Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 263 of 707

Table I. Bath Parameters for an Acid Zinc-Nickel Bath Parameters Rack Zinc chloride Nickel chloride Potassium chloride Ammonium chloride pH Temperature Cathode current density Anodes 130 g/L 130 g/L 230 g/L — 5.0-6.0 24-30° C 0.1-4.0 A/dm2 Barrel 120 g/L 110 g/L — 150 g/L 5.0-6.0 35-40° C 0.5-3.0 A/dm2 Zinc and nickel separately. In some cases, separate rectifiers and bussing are required. Table II. Bath Parameters for an Alkaline Zinc-Nickel Bath Parameters Zinc metal Nickel metal Sodium hydroxide Zinc/Nickel ratio Temperature Cathode current density Anode current density Anodes High & Low Nickel 8.0 g/L 1.6 g/L 130 g/L 5.0:1 23-26° C 2-10 A/dm2 5-7 A/dm2 Nickel-plated (25 microns) on steel anodes • Adherent conversion coating • Low dissolution rate of passivate film in neutral salt spray (NSS) testing Corrosion of Steel Plated with Zinc and Zinc Alloy Substrates are protected through the electrodeposition of zinc and zinc alloys. Due to their poor ionization tendency, zinc and zinc alloys sacrificially dissolve prior to the substrate. Corrosion by-products provide a very stable barrier film to pro- tect the electrodeposited coating from the corrosive environment. Zinc-nickel can be plated from acid or alkaline (cyanide free) solutions. The acid bath typically provides a nickel content of 10% to 14% as compared to alkaline for- mulations that will yield 5% to 8% nickel or 10% to 17% nickel. Corrosion protection increases with increasing nickel content approaching 17%. Beyond that level, the zinc-nickel deposit becomes more noble than the substrate, thus losing its corrosion protection properties (see Table I). Additionally, at a nickel content above 10%, the deposit has only one crystal structure, phase. The deposit from the acidic solu- tion tends to have less uniform thickness distribution and a higher alloy com- position variation from high to low current density areas than deposits from an alkaline electrolyte. The alkaline bath produces a columnar structure with a lower tensile stress as compared to the laminar structure as deposited from the acidic electrolyte. Thus, the alkaline system maintains better deposit integrity when the part is formed, bent, or crimped after plating. The alkaline high zinc nickel process is non-embrittling to high-strength steels and can meet the requirements for a non-embrittling process per ASTM F 519 as a suitable replacement for cadmium.This bath is very simple to operate, being quite similar to conventional alkaline noncyanide zinc processes (Table II). 262

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