TOPIC 12TOPIC 12

Corrosion

CorrosionCorrosion

When a metal corrodes it loses electrons When a metal corrodes it loses electrons and forms positive metal ions.and forms positive metal ions.

When iron metal corrodes it forms an Iron When 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

Two factors are required for Two factors are required for Corrosion Corrosion

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 through Gives the electrons a medium to travel through on their way away from the metal.on their way away from the metal.

Half-EquationsHalf-Equations Corrosion of Iron is a three step processCorrosion of Iron is a three step process First StageFirst Stage Fe Fe 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

Three things speed up the rate of Three 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 or electrons 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 the moisture, form acids which then attack the metal.metal.

Heat:Heat: Electrons have more energy and get Electrons have more energy and get carried away from the metal quicker.carried away from the metal quicker.

Factors which prevent corrosionFactors which prevent corrosion

Barrier or Physical methods:Barrier or Physical methods:

Painting or greasing stops Painting or greasing stops oxygen and water getting oxygen and water getting at the underlying metal. at the underlying metal. This is probably the This is probably the easiest and cheapest easiest and cheapest method.method.

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.

Chemical methods (continued)Chemical methods (continued)

Galvanising:Galvanising: This involves dipping the metal in This involves dipping the metal in Zinc.Zinc.This This

is good for two reasons as it acts like a barrier is good for two reasons as it acts like a barrier method (with the zinc coating stopping method (with the zinc coating stopping oxygen and water getting at the underlying oxygen and water getting at the underlying metal). If the metal is scratched, then metal). If the metal is scratched, then sacrificial protection takes over as the Zinc is sacrificial protection takes over as the Zinc is higher in the electrochemical series than the higher in the electrochemical series than the metal it is coating i.e.. The zinc gives the metal it is coating i.e.. The zinc gives the lower metal a constant supply of electrons. lower metal a constant supply of electrons.

Chemical Protection Chemical Protection (continued)(continued)

ElectroplatingElectroplating: Using electrical methods to coat : Using electrical methods to coat the main metal to be protected with a thin layer the 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.