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Electroplating

Electroplating is the process of transferring metal from one object to another using electricity in a solution of metal salts.
In jewellery, electroplating is frequently used to plate a precious metal such as silver, gold or platinum onto another metal. White gold jewellery is often plated with rhodium to provide a harder, better wearing surface to wedding rings, for example.

Cheap jewellery is often silver or gold electroplated over a base metal such as brass or copper. The thickness of the plating is typically only a few microns (one thousandths of a millimetre). It looks great initially, but the layer is so thin that it will wear through in a matter of months. It is not suitable for items that are worn regularly, but may be adequate for infrequently worn or costume jewellery.

The actual process is quite basic in theory, but requires considerable knowledge and skill to perfect and produce great results consistently. In simple terms, this is how it works: A cathode (the metal part to be plated) and an anode (the metal to be coated over the plate) are immersed in a solution of metal salts (an electrolyte) at a distance from each other. A direct current (DC) power source is connected, negative (-) to the cathode and positive (+) to the anode.

Current flows through the solution oxidising the metal on the anode into metal ions which dissolve into the solution. At the cathode, the dissolved ions leave the solution and bind to the cathode. This causes a layer of the anode metal to form over the cathode surface. The amount of metal leaving the anode is the same as the amount of metal being deposited on the cathode.

To ensure good adhesion of the plating, the cathode must be cleaned prior to processing. Any oils present on the surface can prevent the plating from binding successfully.

Some metals have poor adhesion to other metals and cannot be plated directly. In this case a metal that binds well to both the other metals will be plated first. Nickel for example, does not bind well to zinc alloys, but both will bind well with copper. In this case the zinc would first be copper plated and then nickel plated. (Double plating)

A strike or flash plating is when a very thin layer (less than 0.1 microns) is applied to the cathode. This is a slow process using a low concentration solution but creates a good quality plating that can be used to create an excellent foundation for further plating (using faster methods) and may be the method used where double plating is required to create the initial layer.

Brush plating is not done in solution, but rather with a brush that has been saturated with the plating solution. The brush will have a stainless steel core which is connected to the positive of the power supply and current flows through the solution soaked bristles causing plating to occur where the brush makes contact with the cathode. It is more difficult to get a completely even plating with this method, but it does allow the plating to be applied to selected areas with reasonable accuracy.

The correct electrolyte must be used for the type of metal being transferred. They are typically a salt of the material to be plated (chloride, sulphite, phosphate, carbonate) or potassium cyanide based solutions. The electrolyte can also contain other chemicals (acids) to prevent the formation of hydrogen cyanide gas which is lethal and occurs when the pH of the solution goes above 8 (slightly alkaline).

The speed of the plating process depends mainly upon the concentration of the electrolyte and the strength of the electrical current. A higher concentration or current will produce a more rapid rate of plating. The thickness of the plating is dependent on the speed of the process and the length of time the process continues.

 

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