Process description: Plating precious metal
Back to Plating precious metal in metal industry
- Precious metal plating
Silver and gold have been deposited electrolytically since the first half of the nineteenth century. More recently, processes for the so-called platinum metals (palladium, rhodium, ruthenium and platinum) have also been developed. Thin layers of less than 1 μm are used to make a wide range of items appear valuable without commensurate cost. They also provide stain and corrosion resistance. Other specific technical properties such as conductivity, hardness and wear resistance have led to their wide-spread application in the electric and electronic industries.
- Silver
The silver plating of cutlery and hollowware is one of the oldest decorative applications of electroplating. Bright silver deposits are widely used for jewellery as well as on ornaments, trophies, giftware, etc. Due to its high conductivity and mechanical and chemical properties, silver layers are found on many components of electrical and electronical devices. Also, silver-palladium layers as a substitute for gold on electrical connectors have recently gained importance.
The majority of silver electrolytes are based on potassium-silver cyanide (potassium dicyanoargentate). A silver content of 30 – 65 g/l with free potassium cyanide of 100 – 160 g/l and potassium carbonate (15 – 20 g/l) is necessary for good plating performance. However, a potassium carbonate concentration of over 200 g/l makes it necessary to discard the electrolytes.
Attempts to replace cyanide, for example by thiosulphate and alternative complexing agents have had limited success due to lower stability of the bath, high cost and cosmetic differences of the deposit.
- Gold
Gold can be plated using rack, barrel or high speed equipment either as a very pure metal or as an alloy. It finds use in a wide range of industries such as connectors, printed circuit boards, integrated circuits, semiconductor manufacture, bathroom fittings, giftware, tableware, buttons, watches, pens, jewellery, and spectacle frames.
The majority of gold plating solutions in commercial plating facilities, whether the deposit has a decorative or a functional requirement are based on mildly acid solutions of gold potassium cyanide as potassium cyanoaurate, KAu(CN)2. These solutions typically contain 2 – 6 g/l of gold together with buffering agents to maintain the pH. These solutions often contain alloying metals such as cobalt, nickel, iron or indium. These harden, increase the wear resistance, and brighten the deposit. They also give distinctive colours.
A gold strike solution is often used to deposit the initial layer of gold to promote adhesion. Strike solutions usually based on gold potassium cyanide at 1 – 2 g/l but may use gold (III) potassium cyanide (KAu(CN)4) at a pH of around 1. These are often used on difficult to plate base materials such as stainless steel.
Alkaline solutions containing free cyanide are occasionally used for jewellery plating and for electroforming.
There are non-cyanide gold electrolytes, which are based on one of the gold sulphite complexes, which generally operate in mildly alkaline conditions, these typically contain 8 – 15g/l of gold together with organic or inorganic brightener systems.
- Palladium and alloys
Layers of palladium-nickel alloy (75 – 80% Pd/25 – 20% Ni) are hard, extremely ductile and have a white colour with good corrosion resistance. They are suitable for coating spectacle frames and writing implements.
Pure palladium deposits are also used to substitute for nickel as a diffusion barrier below a final gold flash on spectacle frames, jewellery and buttons. Sandwich layers of nickel, palladium or palladium/nickel alloy and gold flash have proven applicability on printed circuit boards and connectors as substitutes for hard gold layers.
The most common formulation is based on the complex tetraamine palladium (II) dichloride (4 – 20g Pd/l), containing ammonia. Alloying metals are nickel, cobalt and silver.
- Rhodium
Very thin rhodium deposits on silver have a silvery-white colour and are used to prevent staining. Their hardness and wear resistance properties are suitable for technical applications such as on reed contacts and on heavy duty connectors. Rhodium is also applied on reflectors for optical equipment and in aerospace applications.
Electrolytes are based on rhodium (III) sulphate or rhodium (III) phosphate, with a rhodium content of 2.5 – 20 g/l depending on their application for decorative (thickness of layer 0.05 – 0.5 μm) or for technical purposes (thickness of layers 0.5 - >8 μm), where selenium and sulphites are added to prevent cracking.
- Platinum
Thin platinum layers are applied for decorative purposes, with thicker layers on electrical devices and on equipment for the chemical industry. Non-soluble platinised titanium anodes and anode baskets are in common use in electroplating.
Acid electrolytes are based on chloride, sulphate, nitrate and nitrite complexes of platinum, alkaline electrolytes on phosphate, ammonia and sodium hydroxide complexes, with metal contents of 6 – 40 g/l.
Source: BAT Surface Treatment of Metals and Plastic, Aug. 2006.
