In this paper, we shall concentrate on the electroplating of gold and gold alloys and rhodium – one of the platinum group of metals with a good white colour and tarnish resistance – for decorative applications.
Electroplating is a method to put a metal coating onto an object, in our case a piece of jewellery, by placing it in a solution containing the metal to be plated and passing an electrical current through the piece and the solution. It is possible to electroplate coatings of most pure metals and even some alloys.
Electroplating is a comparatively quick and easy process to carry out and does not require major investment in costly equipment. It can be done successfully with very simple, basic equipment. Finished carat gold jewellery may be electroplated with gold for several reasons:
Rhodium is often used to give a good white colour to white gold jewellery (which is often not a good white colour) or is used selectively on yellow gold jewellery to give local areas of whiteness, often around gem stone settings, and also to plate the master model made in silver used for making the rubber mould in investment casting.
In contrast, electropolishing is the opposite to electroplating: we remove metal from the surface of our jewellery by passing an electrical current in the opposite direction and, if we get the conditions right, we can do this in a way that leaves a polished surface. Many jewellery producers use electropolishing as part of their total finishing process in gold jewellery manufacture. In this paper we shall discuss the following aspects:
Much of this information is contained in our two WGC publications – The Technical Manual and the Finishing Handbook.
Electroplating and electropolishing is carried out in an electrolytic cell, Fig.1. This comprises two electrodes that are electrically connected and immersed in a solution – called an electrolyte. When an electrical current is passed through the cell, metal dissolved in the electrolyte is deposited on the negative electrode – the Cathode – whilst the metal of the positive electrode – the Anode – may be removed and dissolved in the electrolyte. Thus, metal passes from the anode into solution in the electrolyte and is then deposited on the cathode.
|Figure 1 – Schematic: electrochemical plating cell|
Thus, if we make the cathode the piece of jewellery we wish to plate and the electrolyte contains gold, then we can deposit gold on our jewellery item. On the other hand, if we make our piece of jewellery the anode in a suitable electrolyte capable of dissolving gold, then, under the right electrical conditions, we can remove the surface selectively to obtain a polished surface. A typical electrolytic cell is shown in Fig. 2 and can enable several pieces to be electroplated simultaneously.
|Figure 2 – Typical electroplating bath|
Often, in electroplating, we use an inert anode, where metal is not dissolved away, and control the concentration of the depositing metal in the electrolyte solution by direct additions of the appropriate metal as a salt to the electrolyte.
The amount of metal – in our case gold – deposited is governed by Faraday’s law which says that : The weight of metal deposited is proportional to the quantity of electricity passed.
The quantity of electricity is defined as the current (in amps) multiplied by the time (in hours). The weight of metal deposited for a given quantity of electricity will be different for different metals which is related to their atomic number and valency through a factor called the electrochemical equivalent.
This Faraday law is very useful in calculating and controlling the amount (weight or thickness) of metal deposited on a piece of jewellery. Obviously, at a constant electroplating current (and salt concentration in the electrolyte), thickness of the electroplate is directly proportional to the plating time. Double the plating time and you double the thickness.
For decorative applications, we usually require a uniform thickness of electroplate over our complex shaped item. This can be a problem at sharp edges and recessed surfaces, for example. We usually also want a bright deposit, with good adhesion to the underlying item. We do not want the electroplated deposit to be highly stressed with a tendency to crack and spall. We may want to plate at high speeds and still retain a good uniform bright surface. We do not want a porous or micro-cracked coating which could allow corrosion or tarnishing of the item during subsequent wear.
If we are co-depositing more than one metal, i.e. a carat gold, we also want good control of composition – a uniform gold content over all the surface and throughout the thickness, for example.
So how do we control these factors? Well, we achieve this through a number of ways:
Firstly, the electrolyte. A good electrolyte will contain the metal (or metals) to be deposited in solution in a sufficient concentration. In cyanide based gold baths, this will be in the form of gold potassium cyanide salt. It will also contain other additives to give good plating properties, These include, for example, additives to improve:
These additives are usually proprietary to the electroplating salt manufacturers and it is difficult to find information on what they are. They are frequently organic chemical compounds.
During plating, it is usual to agitate or stir the electrolyte to maintain optimum plating conditions and uniformity of composition.
The anode area and position are important to efficient electrodeposition and uniformity of deposit. There is a tendency for plating to be thicker on cathode areas closest to the anode and thinner in areas hidden (or out of line of sight) from the anode. Correct positioning of the anodes (more than one may be used) and a large anode area (compared to cathode area) is desirable for good plating.
