Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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navigating 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 reduc- tion of chromium(VI) to chromium(III). In the course of this reaction, oxonium ions are consumed: Cr2 O7 2CrO4 2– + 3Zn + 14H3 o2Cr3+ 2– + 3Zn + 16H3 o2Cr3+ O+ O+ + 21H2 + 24H2 O + 3Zn2+ O + 3Zn2+ Due to the consumption of oxonium ions, a pH-gradient toward more alka- line pH next to the zinciferous surface develops. In this zone of elevated pH the generated chromium(III) compounds hydrolyze, thereby generating the respec- tive µ-oxo-bridged and µ-hydroxo-bridged polynuclear chromium(III) com- plexes.6 Chromium(VI) compounds from the solution are adsorbed on the sur- face 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 resis- tance can be attributed to silver particles being abundantly present in the chro- mate layer and also in contact with the zinc surface. Consequently, the zinc in con- tact 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 chro- mate 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 per- formance. A common, simple example being part of oxidant mixtures in passi- vates is nitrate. Zinc reduces nitrate according to the formulas given (Dikinis V, 290

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