Electroplating: Definition, Working Principle, Application

When you buy a brand new bicycle, it has a shiny handlebar and wheel rims. However, if we accidentally scratched them, the shiny coating comes off, revealing a not-so-shiny surface beneath it. It is also observed in the case of artificial gold jewellery, which appears to be made from gold, but with repeated use, the gold coating wears off, revealing another metal beneath. In all these cases, there is a layer of coating of another metal. How can we deposit a layer of one metal on another? What is Electroplating? What are the benefits of doing so?

Electroplating is the process of depositing a metal coating on an object by providing a negative charge to it and immersing it in a metal salt solution. The metal salt comprises positively charged metal ions that are attracted to the negatively charged object’s surface and “reduced” to a metallic state. In other terms, electroplating is the electrolysis-based technique of covering a metal object with a thin layer of another metal. Continue reading to know more.

What is Electroplating?

Electroplating is also known as electrodeposition. It is the process of depositing a thin layer of one metal onto another using an electric current. This thin layer of metal being deposited onto the surface of a workpiece is called the substrate. Electroplating can be used to change the physical properties of an object, such as increased wear resistance, corrosion protection, or aesthetic appearance, as well as increased thickness.
Let us understand this with an example:-When electric current is passed through a copper sulfate solution, then copper sulfate dissociates into copper and sulfate, and the copper gets deposited to the electrode that is connected to the negative terminal of the battery.

How does Electroplating Process Work?

In the process of electroplating, the anode (Metal to be used for electroplating) is connected to the positive terminal, and also the cathode (Object to be plated) is connected to the negative terminal. Then, it is connected to an external supply of direct current after immersing both (anode and cathode) in a solution that contains an electrolyte of salt of the metal to be used for electroplating. When \(DC\) power is applied, the anode is oxidized, and its metal atoms dissolve in the electrolyte solution, and these dissolved metal ions are further reduced at the cathode and then form a coating. The current passing through the circuit is adjusted to control the rate of deposition. In the process of electroplating, the speed at which the anode is dissolved equals the speed at which the cathode is plated.

Some important terms related to electroplating are:-

1. Anode: It is a positively charged electrode in the circuit. This is made of the metal that will form the plating.
2. Cathode: The cathode in the electroplating circuit is part that needs to be plated with another metal. It is also called the substrate. This part is the negatively charged electrode within the circuit.
3. Solution: The electrodepositing reaction takes place in an electrolytic solution made by dissolving the salt of the metal to be deposited. Sometimes this solution contains one or more metal salts, usually including copper sulfate, to facilitate the flow of electricity.

Faraday’s Laws of Electrolysis

Michael Faraday was the first scientist who explained the quantitative aspects of electrolysis. After his extensive investigations on electrolysis of solutions and melts of electrolytes, Faraday gives two laws of electrolysis:
Faraday’s First Law: The amount of chemical reaction which occurs at any electrode during electrolysis by a current is proportional to the quantity of electricity passed through the electrolyte (solution or melt).
Faraday’s Second Law: The amounts of different substances deposited by the same quantity of electricity passing through the electrolytic solution are proportional to their chemical equivalent weights (Atomic Mass of Metal \(÷\) number of electrons required to reduce the metal). So, the amount of electricity (or charge) required for oxidation or reduction depends on the stoichiometry of the electrode reaction.
For example, in the reaction: \(\rm{Ag}^+ (aq) + e^- \to \rm{Ag}(s)\)
One mole of the electron is required for the reduction of one mole of silver ions. We know that the charge on one electron is \(1.602 × 10^{-19}\,\rm{C}\). Therefore, the charge on one mole of electrons is equal to: \(N_A × 10^{-19}\,\rm{C} = 6.02 × 10^{23}\,\rm{mol}^{-1} × 1.602 × 10^{-19}\,\rm{C} = 96487\,\rm{C}\,\rm{mol}^{–1}\). This quantity of electric charge is called Faraday and is represented by the symbol \(F\). For approximate calculations, we use \(1\,\rm{F} ≅ 96500\,\rm{C}\,\rm{mol}^{-1}\).
Similarly for other electrode reactions:
\(\rm{Mg}^{2+} (l) + 2e^- \to \rm{Mg}(s)\)
\(\rm{Al}^{3+} (l) + 3e^- \to \rm{Al}(s)\)
It is obvious that one mole of \(\rm{Mg}^{2+}\) and \(\rm{Al}^{3+}\) require \(2\,\rm{mol}\) of electrons \((2\,\rm{F})\) and \(3\,\rm{mol}\) of electrons \((3\,\rm{F})\), respectively. The charge passed through the electrolyte during electrolysis is equal to the product of current and time.