The electrical conditions during plating are also important for plating quality. In particular, the current density (the current divided by surface area of the piece) plays an important role, particularly in alloy plating where deposit composition is controlled by current density. If the current is too high, the plating speed is increased but one may get a porous, dendritic deposit rather than a bright one and it may be accompanied by gas evolution which affects the surface finish. If it is very low, then the deposit may not have a good appearance and plating will be slow.
The temperature of the electrolyte can also play a role in getting good plating, particularly in alloy plating. Follow the electrolyte supplier’s recommendations.
For good quality electroplating and good adhesion of the deposit, the condition of the surface to be plated is important. Most plating defects arise from unclean surfaces prior to plating. The surface to be plated must be clean and free from grease, dirt, oxides and tarnish films, polishing compounds, etc. Greasy, dirty surfaces will not be wetted by the electrolyte and may not be plated. It also helps to have a smooth polished surface, free from defects and imperfections, if one wants a bright polished electroplated deposit.
Plating should not be used to hide defects and to improve the surface polish (reduction in surface roughness). Defects to be avoided include casting porosity, inclusions and embedded polishing compounds, scratches and tool marks, and pitting from over-pickling.
The surface to be plated (the ‘substrate’) can be prepared by normal polishing techniques and then cleaned in several ways:
In practice just one, or possibly two, techniques are used, for example degreasing and acid pickling, followed by rinsing in water and drying. Many proprietary cleaners are alkaline with wetting agents and surfactants added. Deionised or distilled water should be used as the final rinse before drying to prevent any deposits from the water being left on the surface.
Many electrolytes are based on cyanide. This is particularly true for gold. Cyanide is very poisonous and must be handled with great care.
A golden rule is never to allow drinking and eating in an electroplating facility and to have very strict control and procedures in the plating shop. Protective overalls and visors should be worn and changed regularly. Cleanliness is vital. For safety, cyanide electrolytes and plating salts should be kept in locked cupboards. Keep cyanides and acids apart from each other. Acid will react with cyanide to liberate deadly hydrogen cyanide gas!
Old electrolytes, as well as cleaners and rinse waters must be disposed of safely and NOT thrown away down the sink or drain. The consequences of doing so are too awful to contemplate!
Acid based, non-cyanide electrolytes must also be handled with care.
All reputable salt or electrolyte manufacturers will provide Materials Safety Data Sheets on their products and give good advice on health and safety procedures
There are many electroplating systems on the market for putting pure gold and gold alloy deposits on to gold jewellery and on to base metals for decorative applications. There are also many others for technical applications such as electrical contacts and connectors, where the coating properties must have a certain technical performance.
The electrolytes can be classified into cyanide and non-cyanide based and may contain small alloying additions to control colour and other properties. All cyanide-based electrolytes are based on the use of gold potassium cyanide salt, KAu(CN)2, which contains about 68% gold. However, most electrolytes do not contain anything like this concentration of gold. Some electrolytes are acid, others neutral and others are alkaline, as shown in the classification in Table 1.
|Electrolyte Type||pH||Gold complex||Alloying Metals|
|Alkaline||8 – 13||KAu(CN)2||Cu, Cd, Ag, Zn|
|Neutral||6 – 8||KAu(CN)2||Cu, Cd, Ag|
|Weakly acid||3 – 6||KAu(CN)2||Co, Ni, In, Fe|
|Acid||0.5 – 2.5||KAu(CN)4||Co, Ni, In, Sn|
|Cyanide-free, alkaline||8 – 10||Na3Au(SO3)2||Co, Ni, In, Sn|
Table 1 – Electrolytes for gold alloy electroplating
The range of colours possible and bath and deposit characteristics of electroplating systems from one well-known manufacturer are shown in Figures 3 – 5. Note the optimum bath temperature is often above ambient. The gold concentration is quite low – about 0.1 – 7.0 g/l and the speed of plating ranges typically from about 10 – 75 mg/amp/min. The time to plate 1 micron thickness ranges from 3 – 15 mins.
|Gold content, g/l||8 – 108||12 – 16||1 – 2||–|
|Bath temperature||60 – 70°C||50°C||70 – 75°C||50°C|
|pH||6 -7||6||7.5 – 8||7|
|Plating rate,µm/min||0.1 – 0.6||0.5||0.6 – 24||0.1 – 0.2|
|Current density, A/dm2||0.2 – 1.0||ca. 0.8||ca. 1 – 40||ca. 1.0|
|Additives||As/Ti /Pb||As||No As,Ti or Pb||–|
|Salts/acids||Citrate, phosphate, phosph. acid||Citrate, phosphate||Phosphate, phosph. acid||Phosphate|
|Purity, %gold||99.9 – 99.99||99.9||99.9||99.9|
|Hardness, HV||70 – 90||250||70 – 100||100|
|Application||Electronics||Elec. Contacts, Decorative||Electronics||Decorative|
Table 2 – Fine gold electroplating baths
Table 2 (above), shows some pure gold plating baths based on gold potassium cyanide salt from another well known German manufacturer.
