Corrosion occurs when a metal loses electrons and forms positive metal ions, reacting with oxygen and water. For iron, this forms rust. Corrosion happens faster in the presence of salt, pollutants, and heat, as they help carry electrons away from the metal. Common prevention methods include barriers like painting or greasing, sacrificial protection using more reactive metals, cathodic protection with batteries, and coating metals like galvanizing or electroplating.

1. TOPIC 12TOPIC 12
Corrosion

2. CorrosionCorrosion
 When a metal corrodes it loses electronsWhen a metal corrodes it loses electrons
and forms positive metal ions.and forms positive metal ions.
When iron metal corrodes it forms an IronWhen iron metal corrodes it forms an Iron
oxide layer, which is better known as rust.oxide layer, which is better known as rust.
Only Iron rusts all other metals corrodeOnly Iron rusts all other metals corrode

3. Two factors are required forTwo factors are required for
CorrosionCorrosion
1.1. Oxygen:Oxygen:
2. Water:2. Water:
It helps to carry the electrons away from the metal.It helps to carry the electrons away from the metal.
Gives the electrons a medium to travel throughGives the electrons a medium to travel through
on their way away from the metal.on their way away from the metal.

4. Half-EquationsHalf-Equations
 Corrosion of Iron is a three step processCorrosion of Iron is a three step process
 First StageFirst Stage FeFe Fe2+
+2e
The test for
Fe2+
ions is that it
turns Ferroxyl
Indicator from
Green to Dark
Blue
Second stage: Fe2+
Fe3+
+e
Third Stage:
The Fe3+
ions combine with oxygen
from the Air and Water to form Iron
Oxide Fe2O3 which is rust

5. Three things speed up the rate ofThree things speed up the rate of
CorrosionCorrosion
SaltSalt: salt contains Ions which help to carry the: salt contains Ions which help to carry the
electrons away from the metal. Glucose orelectrons away from the metal. Glucose or
ethanol do not contain ions.ethanol do not contain ions.
PollutantsPollutants: Gases in the air mix with: Gases in the air mix with
moisture, form acids which then attack themoisture, form acids which then attack the
metal.metal.
Heat:Heat: Electrons have more energy and getElectrons have more energy and get
carried away from the metal quicker.carried away from the metal quicker.

6. Factors which prevent corrosionFactors which prevent corrosion
Barrier or Physical methods:Barrier or Physical methods:
Painting or greasing stopsPainting or greasing stops
oxygen and water gettingoxygen and water getting
at the underlying metal.at the underlying metal.
This is probably theThis is probably the
easiest and cheapesteasiest and cheapest
method.method.

7. Chemical methods of protectionChemical methods of protection
Sacrificial Protection: Involves two metals. The metal
higher up in the Electrochemical Series donates
electrons the metal lower down thus stopping it from
corroding. As a result it corrodes quicker itself than it
normally would.
Cathodic Protection: The metal to be protected is
attached to the negative end of a battery resulting in a
constant flow of electrons to metal thus preventing
corrosion.

8. Chemical methods (continued)Chemical methods (continued)
 Galvanising:Galvanising:
 This involves dipping the metal inThis involves dipping the metal in Zinc.Zinc.ThisThis
is good for two reasons as it acts like a barrieris good for two reasons as it acts like a barrier
method (with the zinc coating stoppingmethod (with the zinc coating stopping
oxygen and water getting at the underlyingoxygen and water getting at the underlying
metal). If the metal is scratched, thenmetal). If the metal is scratched, then
sacrificial protection takes over as the Zinc issacrificial protection takes over as the Zinc is
higher in the electrochemical series than thehigher in the electrochemical series than the
metal it is coating i.e.. The zinc gives themetal it is coating i.e.. The zinc gives the
lower metal a constant supply of electrons.lower metal a constant supply of electrons.

9. Chemical ProtectionChemical Protection (continued)(continued)
 ElectroplatingElectroplating: Using electrical methods to coat: Using electrical methods to coat
the main metal to be protected with a thin layerthe main metal to be protected with a thin layer
of of another. Common examples are: Gold –of of another. Common examples are: Gold –
plating, Silver- plating and Chrome – plating.plating, Silver- plating and Chrome – plating.

10. Chemical ProtectionChemical Protection (continued)(continued)
 ElectroplatingElectroplating: Using electrical methods to coat: Using electrical methods to coat
the main metal to be protected with a thin layerthe main metal to be protected with a thin layer
of of another. Common examples are: Gold –of of another. Common examples are: Gold –
plating, Silver- plating and Chrome – plating.plating, Silver- plating and Chrome – plating.