72– + 3Zn + 14H3O+ ␱ 2Cr3+ + 21H2O + 3Zn2+ 2CrO42– + 3Zn + 16H3O+ ␱ 2Cr3+ + 24H2O + 3Zn2+ Due to the consumption of oxonium ions, a pH-gradient toward more alkaline pH next to the zinciferous surface develops. In this zone of elevated pH the generated chromium(III) compounds hydrolyze, thereby generating the respective μ-oxo-bridged and μ-hydroxo-bridged polynuclear chromium(III) complexes.6 Chromium(VI) compounds from the solution are adsorbed on the surface of these polynuclear chromium-complex layers (Fig. 2) making up the chromate conversion coating.7 Investigation of the surface of a standard hexavalent chromate on zinc by means of SEM shows the typical fissured, "dry river bed-like" surface of the conversion layer (Fig. 3). Black chromate conversion coatings can be categorized into zinc and zinc alloy processes. Black CCCs on non-alloy zinc use silver evenly spread in the conversion layer as a black pigment. Yellow chromates on zinc usually show white corrosion products after 240– 500 h in neutral salt spray testing according to ISO 9227.8 The corrosion protection of black chromates is found to be noticeably reduced compared to that of yellow chromates with a similar conversion layer thickness. This decreased corrosion resistance can be attributed to silver particles being abundantly present in the chromate layer and also in contact with the zinc surface. Consequently, the zinc in contact with the noble metal is forced to corrode by means of contact corrosion. The other category of black chromates includes those being applied on zinc alloys like zinc–nickel, zinc–cobalt, or zinc–iron. The black pigment in the chromate layers on these alloys consists of iron, nickel, or cobalt, and their respective oxides, produced on dissolution of some zinc in the acidic process solution. Although these metals are nobler than zinc, the overall corrosion protection of the black CCCs achieved on these base metals is much better. Chromium(VI)-based post-dip solutions were commonly used on such chromates, giving about 240–360 h to white corrosion in neutral salt spray testing according to ISO 9227. Passivate conversion coatings. Layer growth in hexavalent CCCs depends on the oxidizing effect of chromates on zinc, consuming acidity and increasing the pH on the zinc/solution interface. In trivalent passivates, an alternative oxidant taking the role of the chromate is needed. The proper choice of oxidizing agents is crucial for the passivate's 332

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