This illustrates the high purity of the deposit and how the properties of the deposit are influenced by plating conditions and electrolyte composition. Note the high hardness values compared to bulk pure gold.
For jewellery application, a deposit thickness of about 0.5 – 5.0 microns is typical, but very thin ‘flash’ coatings may be used where cost is more important than quality.
If one is gold plating onto base metals, it is common practice to first electroplate with a thin flash or ‘strike’ coat of copper to provide a good key , then an undercoat of nickel, bronze or tin. The purpose of these underlayers is to provide levelling and brightening to the substrate and to inhibit migration of underlying copper into the gold layer, causing it to turn redder. With the European Directive against use of nickel, there is a trend to use bronze (copper-tin – zinc) or tin or palladium as the underlayer.
Often, a ‘strike’ gold layer is then deposited of about 0.1 microns thickness before the full gold layer is electroplated from a different gold electrolyte. These are known as duplex systems.
In selecting an electrolyte and plating system, it is good practice to seek advice from your plating materials supplier. They can advise on what is most suitable for your needs. Plating, of course, removes gold from the electrolyte. Therefore, it is important to maintain the correct concentration of salt in the electrolyte. Additions of salt should be made periodically. This requires an ability to measure the gold concentration in the bath.
Between each stage of surface preparation and electroplating, it is important to rinse the items being plated before moving to the next stage. This prevents contamination of the new bath and loss of precious metal salt. This is known as ‘drag-out’. Of course, after completion of the total process, the item should be rinsed and dried. Do not use tap water as this will leave deposits on the surface after drying.
Rhodium is a platinum group metal with a good white colour and is hard and tarnish resistant. For jewellery purposes, we desire a bright deposit, defect-free and hard and there are several suitable rhodium plating systems on the market. These are sulphate type baths and are very acidic.
Usually, deposit thickness of about 0.5 microns, but up to 2-3 microns, is plated on gold jewellery to give the required surface characteristics. There is a tendency for internal stress to build up in the deposit as thickness increases, resulting in cracking eventually.
For the high carat golds, a thicker layer of rhodium is plated directly on the substrate, but for low carat golds, a nickel interlayer is plated first, allowing a thinner, cheaper rhodium deposit without losing colour and providing good corrosion resistance.
As with gold, good surface preparation is required to provide a clean surface for quality electroplating. The following practice is recommended:
Plating should take from 30 seconds to 2 minutes, depending on thickness desired. Inert anodes of platinum are used at 4-5 cm distance with a surface area at least as big as the cathode. The bath should be well agitated or stirred.
Periodic replenishment of the rhodium in the bath is necessary and this is done with special rhodium replenishment solutions which have a high rhodium concentration and low acidity. It is important to avoid contamination of the electrolyte by other metals, so good rinsing and use of non-metallic tanks is recommended.
A typical rhodium electroplating system has the characteristics shown in Table 3.
The extremely high hardness of the deposit is notable. This is an advantage in rhodium plating master models in silver for investment casting and electroforming as it enables a high degree of polish to be obtained on the model, with benefit down the line to the casting or electroform.
Electrolyte additives such as magnesium sulphate, selenic acid and sulphites are often used to control internal stress build-up.
|Rhodium content||1.5 – 2.5 g/l|
|Bath temperature||40 – 50°C|
|Plating rate||2 mg/A/min|
|Current density||1.5 – 5.0 A/dm2|
|Deposit purity, rhodium %||99.9|
|Time to deposit 1 micron||30 secs.|
|Hardness of deposit||HV 950|
Table 3 – Typical bright rhodium electroplating system
Basically, electroplating is a simple process and can be performed in simple glass beakers with a simple d.c. electrical supply. However, if good consistent quality is desired, it is preferable to use purpose-made equipment, which will include:
Whether one is only plating on a small scale on a bench or on a mass production scale, there are many suppliers of purposemade equipment to suit all needs. Some examples are shown in Figure 6. They can often be viewed at the major jewellery shows, e.g. at Basel and Vicenza in Europe. Prices do vary significantly, but it is possible to buy suitable equipment quite cheaply, or even to find a local fabricator to make one tailored to your needs.
|Figure 6 – Typical commercial electroplating equipment, for small bench operations (a) range of sizes (b) in use|
It is appropriate to make a comment on masking of surfaces so that electroplating is only done in areas where it is wanted, e.g. around gem stone settings. This is done by painting on an organic lacquer (often pink in colour) to those areas where plating is not wanted and allowing it to dry. After plating, it can be easily removed with an organic solvent such as acetone. There are many commercial products on the market.
