Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 266 of 707

Figure 3: Corrosion resistance of tin-zinc alloys in salt spray in accordance with ASTM B117. Adhesion of the conversion coating film onto zinc alloy electrodeposits is superior to zinc plating. This comes from an "anchor function" of the second met- al. Tin, nickel, iron, and cobalt do not dissolve in the passivating solution. Another application where zinc-nickel was found to offer excellent protec- tion in combination with a topcoat is for the plating of fasteners that are to be used in contact with aluminum, safely replacing cadmium electrodeposits. ZINC-COBALT Commercial zinc-cobalt baths are essentially conventional low ammonium or ammonium-free acid chloride zinc baths, with the addition of a small amount of cobalt. The resulting deposit is generally up to about 1% cobalt, with the balance being zinc. This bath has a high cathode efficiency and high plating speed, with reduced hydrogen embrittlement compared with alkaline systems, but the thick- ness distribution of the deposit varies substantially with the current density. An alkaline bath comparison is provided (see Tables III and IV for acid and alkaline bath parameters). Acid cobalt baths have many variables that can affect the cobalt codeposition percentage. These variables include cobalt concentration, zinc concentration, temperature, agitation, pH, current density, and chloride concentration. Zinc-cobalt deposits will accept trivalent and hexavalent blue bright, yellow iridescent, and nonsilver black chromate conversion coatings. Higher corrosion per- formance with trivalent passivates is not achievable on zinc-cobalt electrodeposits. ZINC-IRON The primary advantages of zinc-iron are low cost and the ability to develop a deep uniform black conversion coating from a nonsilver passivate. Additionally, the alloy has good welding characteristics and workability, and can readily be used on 265

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