gcse Useful metals, uses aluminium anodising, uses of steel alloys, titanium extraction and production igcse O Level chemistry revision notes

4. How can metals be made more useful? e.g. iron and aluminium

Metals can be mixed together to make alloys to improve the metal's properties to better suit a particular purpose. Quite often the presence of different atoms stops the layers of the metal sliding over each other when stressed so making the metal tougher (see Metal Structure for more details).

Pure copper, lead, gold, iron and aluminium are too soft for many uses and so are mixed with small amounts of similar metals to make them harder for everyday use.

Aluminium can be made more resistant to corrosion by a process called anodising. Iron can be made more useful by mixing it with other substances to make various types of steel. Many metals can be given a coating of a different metal to protect them or to improve their appearance.

Aluminium is a reactive metal but it is resistant to corrosion. This is because aluminium reacts in air to form a layer of aluminium oxide which then protects the aluminium from further attack.
- This is why it appears to be less reactive than its position in the reactivity series of metals would predict.
For some uses of aluminium it is desirable to increase artificially the thickness of the protective oxide layer in a process is called anodising.
- This involves removing the oxide layer by treating the aluminium sheet with sodium hydroxide solution.
- The aluminium is then placed in dilute sulphuric acid and is made the positive electrode (anode) used in the electrolysis of the acid.
- Oxygen forms on the surface of the aluminium and reacts with the aluminium metal to form a thicker protective oxide layer.
Aluminium can be alloyed to make 'Duralumin' by adding copper (and smaller amounts of magnesium, silicon and iron), to make a stronger alloy used in aircraft components (low density = 'lighter'!), greenhouse and window frames (good anti-corrosion properties), overhead power lines (quite a good conductor and 'light'), but steel strands are included to make the 'line' stronger and poorly electrical conducting ceramic materials are used to insulate the wires from the pylons and the ground.
- There is a note about the bonding and structure of alloys on another page.
The properties of iron can be altered by adding small quantities of other metals or carbon to make steel.
Steels are alloys since they are mixtures of iron with other metals or with non-metals like carbon or silicon.
- Most metals in everyday use are alloys.
- Iron from the blast furnace contains about ~96% iron with ~4% of impurities including carbon, silica and phosphorus.
- In this state the cast iron is too hard and too brittle for most purposes.
- Cast iron is hard and can be used directly for some purposes eg manhole covers because of its strength in compression.
- However, if all the impurities are removed, the resulting very pure iron is too soft for any useful purpose.
- Therefore, strong useful steel is made by controlling the amount of carbon and selected metals to produce an alloy mixture with the right physical properties fit for a particular application e.g. steel for car bodies, chrome stainless steel, extremely hard and tough tungsten-iron steel alloys etc.
Making Steel:
- (1) Molten iron from the blast furnace is mixed with recycled scrap iron
- (2) Then pure oxygen is passed into the mixture and the non-metal impurities such as silicon or phosphorus are then converted into acidic oxides (oxidation process) ..
  - e.g. Si + O₂ ==> SiO₂, or 4P + 5O₂ ==> P₄O₁₀
- (3) Calcium carbonate (a base) is then added to remove the acidic oxide impurities (in an acid-base reaction). The salts produced by this reaction form a slag which can be tapped off separately.
  - e.g. CaCO₃ + SiO₂ ==> CaSiO₃ + CO₂ (calcium silicate slag)
- Reactions (1)-(3) produce pure iron.
- Calculated quantities of carbon and/or other metallic elements such as titanium, manganese or chromium are then added to make a wide range of steels with particular properties.
- Because of the high temperatures the mixture is stirred by bubbling in unreactive argon gas!
- Economics of recycling scrap steel or ion: Most steel consists of >25% recycled iron/steel and you do have the 'scrap' collection costs and problems with varying steel composition* BUT you save enormously because there is no mining cost or overseas transport costs AND less junk lying around! (NOTE: * some companies send their own scrap to be mixed with the next batch of 'specialised' steel they order, this saves both companies money!)
Different steels for different uses:
- High % carbon steel is strong but brittle.
- Low carbon steel or mild steel is softer and is easily shaped and pressed e.g. into a motor car body.
- Stainless steel alloys contain chromium and nickel and are tougher and more resistant to corrosion.
- Very strong steels can be made by alloying the iron with titanium or manganese metal.

There is a note about the bonding and structure of alloys on another page.

However, apart from expensive stainless steels using chromium and nickel mixed with iron, most steel alloys readily rusting leading to potential structural weakness and failure or the extra costs involved in protecting the steel.

Steel can be galvanised by coating in zinc, this is physically done by dipping the object into a bath of molten zinc. On removal and cooling a thin layer of zinc is left on. The zinc chemically bonds to the iron via the free electrons of both metals - its all the same atoms to them! It can also be done by electroplating (details below).

Steel (and most metals) can be electroplated.
- The steel object to be plated is made the negative electrode (cathode) and placed in a solution containing ions of the plating metal.
- The positive electrode (anode) is made of the pure plating metal (which dissolves and forms the fresh deposit on the negative electrode).
- Nickel, zinc, copper, silver and gold are examples of plating metals.
- The details of copper purification amount to copper plating, so all you have to do is swap the pure negative copper cathode with the metal you want to coat (e.g. Ni, Ag or Au or any material with a conducting surface). Swap the impure positive copper anode with a pure block of the metal you want to form the coating layer. The electrodes dip into a salt solution of nickel, zinc, copper, silver or gold ions etc. and a low d.c. voltage passed through. If M = Ni, Cu, Zn ....
  - At the positive (+) anode, the process is an oxidation, electron loss, as the metal atoms dissolve to form metal(II) ions.
  - M_(s) ==> M²⁺_(aq) + 2e^-
  - at the negative (-) cathode, the process is a reduction, two electron gain by the attracted metal(II) ions to form neutral metal atoms on the surface of the metal being coated.
  - M²⁺_(aq) + 2e^- ==> M_(s)
  - For silver plating it is Ag⁺, Ag and a single electron change.
  - Any conducting (usually metal) object can be electroplated with copper or silver for aesthetic reasons or steel with zinc or chromium as anti-corrosion protective layer.
Many other metals have countless uses e.g. zinc
- Zinc is used to make the outer casing of zinc-carbon-weak acid batteries.
- Zinc is alloyed with copper to make the useful metal brass (electrical plug pins). Brass alloy is stronger and more hardwearing than copper AND not as brittle as zinc.
See also extraction/recycling of iron and extraction/recycling of aluminium
Titanium manufacture and uses are described below in section 5. below
More GCSE/IGCSE Chemistry notes on metals:







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