What is corrosion
The process by which metals are slowly broken down by reacting with substances in their environment eg rusting
Describe the method to identity which conditions are required for rusting
Test tube 1: OXYGEN + WATER
- iron nail in DISTILLED water
- open to air
- will RUST
Test tube 2: WATER, NO OXYGEN
- iron nail in distilled water (boiled to remove oxygen)
- layer of oil (which stops air from dissolving into it)
Test tube 3: OXYGEN, NO WATER
- iron nail with calcium chloride (which removes any water vapour from the air)
- open to air
• leave test tubes for several days
• look for any changes
Variables for the method of determining the conditions for rusting (3 test tubes)
IV: changing the conditions
DV: does the nail rust or not
CV: size of nail, type of metal (iron), test tube, time
Meaning of brittle
Hard material but liable to break easily
Meaning of alloy
a mixture of two or more elements, where at least one element is a metal. Many alloys are mixtures of two or more metals. They make the material stronger.
Meaning of rusting
an example of corrosion. It occurs when iron or steel reacts with oxygen and water:
iron + oxygen + water → hydrated iron(III) oxide
Meaning of galvanisation
When iron is coated in zinc.
- 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.
What are the three things that make iron rust faster
Salt
Oxygen
Water (moisture)
Why does aluminium oxide protect the aluminium metal?
Aluminium oxide forms an insoluble layer which stops oxygen and water from reaching the aluminium, protecting it from further corrosion
What are the six ways to reduce rusting? What do they all do?
- apply a barrier coating such as paint
- oil/grease
- electroplating
- galvanising
- plastic
- sacrificial protection
Stops both oxygen and water from reaching the surface
examples of barrier methods of protection against corrosion
painting, greasing, electroplating iron
Meaning of sacrificial protection
- Iron can be protected from rusting if it is in contact with a more reactive metal, such as zinc.
- The more reactive metal oxidises more readily than iron, so it ‘sacrifices’ itself while the iron does not rust.
- Once the sacrificial metal has corroded away, it can simply be replaced.
2 examples of where sacrificial protection is used
Ship hulls
Underground pipes
Why does an iron pipe buried in clay soil rust more quickly than one buried in sandy soil?
Clay holds moisture
Modern cars often have engines made of aluminium. Explain why such engines are never bolted directly onto the steel shell of the car?
The two metals will form a galvanising reaction if they touch, making the aluminium corroded faster
Why is a piece of magnesium connected to the underground petrol tanks at filling stations?
The magnesium acts as a sacrificial metal as it’s more reactive than iron, so it corrodes instead of the steel tank rusting
Explain why rusting is considered to be a redox reaction.
Since iron loses electrons (oxidation) and oxygen gains electrons (reduction)
Why does a piece of iron connected to the negative terminal of a battery never rust?
it is connected to the negative terminal, the iron gains electrons instead of losing them, so it cannot be oxidised and therefore doesn’t rust.
Why do iron or steel objects near the sea rust more quickly than those found inland?
because sea water contains salt, which makes it easier for water and oxygen to react with the metal
The term rusting is used to describe the…
corrosion of iron and iron alloys. Many other metals corrode, but only iron and its alloys are said to rust
rust is formed when iron objects react with…
oxygen in the presence of water or moisture
if oxygen or water can be completely removed then…
rusting will not occur
different forms of rust are made depending on the…
conditions present.
rusting is undesirable and over time iron objects can become…
brittle and crumbly and can eventually disintegrate
