Silver plating [SubsTech]

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Symbol: Ag

Atomic number: 47

Atomic weight: 107.87

Classification: Metal

Crystal structure: Face Centered Cubic (FCC)

Characteristic properties of silver:

• Very high thermal conductivity (highest of all metals);

• Very high electrical conductivity (highest of all metals) and low contact electrical resistance;

• High ductility

• Good chemical resistance (silver is attacked only by nitric acid , hot sulfuric acid and ozone);

• Bright lustrous appearance.

The metals substrates which are plated by silver:

• Gold

• Copper
• Nickel
• Steels . Prior to silver plating the steel surface is coated by copper/copper-silver from copper/copper-silver strike solution.
• Aluminum . Prior to silver plating aluminum surface is zincated.

The most reliable and widely used process of silver deposition is cyanide silver plating.

Cyanide silver is the Electroplating process utilizing an electrolyte containing silver cyanide solution and some free cyanide and operating at PH value not less than 8.
Free cyanide prevents precipitation of silver cyanide salt from the solution, provides electrical conductivity of the electrolyte and helps dissolution of silver anodes.
Cyanide silver plating is used for decorative and engineering applications.
Common cyanide silver process includes two stages: silver strike coating followed by general silver plating.

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Silver strike is a very thin deposition of silver from an electrolyte containing low concentration of silver ions and high concentration of free cyanide as complexing agent.
Silver is a noble metal having high value of standard electrode potential. When a metal with lower electrode potential (less noble) is dipped into a silver cyanide solution the metal atoms are displaced by silver ions, which are deposited on the surface. This electroplating process is called immersion deposition. Immersion deposition is characterized by weak adhesion to the substrate.
Silver strike prevents immersion deposition of silver providing good adhesion of the coating to the workpiece surface. Silver strike coating has dull powdery appearance and low mechanical strength therefore it is used only as an adherent layer over-coated by bright silver.

Silver strike process

Bath compositions for silver strike

• Silver(Ag): 0.3 oz/gal (2.2 g/l)

• Potassium cyanide (KCN): 13 oz/gal (100 g/l)

• Water: reminder

• Silver cyanide (AgCN): 0.5 oz/gal (4 g/l)

• Potassium cyanide (KCN): 13 oz/gal (100 g/l)

• Water: reminder

• Silver cyanide (AgCN): 0.6 oz/gal (4.5 g/l)

• Sodium cyanide (NaCN): 10 oz/gal (75 g/l)

• Water: reminder

• Silver potassium cyanide (KAg(CN)2): 1 oz/gal (7.5 g/l)

• Potassium cyanide (KCN): 11 oz/gal (80 g/l)

• Water: reminder

Copper-silver strike for steel substrates

• Silver potassium cyanide (KAg(CN)2): 0.4 oz/gal (3 g/l)

• Copper cyanide: 1.25 oz/gal (10 g/l)

• Potassium cyanide (KCN): 11 oz/gal (80 g/l)

• Water: reminder

Silver strike for nickel/nickel coated substrates

• Silver cyanide (AgCN): 0.12 oz/gal (1 g/l)

• Sodium cyanide (NaCN): 6.7 oz/gal (50 g/l)

• Water: reminder

Operating conditions:

• Anodes: silver or stainless steel

• Cathode current density: 20-25 A/ft2 (2.2-2.7 A/dm2)

• Voltage: 6-8 V

• Temperature: room

• Striking time: 0.5-1 min, 5 min for barrel plating

• The plated parts should be electrically connected before entering the bath )prevent immersion deposition)

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Prior to silver plating the parts are treated in a silver strike solution.
The Grain structure and the level of brightness (bright, semi-bright, dull) of the cyanide platings is controlled by additives (brightening agents). Sulfur containing organic materials (products of reaction organic compounds with carbon disulfide, ammonium thiosulfate, sodium thiosulfate).

Cyanide silver bath compositions:

• Silver cyanide (AgCN): 4 oz/gal (30 g/l)

• Potassium cyanide (KCN): 6 oz/gal (45 g/l)

• Potassium carbonate (K2CO3): 4 oz/gal (30 g/l)

• Water: reminder

• Silver(Ag): 5 oz/gal (37.5 g/l)

• Free potassium cyanide (KCN): 15 oz/gal (110 g/l)

• Potassium carbonate (K2CO3): 13 oz/gal (100 g/l) maximum

• Water: reminder

• Silver(Ag): 4 oz/gal (30 g/l)

• Free potassium cyanide (KCN): 16 oz/gal (120 g/l)

• Potassium hydroxide (KOH): 0.8 oz/gal (6 g/l) maximum

• Water: reminder

Barrel plating:

• Silver(Ag): 3 oz/gal (22.5 g/l)

• Free potassium cyanide (KCN): 15 oz/gal (110 g/l)

• Potassium carbonate (K2CO3): 13 oz/gal (100 g/l) maximum

• Water: reminder

Operating conditions:

• Anodes: silver or stainless steel

• Cathode current density: 25 A/ft2 (2.5 A/dm2)

• Temperature: room

• Cathode agitation: cathode rod movements 6-15 ft/min (2-6 m/min)

• Filtration: at least 1 turnover/hour

• Silver consumption: 0.13 oz/A*hr (4 g/A*HR)

• Time for deposition 1 µm (0.04 µinch) at 1.5 A/dm2: 1 min.

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Environmental and safety considerations have stimulated development of cyanide-free silver plating formulations.

Some of these new non-cyanide processes have been already implemented in the electroplating industry.

Besides of non-poisonous composition cyanide-free silver plating provides good adhesion without silver strike treatment.

Non-cyanide bath compositions:

• Silver(Ag): 0.5-1.0 M

• Potassium carbonate (K2CO3): 0.2 M

• Succinimide (C4H5NO2): 0.5-0.7 M

• Water: reminder

• Silver iodide: 8 oz/gal (60 g/l)

• Sodium iodide: 67 oz/gal (500 g/l)

• Polyvinyl alcohol 0.13 oz/gal (1 g/l)

• Sodium thiosulfate 0.16 oz/gal (1.2 g/l)

• Water: reminder

• Silver as silver chloride: 5.3 oz/gal (40 g/l)

• Sodium thiosulfate 67 oz/gal (500 g/l)

• Potassium metabisulfite 4 oz/gal (30 g/l)

• Water: reminder

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