Transition metals, alloys and corrosion

• Typical properties of transition metals:

  • High melting points.
  • High density.
  • They form coloured compounds.
  • Can act as catalysts.
• Iron (II) hydroxide forms pale green compounds.
• Iron (III) hydroxide forms orange-brown compounds.
• Iron (II) oxide forms red-brown compounds.
• When sodium is cut its shiny surface quickly turns dull as sodium oxide forms, sodium + oxygen -> sodium oxide.

  4Na(s) + O2(g) → 2Na2O(s)
• Corrosion occurs when metals oxidise - becoming weaker overtime.
• Rusting occurs when iron/steel reacts with oxygen + water. Iron + oxygen + water -> hydrated iron (III) oxide.
• Iron/steel objects (like nails) do not rust when oxygen is absent (layer of oil) or water is absent (using a desiccant like calcium chloride).
• Rusting can be avoided by storing the metal in an atmosphere of inept nitrogen or argon.
• Water can be excluded by storing the metal with a desiccant (e.g. calcium chloride) to absorb water vapour.
• Physical barriers to rusting include:

  • Paint.
  • Oiling and greasing.
  • Coating with plastic.
• Electroplating uses electrolysis to put a thin layer of metal on the object:

  • The cathode is the iron/steel object.
  • The anode is the plating metal.
  • The electrolyte contains ions of the plating metal.
• Sacrificial protection is where iron or steel is protected by being covered in a more reactive metal like zinc, so it oxidises instead.
• Galvanising is where iron is coated in zinc - which acts as a sacrificial metal - working even if it is scratched.
• An alloy is a mixture of two or more elements, where at least one is a metal.
• Brass is an alloy of copper and zinc - it and copper resist corrosion and conduct electricity well (copper is better and used in electrical wiring). Brass is stronger so is used for pins in electrical plugs.
• Solid metals have a regular lattice structure thats layers slide over eachother when force is applied. When different size atoms create the lattice it is more difficult for layers to slide around so alloys are very strong as the layers aren’t uniform.
• Alloy steels are produced by mixing iron with one or more other metals.
• Mild steel is iron + carbon. It is malleable and ductile - used for car body parts - protected from corrosion by galvanising and painting.
• Tool steel is iron + tungsten. It is hard and resistant to high temperatures - it is used for drill bits and is not easily damaged by heating or friction.
• Stainless steel is iron and chromium. It is hard and resistant to rusting.
• Magnalium is made of aluminium and magnesium. Aluminium is low density/light weight - magnesium also has this property but is very strong. So it is used for the manufacture of aircraft and ships.
• Aluminium doesn’t react with water and it’s surface is coated with a natural layer of aluminium oxide so resist corrosion.
• Aluminium foil is used as it doesn’t react with substances in food and is very light and malleable.
• Gold is very soft and malleable - and very unreactive - so it is used in jewellery (where it is mixed with copper to make stronger) as it resists corrosion, staying shiny.