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Corrosion

Corrosion. 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. Two factors are required for Corrosion .

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Corrosion

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  1. Corrosion • 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

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

  3. Half-Equations • Corrosion of Iron is a three step process • First Stage Fe Fe2+ +2e The test for Fe2+ions is that it turns Ferroxyl Indicator from Green to Dark Blue Fe2+ Fe3+ +e Second stage: Third Stage: The test for the Fe3+ ion is that it turns Ammoniun ThiocyanateRed The Fe3+ ions combine with oxygen from the Air and Water to form Iron OxideFe2O3which is rust

  4. 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. • Heat: Electrons have more energy and get carried away from the metal quicker. • Pollutants: Gases in the air mix with moisture, form acids which then attack the metal.

  5. 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.

  6. Chemical methods of protection • 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. • 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.

  7. Chemical methods (continued) • 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.

  8. Anodising: This used to protect Aluminium from corroding. Unusually it works by increasing the thickness of the oxide layer covering the Al metal. It is generally done by electrolysing the Al in bath of sulphuric acid. 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. Chemical Protection(contd)

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