Remember that such lacquers are inflammable and must be stored in well closed containers. More details of masking lacquers are given in the WGC publication, The Finishing Handbook.
The equipment for electropolishing is very similar to that for electroplating as seen in the sketch, Figure 7, and is manufactured by the same companies. The cathode is normally stainless steel or titanium as is the anode frame, which has platinum suspension wires or hooks. This anode frame may need to be agitated. Again the bath is heated, in this case by an immersion heater, and there is fume extraction. A D.C. power supply supplying low voltage (6- 15 V) and a high current is needed to give a current density in the range 100 – 150 A/dm2. Typical bath temperature is up to 80°C and a system for stirring the electrolyte is also necessary.
|Figure 7 – Schematic: electropolishing cell|
To explain how electropolishing is achieved, it is necessary to examine the anode polarisation curve which plots current density against applied voltage, Figure 8. Such curves are characteristic for each electrolyte and metal item. If we operate the electrolytic cell at the low voltage portion of the curve A – B, nothing much happens to our jewellery. At higher voltages, in the region B – C, etching of the surface occurs and this will reveal details of the metallographic structure of the surface under the microscope. In the region D – E, the current density remains constant, despite increasing voltage. This is the range where good electropolishing takes place. This is where we operate the process! At voltages higher than E, the current density increases rapidly and there is gas evolution at both cathode and anode which is undesirable for a good polished surface.
|Figure 8 – Electropolishing: anode polarisation curve|
The mechanism of electropolishing is complex and it is not appropriate to discuss it here. However, the rough surface is levelled in the process and a good bright smooth surface can be achieved, as shown in the examples in Figure 9.
|Figure 9 – a) Electropolished jewellery: as cast 14 ct gold (right) and
after electropolishing (left)
There are many factors which influence the process, including jewellery alloy, electrolyte composition, temperature, current density & voltage and time.
There are several proprietary electropolishing systems on the market for electropolishing gold alloys in the range 8ct up to 24ct, many using the safer, cyanide-free, weakly acid electrolytes, operating at temperatures up to 80°C. The older systems use cyanide-based electrolytes, operating at 80–90°C. As with electroplating, it is important to rinse the jewellery after electropolishing and to dry it.
Clearly, gold is dissolved from the surface in the process. This is small if the initial surface is good. Chains and all types of jewellery can be electropolished. The process does not discolour the jewellery, even at solder lines. Good rinsing and the use of a brightening chemical solution are recommended after electropolishing.
The gold that is dissolved in the electrolyte from electropolishing can be recovered. For cyanide-freesolutions, the electrolyte is treated with sodium hydroxide until a pH of 5 is attained. Then a special reducing compound is added and gold is precipitated from solution. It is allowed to settle and filtered off. More sodium hydroxide is added to the remaining solution until pH 5-7 is reached and then safely disposed of down the drain. The gold slime filtered off is dried, mixed with borax flux and melted. It is poured off and allowed to solidify into a small bar or button. For cyanide solutions, the gold can be precipitated by additions of zinc or aluminium dust.
Electropolishing of gold jewellery can be done as a single finishing step but, more often, it is part of a multistep process involving mechanical polishing as well.
We have discussed the basic principles of electroplating and some of the factors affecting the process. We have also discussed the equipment requirements.
As we have seen, electroplating of jewellery is a very versatile process and one can obtain gold coatings of varying colour, appearance, properties and caratage as well as pure gold. It is a quick, cheap and easy process to operate.
It does not require expensive equipment, but it is worthwhile to buy good quality electroplating salts from reputable suppliers. Such salts are specially formulated to give good performance.
Many gold plating processes use toxic cyanide electrolytes. Care must be taken in their use and disposal.
We have also discussed the basics of electropolishing. It is a process finding increasing use once more, often in combination with mechanical polishing.
There are several books on electroplating and on gold electroplating. Many manufacturers also have useful literature. See also the WGC publications mentioned above. There are some papers in the Proc. Santa Fe Symposia, Gold Bulletin, etc.
I am very grateful for information and advice from Degussa Galvanotechnik GmbH, Germany, Enthone-Omi S.A., France and W.C. Heraeus GmbH, Germany. Some diagrams have been taken from other WGC publications.