Preventing corrosion
Removing substances that cause rusting
A corrosion process in which iron or steel reacts with oxygen and water. can be prevented by keeping oxygen or water away from the iron or steel:
- oxygen can be excluded by storing the Shiny element that is a good conductor of electricity and heat, and which forms basic oxides. in an atmosphere of A substance is unreactive or inert if it does not easily take part in chemical reactions. nitrogen or argon
A A substance that absorbs water vapour, helping to keep air or objects dry. is a substance that absorbs water Vapour is a cloud of liquid particles. Steam is water vapour., so it keeps the metal dry.
Physical barriers to oxygen and water
Many methods of rust prevention rely on creating a physical barrier to oxygen and water. These include:
- painting
- oiling and greasing
- coating with plastic
Different methods are used depending on the situation.
Question
Explain why a bike chain is protected from rusting by oiling it, rather than by painting it.
The oil lubricates the chain, helping it to move smoothly. Paint just flakes off when the bike is ridden, exposing the steel chain to air and water again.
Electroplating
Using electrolysis to deposit a thin layer of metal onto another metal, usually to improve its appearance or corrosion resistance. involves using The decomposition (breakdown) of a compound using an electric current. to put a thin layer of a metal on the object:
- the The negative electrode during electrolysis. is the iron or steel object
- the The positive electrode during electrolysis. is the plating metal
- the A substance which, when molten or in solution, will conduct an electric current. contains Electrically charged particle, formed when an atom or molecule gains or loses electrons. of the plating metal
For example, steel cutlery can be electroplated with silver using a silver anode and silver nitrate solution. Electroplating improves the When chemicals in the water dissolve minerals in the rocks, causing them to break up (also called solution). resistance of metal objects.
Sacrificial protection
Iron can be protected from rusting if it is in contact with a more The tendency of a substance to undergo a chemical reaction. metal, such as zinc. The more reactive metal Chemical substances are oxidised by the addition of oxygen, removal of hydrogen or the removal of electrons. more readily than iron, so it ‘sacrifices’ itself while the iron does not rust. Once the A more reactive metal than iron, attached to an iron or steel object to prevent the object rusting. has corroded away, it can simply be replaced.
Worked example
Three nails are left in contact with air and water for a few days. A nail wrapped in magnesium does not rust. A nail alone rusts but a nail wrapped in copper rusts more. Explain these observations.
- Magnesium is more reactive than iron. It oxidises more readily than iron so the nail does not rust.
- Iron is more reactive than copper. This means it oxidises more readily than copper, so it rusts faster than the nail alone.
Galvanising
When iron is coated in zinc, the process is called Coating iron or steel with a layer of zinc to prevent rusting.. The zinc layer stops oxygen and water reaching the iron. Zinc is more reactive than iron, so it also acts as a sacrificial metal. This protection works, even if the zinc layer is scratched.
Question
The inside of a steel food can is electroplated with tin, a less reactive metal than iron. It provides a physical barrier to oxygen and water, stopping the can rusting.
Explain why the inside of the can rusts very quickly if the layer of tin is broken.
Oxygen and water can reach the steel when the layer is broken. Iron is more reactive than tin, so it oxidises more readily than tin. The damaged can rusts faster than if the tin was not there at all.