Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 434 of 707

Fig. 5. Scanning electron micrographs of Cr(6) at left and Cr(3) at right. Passivation films after heat treatment at 200O C for 1 hr at 1,000[times]. Heat Resistance An advantage of trivalent chrome passivation is its superior resistance to high tem- peratures. Unlike hexavalent chrome passivation films, they can be heated to 200O original resistance. Hexavalent chromate films dehydrate and fail entirely when heat- ed above 55O C or more for extended periods of time and still maintain up to 70% of their C for more than a few minutes. Figs. 4 and 5 show the surface analy- sis of both types of conversion coatings before and after heat treatment. After the coatings have formed and dried, hexavalent chromate films show a pattern of cracks or fissures as a result of partial dehydration of the adsorbed Cr(6) content. Upon heat treating, total dehydration takes place and Cr6+ is reduced to Cr3+ , widening and deepening the cracks, exposing zinc, and resulting in premature cor- rosion failure. By contrast, the trivalent chrome passivation film, consisting of the more stable oxidation state Cr(3) compounds, is more homogeneous and crack free. It remains unchanged after heat treatment. This property is used to great advan- tage when zinc plated parts must be heat treated for hydrogen embrittlement relief. This is done typically without passivation, which would otherwise be destroyed. The need to replate with a thin layer of zinc after baking to apply an adherent conversion coating is eliminated. Parts passivated with trivalent chrome can be heat treated with no change in appearance and minimum loss of corrosion protection. The choice of sealers and topcoats must be carefully considered if parts are to be heat treated as some types of sealers could reduce this advantage either by corrosive chemical attack or by dehydrating and inducing cracking in the underlying passivation film. Appearance Trivalent chrome passivation produces a range of colored films. Thin layers are typically iridescent blue, while thicker coatings are pale green to yellow blue depending on whether the zinc is alloyed and the specific alloying element. Since hexavalent chrome is the source of yellow color in conventional conversion coatings, this color is not usually available in Cr(6)-free coatings unless induced by dyes or other metals and their oxides. The use of transparent sealers and topcoats can modify the appearance of the coating producing silver-white or pale- colored films free of iridescence. Black coatings may be obtained with specially modified trivalent chrome passivating solutions containing metals, such as cobalt or iron, but are difficult to control. Deep uniform black finishes are best produced on zinc-iron alloys. Other alternatives for black finishes over trivalent 433

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