Corrosion

• Metals

  • Shiny element that is a good conductor of electricity and heat, and which forms basic oxides
• Oxidise

  Chemical substances are oxidised by the addition of oxygen, removal of hydrogen or the removal of electrons
• Reactive metals


  • Sodium
• Reaction of sodium with oxygen

  Sodium + oxygen → sodium oxide 4Na(s) + O2(g) → 2Na2O(s)
• Corrosion

  When a metal continues to oxidise, becoming weaker over time and eventually turning into metal oxide
• Rusting


  • A corrosion process in which iron or steel reacts with oxygen and water
• Rusting of iron or steel

  Iron + oxygen + water → hydrated iron(III) oxide
• An iron or steel object only rusts if oxygen and water are present
• Experiment showing rusting


  Calcium chloride absorbs water, nail rusts in the presence of oxygen and water
• Iron is oxidised when it forms rust because it gains oxygen during rusting
• Where can metals be oxidised?

  In air
• What do metals form when they react with oxygen?


  Metal oxides
• How can we tell a metal is oxidised by its appearance?


  Its freshly exposed shiny surface turns dull (as a thin layer of metal oxide is formed)
• What type of metals will not react with oxygen in the air?


  Unreactive metals such as gold
• What is corrosion?

  Corrosion happens when a metal continues to oxidise as it will become weaker over time, eventually all of ir may become metal oxide
• Give an example of corrosion

  Rusting
• When does rusting occur?


  When iron OR steel reacts with oxygen AND water

  iron+oxygen+water->hydrated iron (III) oxide
• Explain the rusting experiment?


  The iron nail only rusts in the left-hand test tube as it contains BOTH oxygen and water

  • Middle: has water but NO oxygen (oil prevents air from entering the test tube)
  • Right: has oxygen (air) but NO water (calcium chloride absorbs the water in the test tube)

  
• Generally, how can rusting of metals be prevented?

  • Keeping oxygen OR water AWAY from iron or steel
  • Oxygen: can be excluded by storing the metal in an atmosphere of unreactive nitrogen/ argon
  • Water: can be absorbed by desiccant, which keeps the metal dry
• Name three examples of physical barriers to oxygen and water?

  
  

  • Painting
  • Oiling (greasing)
  • Coating (with plastic)
• Explain electroplating.

  • using electrolysis to put a thin layer of a metal on an object ->improves the corrosion resistance of metal objects
  • Cathode (-ve): iron/steel object
  • Anode (+ve): plating metal
  • Electrolyte: contains ions of the plating metal
  • eg: Electroplate steel cutlery with silver using silver anode with silver nitrate solution
• Explain sacrificial protection.

  • Protect iron from rusting by putting it in contact with a more reactive metal (eg: zinc)
  • The more reactive metal oxidises more readily than iron, which ‘sacrifices’ itself while the iron does not rust
  • Once the sacrificial metal has corroded away, it can simply be replaced
• Sacrificial protection experiment

  Three iron nails are left in contact with water and air for a few days, predict what will happen.

  • The iron nail will rust as it is in contact with water and air
  • The nail wrapped with magnesium will not rust as magnesium is more reactive than iron, which oxidises more readily than iron so the nail will not rust
  • The nail wrapped with copper will rust the most as copper is less reactive than iron, which iron oxidises more readily than copper, thus the nail will rust faster than the iron nail

  
• Explain Galvanising

  • Galvanisation: iron is coated in zinc
  • Zinc layer stops oxygen and water reaching the iron
  • Zinc is more reactive than iron ->also acts as sacrificial metal
  • This protection works even if the zinc layer is scratched
• Desiccant


  substance that absorbs water vapour