Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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some black passivates. Tridur Finish 300 can be applied in both rack and barrel applications. The bath parameters are the same for both methods and depend only on the composition of the underlying conversion coating. Tridur Finish 300 is no substitute for sealers in general. Usually the corrosion protection that can be expected from a chromium-based post-dip can be classi- fied as slightly below that of a film-building sealer based on polymer dispersions or solutions (e.g., Corrosil Plus 501). The corrosion behavior was analyzed by recording polarization curves ±50 mV around the open circuit potential in a three-electrode set-up, including a platinum counter electrode, a Ag/AgCl (3M KCl) reference electrode, and the sample as the working electrode. The samples were immersed in aerated solutions of 50 g/l sodi- um chloride adjusted to pH 7. After 3 min of equilibrium time the open circuit potentials (ocp) were measured and the sample was then polarized from –50 to 50 mV vs. ocp at a sweep rate of 5 mV/s. The data were then plotted on a graph (Fig. 12). The results of the Tafel analysis of the data are summarized in Table 2. The registered corrosion currents correlate with corrosion rates. The surface with only the passivate and no post-treatment applied showed the highest cor- rosion rates. Reduced corrosion rates were observed on the surface with the post-dip applied to the black passivate, and even lower corrosion rates were found with the surface having the polymer-based sealer applied. Also, the sealed surface behaves in an electrochemical manner that is slightly nobler than the post- dipped surface, which itself appears nobler than the passivate surface. This principal sequence in corrosion protection is confirmed by neutral salt spray testing on samples with Tridur ZnNi H1 with Tridur Finish 300 accord- ing to DIN EN ISO 9227 (Table 3). Fig. 17 shows three steel panels plated with a Zn/Ni-alloy (14% Ni, 8 µm), black passivated with Tridur ZnNi H1 and with Tridur Finish 300 applied as the final finish after 1,008 h in neutral salt spray test- ing (DIN EN ISO 9227). Only a small amount of non-voluminous zinc corrosion product formed on the rinsed and dried panels. Torque and tension properties. The friction properties of the new surface were evaluated on M10×50 (thread pitch 1.50) hex head bolts of property class 10.9. The bolts were plated with 8–10 µm of zinc (Protolux 3000) as well as with zinc–nickel (14% Ni, Reflectalloy ZNA) and respectively passivated with a black zinc (Tridur Zn H1) or black zinc–nickel passivate (Tridur ZnNi H1). Tridur Finish 300 (10% v/v for zinc and 20% v/v for Zn/Ni) was applied as the final fin- ish after the passivate treatments. Twelve samples (Zn), respectively 20 bolts (Zn/Ni), were tested on a Schatz Analyse 5413-4504 testing machine at a tight- ening speed of 30 min–1 according to DIN EN ISO 16047. The results are sum- marized in Table 4. Higher friction figures have been determined for the zinc–nick- el surface compared with the zinc surface. With both surfaces the friction behavior is essentially the same as that found with hexavalent chromium–based conversion coatings (e.g., black or yellow chromates) without any sealer or lubri- cant applied. CONCLUSIONS The development of a trivalent chromium–based post-dip solution has been demonstrated and its properties investigated. The post-dip solution does not act 297

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