Calculation of Time Required for Electroplating

To determine the time required to electroplate per mole of metal, we first need to write the electrode reactions to know the number of moles of electron \((n)\) required to reduce the metal.
Then the amount of charge required will be given by,
\(Q = n F\)
Where,
\(Q =\) The amount of electricity or charge in coulombs \(C\),
\(n =\) The number of moles of electrons
\(F =\) The Faraday Constant \(96,500\,\rm{C}\,\rm{mol}^{-1}\).
Now, the time required will be,
\(t = \frac{Q}{I}\)
Where,
\(I =\) The current \((\rm{A})\)
\(t =\) The time \((\rm{s})\)

Electroplating Applications

Electroplating is a very useful process. It is widely used in the industry for coating metal objects with a thin layer of a different metal. The layer of metal deposited has some desired property, which that metal lacks. For example, chromium plating is done on many objects such as car parts, bath taps, kitchen gas burners, bicycle handlebars, wheel rims, and many others. Chromium has a shiny appearance also, it does not corrode. But it is expensive and not economical to made whole objects. So, the object is made from a cheaper metal, and only a coating of chromium over it is deposited.

Jewellery makers do the electroplating of silver and gold on less expensive metals. These ornaments have the appearance of silver or gold but are much less expensive. Electroplated tins are used for making tin cans, for storing foods. Tin is less reactive than iron. Thus, food doesn’t get into contact with iron and is protected against spoilt. In bridges and automobiles, iron is used to provide strength. However, iron tends to corrode and rust. So, a coating of zinc is deposited on iron to guard against corrosion.

Electroplating: Sample Problems

Q.1. An electroplating process uses a solution of chromium(III) sulfate, \((\rm{Cr}_2 (\rm{SO}_4)_3)\), to deposit a thin layer of chromium on the surface of an object. A current of \(6.0\,\rm{A}\) is maintained. How long does it take, in seconds, to deposit \(0.0202\,\rm{mol}\) chromium onto the surface?
Ans: Let first write the balanced chemical equation for the reaction that takes place at the cathode
\(\rm{Cr}^{3+} (aq) + 3e = \rm{Cr}(s)\)
Now, the mole of electron needed:-
\(n (e) = 3 × 0.0202 = 0.0606\)
Then, the charge need to be delivered,
\(Q = n(e) F\)
\(⇒ Q = 0.0606 × 96500\)
\(⇒ Q = 5848\,\rm{C}\)
Hence, the time taken in seconds
\(t = \frac{Q}{I}\)
\(⇒ t = \frac{5848}{6}\,\rm{s}\)
\(⇒ t = 975\,\rm{s}\)

Summary

The term “electroplating” refers to the process of coating metal by passing the \(DC\) current through the electrolyte of salt of the metal to be deposited. In electroplating, electric current is used for reducing the dissolved metal cations at the cathode to have a lean coherent metal coating on the electrode. Following conditions are considered for electroplating a normal electrolytic cell.

1. At the negative terminal (cathode): the metal to be coated is placed.
2. At the positive terminal (anode): the metal we wish to coat with is placed.
3. The electrolyte should be the solution of positive metal ions of the metal we wish to coat with. The concentration of the metal ion within the electrolyte stays constant during the electrolysis process.

The amount of metal electroplated is governed by Faraday’s Laws of Electrolysis. According to it, the amount of metal deposited will be proportional to the quantity of electricity passed through the electrolyte. To deposit one equivalent weight of the metal, approximately \(96500\,\rm{C}\) charge needs to be passed.
Electroplating is an additional step that helps extend the lifetime of an object or accessory. It coats the object to protect it from dust or exposure to the elements, e.g., Chromium plating is performed on various objects like car parts, wheel rims, bath taps, etc. Plating of silver or gold is done on less costly metals by jewellers.

FAQs

We have provided some frequently asked questions on Electroplating here:

Q.1. What factors affect electroplating?
Ans: This process is influenced by a variety of factors. Some of these factors include the surface area of electrodes, temperature, the type of metal and electrolyte used, and also the magnitude of the current applied.

Q.2. What are the uses of electroplating?
Ans: In addition to improving the appearance of the substrate, electroplating is used for multiple other purposes. A primary application is to improve the work pieces’ resistance to corrosion. The plated layer will often serve as a sacrificial coating, meaning it will dissolve before the base material.

Q.3. What are the different metals used for electroplating?
Ans: Metals commonly used in electroplating include zinc, copper, and tin, as well as precious metals such as gold, silver, and palladium. Plating can be performed with individual metals or in various combinations (alloys) that can provide additional value to the electroplating process.

Q.4. What is the difference Between Electroplating and Galvanization?
Ans: Electroplating is a type of galvanization. The key difference between electroplating and galvanization is that the electroplating process can be used to apply any suitable metal to an object, whereas galvanization is used to apply a thin zinc layer on the steel.

Q.5. What is the main principle of electroplating?
Ans: Electroplating is the method of depositing one metal over another in the presence of metal salt (in an aqueous solution). During this process, the water molecule is released as the final product. Hence, electroplating relies on the idea of hydrolysis.

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