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MAKING METALS MORE USEFUL

Doc Brown's KS4 Science GCSE/IGCSE/O Level Industrial Chemistry Revision Notes

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

and 5. Titanium – how it is produced? and what is it used for?

Metals have a wide range of uses but quite often the pure metal is not as useful as when it is mixed with other metals and non–metals to make alloys. The alloy uses of aluminium are described and explained e.g. the uses of duralumin. How is steel made? What do use steel for? Why are so many different steel alloys made? What is titanium like? How is titanium made? What is it manufactured for? What is titanium metal used for?

See also SHAPE MEMORY ALLOYS e.g. Nitinol & Magnetic Shape Memory Alloys

(c) doc bIndex of sections: 1. Limestone, lime – uses, thermal decomposition of carbonates, hydroxides and nitrates  *  2. Enzymes and Biotechnology  *  3. Contact Process, the importance of sulphuric acid  *  4. How can metals be made more useful? (alloys of Al, Fe, steel etc.) * 5. The importance of titanium  *  6. Instrumental Methods of Chemical Analysis * 7. Chemical economics of processes and sociological and environmental issues etc.

(c) doc band other web pages of industrial chemistry notes: Ammonia synthesis/uses/fertilisers * Oil Products * Extraction of MetalsHalogens – sodium chloride Electrolysis * Transition Metals * Extra Electrochemistry

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

 See also Extraction of Aluminum details page

An alloy is a mixture of a metal with other elements (metals or non–metals). Metals can be mixed together to make alloys to improve the metal's properties to better suit a particular purpose. An alloy mixture often has superior desired properties compared to the pure metal or metals i.e. the alloy has its own unique properties and a more useful metal. 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. These mixtures are called alloys and have range of properties that can be tailored to use for specific purposes i.e. match the properties of the alloy to its function.

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 alloys. Many metals can be given a coating of a different metal, or painted, to protect them or to improve their appearance.

  • Aluminium is theoretically 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 and subsequently doesn't react with water and only reacts very slowly with acids.

    • 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 as 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 – anodized.

    • The aluminium oxide layer doesn't flake off like rust does from iron or steel exposing more aluminium to corrosion.

  • Aluminium can be alloyed to make 'Duralumin' by adding copper (and smaller amounts of magnesium, silicon and iron), to make a stronger alloy.

    • Duralumin is 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 (c) doc b chemical bonding and the structure of pure metals/alloys

    • Steel or aluminium for making car bodies?

      • Aluminium is much more costly to produce than steel.

      • BUT aluminium is less dense (lighter) than steel and saves on fuel and therefore the car economy.

      • ALSO, aluminium car bodies will not corrode like steel and will therefore last longer.

      • Overall it appears at the present time that steel car bodies are used more than aluminium ones.

  • 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.

    • The real importance of alloys is that they can be designed to have properties for specific uses.

    • Steel alloys of varying strength and anti-corrosion properties are used in thousands of products and constructions e.g. reinforcing rods in concrete buildings, bridge girders, car engines, domestic appliances from washing machines to electric kettles, saucepans, tools like chisels, ship hulls and superstructure, very hard drill bits,

    • eg low–carbon steels are easily shaped for car bodies, high–carbon steels are hard, and stainless steels are resistant to corrosion etc. and in both cases steel has superior properties compared to iron.

    • Although the metals used in construction are strong, in some situations they may become dangerously weak e.g.

      • If iron or steel becomes badly corroded, there is no strength in rust!, and, the thicker the rust layer, the thinner and weaker the supporting iron or steel layer, hence the possibility of structural failure. Therefore, most iron and steel structures exposed to the outside weather are maintained with a good coating of paint.

      • Also, if metal structures e.g. in aircraft or bridges, are continually strained under stress, the crystal structure of the metal can change so it becomes brittle. This effect is called metal fatigue (stress fractures) and may lead to a very dangerous situation of mechanical failure of the structure.

      • So it is important develop alloys which are well designed, well tested and will last the expected lifetime of the structure whether it be part of an aircraft (eg titanium aircraft frame) or a part of a bridge (eg steel suspension cables).

      • See notes on Corrosion of Metals and Rust Prevention

  • Making Steel:

    • (1) Molten iron from the (c) doc b 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 than iron and more resistant to corrosion than pure iron.

    • Very strong steels can be made by alloying the iron with titanium or manganese metal.

  • There is a note about the (c) doc b 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.

  • Top of page - sub-index and linksSteel 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).

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5. What is titanium and how is it produced ?

Titanium is a very important metal for various specialised uses. It is more difficult  to extract from its ore than other, more common metals.

  • Titanium is a transition metal of low density ('light'), strong and resistant to corrosion.

    • Titanium alloys are amongst the strongest lightest of metal alloys and used in aircraft production.

    • There is a note about (c) doc b the bonding and structure of alloys on another page.

    • As well as its use in aeroplanes it is an important component in nuclear reactor alloys and for replacement hip joints because of its light and strong nature AND it doesn't easily corrode.

    • It is one of the main components of Nitinol 'smart' alloys. Nitinol belongs to a group of shape memory alloys (SMA) which can 'remember their original shape'. For example they can regain there original shape on heating (e.g. used in thermostats in cookers , coffer makers etc.) or after release of a physical stress (e.g. used in 'bendable' eyeglass frames, very handy if you tread on them!). The other main metal used in these very useful intermetallic compounds is nickel.

  • Titanium is extracted from the raw material is the ore rutile which contains titanium dioxide.

  • The rutile titanium oxide ore is heated with carbon and chlorine to make titanium chloride

    • TiO2 + 2Cl2 + C ==> TiCl4 + CO2

  • After the oxide is converted into titanium chloride TiCl4, it is then reacted with sodium or magnesium to form titanium metal and sodium chloride or magnesium Chloride. This is an expensive process because sodium or magnesium are manufactured by the costly process of  electrolysis (electricity is the most costly form of energy).

    • This reaction is carried out in an atmosphere of inert argon gas so none of the metals involved becomes oxidised by atmospheric oxygen.

    • TiCl4 + 2Mg ==> Ti + 2MgCl2 or TiCl4 + 4Na ==> Ti + 4NaCl

    • These are examples of metal displacement reactions e.g. the less reactive titanium is displaced by the more reactive sodium or magnesium.

    • Overall the titanium oxide ore is reduced to titanium metal (overall O loss, oxide => metal)

  • Metals can become weakened when repeatedly stressed and strained. This can lead to faults developing in the metal structure called 'metal fatigue' or 'stress fractures'. If the metal fatigue is significant it can lead to the collapse of a metal structure. So it is important develop alloys which are well designed, well tested and will last the expected lifetime of the structure whether it be part of an aircraft (eg titanium aircraft frame) or a part of a bridge (eg steel suspension cables).

Notes information to help revise KS4 Science Additional Science Triple Award Separate Sciences GCSE/IGCSE/O level Chemistry Revision–Information Study Notes for revising for AQA GCSE Science, Edexcel GCSE Science/IGCSE Chemistry & OCR 21st Century Science, OCR Gateway Science WJEC/CBAC GCSE science–chemistry CCEA/CEA GCSE science–chemistry (and courses equal to US grades 8, 9, 10)


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