Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 264 of 707

Table III. Bath Parameters for an Acid Zinc-Cobalt Bath Parameters Rack Zinc metal Potassium chloride Ammonium chloride Cobalt (as metal) Boric acid pH Temperature Cathode current density Anodes 30 g/L 180 g/L 45 g/L 1.9-3.8 g/L 15-25 g/L 5.0-6.0 21-38° C 0.1-5.0 A/dm2 Pure zinc Barrel 30 g/L 225 g/L — 1.9-3.8 g/L 15-25 g/L 5.0-6.0 21-38° C 1-50 A/dm2 Pure zinc High nickel (10-17%) alloy baths are in use and are specified in the European automotive industry, using alkaline noncyanide technology. Typically, these baths have a lower cathode efficiency than the low nickel baths. Some alkaline elec- trolytes compensate for this lower efficiency by plating at a slightly warmer tem- perature. Higher nickel content in the alloy composition will cause increased passivity and reduced chromium conversion film receptivity. One main reason for the success of zinc-nickel alloy electrodeposits with the major automobile mak- ers is their requirement that neutral salt spray testing on plated parts be conducted after passivation and baking. Additionally, higher levels of nickel in the alloy may mean less ductility of the deposit; however, corrosion resistance may increase up to double that of the low nickel baths. Chromium passivation solutions for high zinc-nickel electrodeposits must be more aggressive in order to form a protective coating on the electrodeposit. Newly formulated passivates, with supplemental top- coats, have proven suitable on higher nickel content zinc-nickel electrodeposits, thus eliminating the need for hexavalent chromates. Zinc-nickel has consistently achieved higher corrosion protection results as shown by accelerated corrosion testing (Erichsen and neutral salt spray), with the exception of the SO2 (Kesternich) test, which favors tin-zinc (Fig. 2). Zinc-nickel at a thickness of 8 microns or less does, however, retain high corrosion resistance after the forming of parts, such as fuel lines, brake lines, hydraulic lines, and fasteners. The ability to contin- ue to deliver good corrosion properties after heat treating has, in some cases, allowed parts to be baked after the application of a trivalent conversion coat- ing, rather than before chromat- ing, eliminating the need for dou- ble handling. Figure 2: Corrosion performance test (NSS) with bending. 263

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