Why objects are electroplated
Definition
Electroplating
The use of electrolysis to coat a metal object with a thin layer of another metal.
Definition
Corrosion
The gradual destruction of a metal as it reacts with substances around it, such as oxygen and water.
- Electroplating uses electrolysis to cover a metal object with a thin, even layer of a different metal.
- The coating is often only a few thousandths of a millimetre thick, so very little of an expensive metal is needed.
- Metal objects are electroplated for two main reasons.
- To improve their appearance: a shiny coating of a metal such as silver, gold or chromium makes an object look attractive and more valuable.
- To improve their resistance to corrosion: a coating of a metal that does not corrode easily stops air and water reaching the metal underneath.
- Electroplating lets a cheap, strong metal such as steel form the body of an object while a thin layer of a costly or decorative metal forms its surface.
- Common examples of electroplating are shown below.
- Silver plating: cutlery and trophies made from cheaper alloys are coated with silver so that they shine like solid silver.
- Chromium plating: steel taps, bicycle handlebars and car trims are coated with chromium, which is hard, shiny and resists corrosion.
- Gold plating: jewellery and electrical connectors are coated with gold, which looks attractive and does not tarnish.
- Tin plating: steel food cans are coated with tin, which stops the steel rusting and does not react with the food.
- Nickel plating: steel objects are coated with nickel to protect them from corrosion, often as a base layer underneath chromium.
Setting up an electroplating cell
- Electroplating is carried out in an electrolytic cell connected to a d.c. power supply.
- The object to be plated is made the cathode, so it is connected to the negative terminal.
- A piece of the plating metal is made the anode, so it is connected to the positive terminal.
- The electrolyte is an aqueous solution of a soluble compound of the plating metal, so it contains positive ions of that metal.
- To plate with copper, the anode is copper and the electrolyte is aqueous copper(II) sulfate.
- To plate with nickel, the anode is nickel and the electrolyte is aqueous nickel(II) sulfate.
- To plate with silver, the anode is silver and the electrolyte is a solution containing silver ions.
- The object must be cleaned thoroughly before plating, because grease, dirt or a layer of oxide on its surface stops the coating sticking evenly.
- The object and the anode are placed facing each other in the electrolyte without touching.
How the coating forms
- When the current is switched on, the positive metal ions in the electrolyte are attracted to the negative object.
- At the surface of the object, the metal ions are turned into metal atoms, which build up as a thin coating.
- At the same time, the metal anode slowly dissolves, releasing new metal ions into the electrolyte.
- The ions used up at the cathode are replaced by ions from the anode, so the concentration of the electrolyte stays about the same.
- Metal is therefore transferred from the anode to the object, so the anode loses mass and the object gains mass.
- A small current passed for a longer time gives a smooth, even coating that sticks firmly.
- A large current deposits the metal too quickly, giving a rough or powdery coating that flakes off easily.
- The object is often turned during plating so that every side faces the anode and receives an even coating.
- A coating only protects against corrosion while it stays unbroken, because a scratch lets air and water reach the metal underneath.
Practical
Copper plating a key
- Aim: to coat a brass key with copper by electrolysis and measure the mass of copper deposited.
- Apparatus: a small brass key, a strip of copper foil, a 250 cm3250\ \text{cm}^3250 cm3 beaker, about 150 cm3150\ \text{cm}^3150 cm3 of aqueous copper(II) sulfate, a low-voltage d.c. power pack, an ammeter, a variable resistor, connecting leads with crocodile clips, emery paper, washing-up liquid, distilled water, paper towels, forceps, a balance reading to 0.01 g0.01\ \text{g}0.01 g and eye protection.
- Method:
- Rub the key and the copper strip with emery paper until both are bright, which removes any surface oxide.
- Wash the key in warm water with a little washing-up liquid to remove grease, rinse it with distilled water and from then on handle it only with forceps.
- Dry the key and weigh it, recording the mass to 0.01 g0.01\ \text{g}0.01 g.
- Pour the copper(II) sulfate solution into the beaker and hang the key and the copper strip on opposite sides, facing each other and not touching.
- Connect the key to the negative terminal and the copper strip to the positive terminal, with the ammeter and the variable resistor in series.
- Switch on and adjust the variable resistor until the current is about 0.2 A0.2\ \text{A}0.2 A, then leave the cell running for 202020 minutes while keeping the current steady.
- Switch off, lift the key out, rinse it gently with distilled water and leave it to dry without rubbing its surface.
- Reweigh the key and subtract the starting mass to find the mass of copper deposited.
- Results:
- The key becomes covered in a pink-brown layer of copper and its mass increases.
- The copper strip becomes thinner and loses mass.
- The blue colour of the solution stays the same, because the copper ions removed are replaced from the anode.
- Watch out:
- A greasy or tarnished key gives a patchy coating that rubs off easily.
- A current much larger than about 0.5 A0.5\ \text{A}0.5 A gives a dark, powdery deposit that does not stick.
- Wiping the key dry can remove some of the coating and make the mass gain too small.
- Safety: wear eye protection, avoid skin contact with copper(II) sulfate solution because it is harmful and an irritant, and keep the voltage low.
Example
Silver plating a spoon
- A spoon made from a cheap nickel alloy is to be given a silver surface.
- The spoon is cleaned to remove grease and is then connected to the negative terminal, making it the cathode.
- A bar of pure silver is connected to the positive terminal, making it the anode.
- The electrolyte is a solution containing silver ions, Ag+\text{Ag}^{+}Ag+.
- Silver ions move to the spoon and form a thin, shiny layer of silver on its surface.
- The silver anode dissolves and replaces the silver ions used, so the same electrolyte can be used again and again.
- The finished spoon looks like solid silver for a small fraction of the cost.
Common Mistake
- Do not connect the object to the positive terminal, because the coating only forms on the cathode.
- Do not choose an electrolyte that lacks ions of the plating metal, such as sodium chloride solution for copper plating.
- Do not say that electroplating makes an object stronger, because its purposes are to improve appearance and resistance to corrosion.
Exam technique
Describing electroplating
- Describe electroplating with three points: the object is the cathode, the plating metal is the anode, and the electrolyte contains ions of the plating metal.
- When a question names a metal, use that metal throughout your answer, for example a nickel anode and aqueous nickel(II) sulfate.
- When asked why objects are electroplated, give both reasons: to improve appearance and to improve resistance to corrosion.
Self review
- Give two reasons why metal objects are electroplated.
- Which electrode is the object to be plated, and which terminal is it connected to?
- What must the electrolyte contain when an object is plated with nickel?
- Why does the anode lose mass during copper plating?
- Why does a small current give a better coating than a large current?
