SC13- transition metals, alloys and corrosion

• typical physical properties of transition metals are:

  1. high melting point
  2. high density
  3. malleable and ductile
  4. good electrical conductors

  typical chemical properties include:

  1. form coloured compounds
  2. used as catalysts
• corrosion is when metals react with substances in the environment and become weaker.

  oxidation of metals results in corrosion- eg iron oxide is weaker than iron.
• some metals (copper and aluminium) do not corrode as their surfaces form a protective oxide layer (tarnish), preventing further reactions.
• corrosion can be prevented by:

  • storing the metal in an unreactive atmosphere
  • coating the metal with paint, plastic, oil or grease
• the corrosion of iron is called rusting- steel can also rust as it is an alloy of iron.

  rusting requires water as well as oxygen.
• rusting can be prevented by:

  using desiccant powder that absorbs water vapour

  • sacrificial protection- more reactive metal is attached to the object, oxygen reacts with it instead and protects iron until sacrificial metal corrodes away.
• electroplating is the use of electricity to cover one metal in a thin layer of another metal.

  it has two main uses:

  • make things more attractive
  • protect against corrosion
• electroplating is performed using electrolysis:

  • attach copper ring to cathode
  • attach pure silver to anode
  • use silver compound as the electrolyte
  • at the anode, atoms in silver plating metals lose electrons and are attracted to the cathode where they become silver atoms after being reduced.
  • silver is deposited on the copper ring.
  • longer current flows- thicker the layer of silver.
• an alloy is a mixture of one metal with another element.

  • steel is an alloy of iron and carbon
  • bronze is an alloy of iron and copper
  • brass is an alloy of copper and zinc
• in a pure metal, positive ions are arranged in neat layers which slide over each other, meaning they are malleable and ductile.

  in alloys, positive ions are different sizes so they cannot slide over each other, so they are stronger and less malleable.
• alloy steels are different alloys of iron.

  • mild steel is iron with a small amount of carbon and manganese, it is still malleable so used for building materials.
  • tool steel is iron with tungsten and molybdenum, it is very strong and used in drill bits.
  • stainless steel contains chromium, and forms a thin and transparent tarnish when oxidised so metal is still shiny.
• uses of different alloys/metals include:

  • aluminium is strong, cheap and conducts electricity so it is used for overhead wires.
  • copper is malleable, ductile and conducts electricity so it is used for wires.
  • gold is shiny, malleable and ductile so it is used for connecters in cables and jewellery.
  • magnalium is stronger and less dense than aluminium, so it is used for light weight metal parts and aircrafts.
  • brass is a good conductor and strong, so it is used for pins in electrical plugs.