TOPIC 12 Corrosion

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

Two factors are required for Corrosion 1. Oxygen: 2. Water: It helps to carry the electrons away from the metal. Gives the electrons a medium to travel through on their way away from the metal.

Half-Equations Corrosion of Iron is a three step process Corrosion of Iron is a three step process First Stage Fe First Stage Fe Fe 2+ +2e The test for Fe 2+ ions is that it turns Ferroxyl Indicator from Green to Dark Blue Second stage: Fe 2+ Fe 3+ +e Third Stage: The Fe 3+ ions combine with oxygen from the Air and Water to form Iron Oxide Fe 2 O 3 which is rust

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

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

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

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