Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Table IX. Stannate Baths Composition and Operating Conditions Rack (g/L) Sodium Stannate Bath: Sodium stannate Tin Free sodium hydroxide Cathode current density, A/ft2 Anode current density, A/ft2 Voltage Temperature, ° F (° C) Potassium Stannate Bath: Potassium stannate Tin Free potassium hydroxide Cathode current density, A/ft2 Anode current density, A/ft2 Voltage Temperature, ° F (° C) Sodium and Potassium Stannate Baths: Anode to cathode ratio Filtration Agitation Pumps and tanks Heaters and coolers reasons: 1. Higher plating rates. 2. Greater conductivity, especially in barrel plating. 3. Less potential for sludge formation. The plater is cautioned to choose one or the other bath, and not to mix them. Potassium hydroxide should be used with the potassium stannate bath and sodium hydroxide with the sodium stannate bath. Although the "mixed" bath is operable and satisfactory results can be obtained, analytical control is quite difficult. Anodes in Stannate Baths Anodes used in stannate baths are made of pure tin, tin alloyed with 1% aluminum (known as "high-speed" anodes), or inert materials such as steel, nickel, or stain- less steel. Advantages of the high-speed anode are its greater current density range and its consistent efficiency at higher current density. The formation and maintenance of the anodic film is critical to the operation of the stannate bath. The primary cause of poor tin deposits is the improper film- ing of the anodes. To film tin anodes, a "surge" of current, at higher than normal current density, must be impressed on them for a few seconds to a minute, after which the current is reduced to its regular value (see Table X). The anodes must be refilmed after each shut-down period prior to plating, as the film dissolves rather Table X. Stannate Anode Film Characteristics Pure Tin Color of film formed Anode current density, A/ft2 Yellowish 15-40 Tin + 1% Al (High Speed) Olive green 30-80 Inert Anode No film No limits 253 90 40 12 15-20 25 3-4 170-180 (77-82) 100 40 15 30-100 30-40 4-8 150-180 (66-82) 1:1 25 µm Dynel or polypropylene cartridges Cathode and solution Stainless steel, low-carbon steel, polypropylene, polyethylene Low-carbon steel, polypropylene, polyethylene Potassium stannate is generally preferred over sodium stannate for the following Barrel (g/L) 180 80 23 5-15 15-25 3-4 165-175 (74-79) 200 80 23 1-100 10-30 4-14 150-180 (66-82)

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