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
- water can be excluded by storing the metal with a A substance that absorbs water vapour, helping to keep air or objects dry. such as calcium chloride
A desiccant is a substance that absorbs 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 also 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.
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 鈥榮acrifices鈥 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.
Question
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
Write a half equation to show the oxidation of zinc to form zinc ions. (Higher tier only)
Zn 鈫 Zn2+ + 2e-