Metal Finishing Guide Book


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Table III. Bath Parameters for an Acid Zinc-Cobalt Bath Parameters Rack Barrel Zinc metal 30 g/L 30 g/L Potassium chloride 180 g/L 225 g/L Ammonium chloride 45 g/L — Cobalt (as metal) 1.9-3.8 g/L 1.9-3.8 g/L Boric acid 15-25 g/L 15-25 g/L pH 5.0-6.0 5.0-6.0 Temperature 21-38°C 21-38°C 2 Cathode current density 0.1-5.0 A/dm 1-50 A/dm2 Anodes Pure zinc Pure zinc noncyanide zinc processes (Table II). 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 electrolytes compensate for this lower efficiency by plating at a slightly warmer temperature. 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 makers 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 topcoats, 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 continue to deliver good corrosion properties after heat treating has, in some cases, allowed parts to be baked after the application of a trivalent conversion coating, rather than before chromatFigure 2: Corrosion performance test (NSS) with bending. ing, eliminating 319

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