Electroplating or electrodeposition is an electrochemical technique that consists, as its name suggests, in depositing metal layers on receiving objects; that is to say, cover them with a metal covering, whose thickness ranges between 5-20 μm.
This technique is also known as electroplating or electrolytic plating, since an electrolytic medium composed of the metal ions of the metal to be deposited is essential for its implementation. Therefore, if we want a copper plating (copperized), we will need soluble copper salts, such as copper sulfate, dissolved in water.
The above reasoning applies to any other metal: an electrolytic gold plating will require gold salts, usually gold cyanides, which can be very toxic. Thus, some objects, such as eating utensils, can be coated with gold to assign them greater value and nobility.
But electroplating goes beyond aesthetics: it confers protection, durability, conductivity, resistance, and many other properties to the surfaces on which the electrodeposition is carried out. Likewise, it allows the saving of natural resources, since it is not necessary to spend large amounts of metal for the coatings.
To electrodeposit a metal on a surface some basic elements are needed, whose dimensions and way of mounting them will vary depending on the batch of objects to be coated.
In the image above, for example, small-scale copper plating is shown, reproducible in any laboratory or space.
First, an electrolytic medium for the metal to be electrodeposited is needed. The copper sulfate salt, CuSO 4 , is very soluble in water, so it contributes Cu 2+ ions that will later be reduced to Cu 0 atoms . These atoms are the ones that will assemble on the receiving surface to give rise to metallic crystals.
The electrolytic medium is essential for the circuit to work and the ionic charges can move or move towards the region where the electrodeposition occurs.
The electrodes are the surfaces on which the electrodeposition takes place from its chemical base. In simple terms there are two electrodes: the cathode, which is where the metal is electrodeposited; and the anode, which is where a chemical reaction takes place that donates electrons to the metal ions that will coat the cathode.
The external circuit is a battery that provides the initial electrons that promote electrodeposition. Its presence means that the reaction is not spontaneous: electricity is needed to generate a chemical change, which is the reduction of dissolved metal ions.
Cleaning the electrodes
The part to be coated must be completely clean. Otherwise, the plating or coating will be defective.
For this, it is necessary to make sure that its surface is entirely hydrophilic, that is, that it does not have grease or any type of dirt. One way to ensure this, after chemical washing, is to wet the part with water, and if the formation of drops of water or a non-uniform fall of the liquid is observed, it means that the electrode is not clean yet.
Reactions and sense of electrons
Electroplating is a redox reaction. Therefore: one species must be reduced, while the other oxidized, or vice versa. In copper plating, we have an anode made of copper, and a cathode made of a Me metal (iron, zinc, tin, etc.).
When we turn on the battery, electrons will flow from inside it to the cathode, which begins to charge negatively.
The negative charge of the cathode attracts the Cu 2+ cations from the electrolytic medium, the reduction taking place:
Cu 2+ (ac) + 2e – → Cu (s)
The battery needs to recover the donated electrons, and the reduced Cu 2+ ions must also be replenished to maintain the neutrality of the electrolytic medium. This is when the anode comes into play: the copper atoms oxidize:
Cu (s) → Cu 2+ (ac) + 2e –
And the released electrons are directed back towards the battery, which in turn mobilizes them towards the cathode so that the electrodeposition continues.
The anode gains a positive charge, which attracts the SO 4 2- ions from the electrolytic medium (see image once more).
That is, the reactions that occur is the reduction and oxidation of copper, the metal that is electrodeposited:
Cu 2+ (ac) + 2e – → Cu (s)
Cu (s) → Cu 2+ (ac) + 2e –
The anode, made of copper, loses mass while the cathode, made of another metal, is covered with a thin coppery layer.
Examples of electroplating
Metals can be “electroplated,” which is to electrolytically coat them with a layer of silver. This is especially beneficial in electronic components that must conduct electricity. Silver can be electroplated on steel, copper, nickel, and other metals.
Tinning is the electrodeposition of tin. It is very useful in the production of iron cans, which are lined with tin to make them resistant to corrosion, and thus not damage the food or products they store.
Brass or tan
Although a bit more elaborate, brass and bronze can also be electroplated on some metals to enhance their appearance.
Electroplating of zinc or zinc plating (not to be confused with hot dip galvanizing) is mostly used to give iron resistance to corrosion and more shiny and attractive surfaces. Thus, its function is aesthetic and functional at the same time, and it is found in body parts such as wheels.
Plastics, previously treated to have a conductive surface, can also be coated with metal, which is like a sheet of aluminum foil adhering to them.
Electroplating or electroplating applications can be divided into functional or aesthetic.
Coating a piece or object is to endow its surface with properties that it alone lacks. For example, the electrodeposition of copper or gold makes the parts they cover conductive, making it ideal in electronic circuits to take advantage of the properties of these metals, without the need to spend tons of gold in the manufacture of whole and solid parts.
Gold, on the other hand, protects surfaces from heat and radiation. It is for this reason that it is widely used in satellites and astronauts’ helmets, as it reflects the sunlight, thus preventing it from hurting your eyes, or quickly heating up.
Chromed or nickel-plated steels are very resistant to corrosion thanks precisely to these metallic layers. That is why electroplating is used in industry to protect sensitive metals from corroding, or that are quickly worn away by mechanical actions.
Metallic veneers make objects shine. Zinc, nickel, and chromium are widely used for this purpose. Likewise, gold and silver are electrodeposited in rings, chains, bracelets, necklaces, earrings, etc., and therefore their electroplating finds many applications in jewelry and ornamentation, to enhance the image and prestige of said garments.