Dr Khalil Khan
5 May 2020
The process of coating a metal object with a thin layer of another metal by the means of electrolysis. The metal used for coating is normally more precious and possesses different material properties such as enhanced corrosion or wear resistance. Examples include tin or nickle on iron. Electroplating is a process that uses an electric current to reduce dissolved metal cations so that they form a thin coherent metal coating on an electrode.
The cations associate with the anions in the solution which is called an electrolyte. These cations are reduced at the cathode to deposit in the metallic, zero valence state. For example, for copper plating, in an acid solution, copper is oxidized at the anode to Cu2+ by losing two electrons. The Cu2+ associates with the anion SO2−4 in the solution to form copper(II) sulphate. At the cathode, the Cu2+ is reduced to metallic copper by gaining two electrons. The result is the effective transfer of copper from the anode source to a plate covering the cathode.
Many plating baths include cyanides of other metals (such as potassium cyanide) in addition to cyanides of the metal to be deposited. These free cyanides facilitate anode corrosion, help to maintain a constant metal ion level and contribute to conductivity. Additionally, non-metal chemicals such as carbonates and phosphates may be added to increase conductivity.
In the UK 1p and 2p coins used to be minted from bronze, an alloy of copper and tin, but were changed in 1992 to copper plated steel. In 2006 an old bronze 2p was actually worth 3p due to increases in the price of copper on the metal markets!