corrosion

• corrosion

  destruction of materuals by chemical substanes in environment
• rusting:

  • rusting is corrosion of iron
  • iron corrodes easily
  • iron + oxygen + water -> hydrated iron oxide
  • iron is oxidised
  • rust flakes off to leave more iron available to rust - all iron in object corrodes
• • corrosion only happens on surface of material when it is exposed to air
• • aluminium corrodes when exposed to air
  • objects made from aluminium not completely destroyed by corrosion
  • aluminium oxide formed during corrosion, but doesn't flake away
  • forms layer of protection that stops any further reaction
• • air and water are needed for iron to rust
  • placing nail in boiling tube with just water, it won't rust
  • water boiled to remove oxygen and oil used to stop air getting in
  • placing nail in boiling tube with air, it won't rust
  • calcium chloride used to absorb any water from air
• • placing iron nail in boiling tube with air and water will cause it to rust
  • the mass of rusty nail will increase
  • iron atoms in nail bond to oxygen and water molecules
  • compound is heavier than iron
• • coating iron in a barrier can be used to prevent rusting
  • painting or coating with plastic can be used
  • greasing is used when moving parts are involved
  • electroplating uses electrolysis to reduce metal ions on iron electrode
• sacrifical protection:

  • placing more reactive metal with iron
  • water and oxygen react with sacrifical metal instead of iron
• • zinc is used to galvanise iron
  • zinc layer is firstly protective
  • if zinc is stratched, zinc around stratch works as sacrificial metal