5d. Alloys to improve properties and problems using metals e.g. corrosion

Doc Brown's Chemistry: Chemical Bonding and structure GCSE level, IGCSE, O, IB, AS, A2 A advanced level US grade 9-12 level Revision Notes

• 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.

  • Stainless steel, alloy of iron with chromium/nickel, is an effective option, but too expensive for most structural applications.

  • 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).

  • 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 - 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).

  • Many metals are easily oxidised and corrode easily when exposed to air (oxygen) and rain (water).

  • For the rest of the corrosion story see notes on Corrosion of Metals and Rust Prevention

 Notes on Alloy Structure and uses via a very simplified 2D diagram

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. 

Aluminium, copper, gold and iron not strong enough for many of their uses.

We need alloys of a specialised composition for particular applications.

An alloy is where a metal is mixed with other metals or non-metals, usually to make a stronger metal.

From several thousand years ago we had learned how to make bronze my mixing tin with copper.

A few % of tin in copper makes a harder metal, better than copper for making axes, knives and swords.

An archaeological period is called the bronze age bronze age (in Britain this was from 3500 to 2500 years ago),.

This was followed by the 'iron age' and later the 'Industrial Revolution' built in steel and powered by coal.

Brass is made by mixing copper with zinc to make a much tougher hard wearing alloy.

Zinc is too brittle and copper is too soft, but the mixture is much stronger, not too soft and not too brittle.

The are multiples alloys of steel make by adding carbon to iron.

You then add other metals like nickel, tungsten or chromium to make specialised steels.

This produces steel alloys with great tensile strength and impressive anti-corrosion properties.

Note that other non-transition metals have important uses e.g. aluminium, lead and tin.

Sometimes we need to lower the melting point of a metal for some specific task.

 Lead melts at 328^oC and tin at 232^oC. The alloy solder is a mixture of the two and melts ~180^oC.

Solder is a good electrical conductor and ideal for 'soldering' together electrical wires.

BUT, you don't want to operate the soldering iron at too high temperature or you might damage the circuit board.

Aluminium has good anti-corrosion properties due to a protective oxide layer always present on its surface.

However, aluminium is too soft for many purposes and so is alloyed with copper,  magnesium, manganese or tin.

An aluminium plus copper alloy is called Duralumin and is much stronger than pure aluminium.

The right-hand side of the diagram below could represent such an alloy.

A particle model of a pure metal and an alloy

1. Shows the regular arrangement of the atoms in a pure metal crystal and the white spaces show where the free electrons are (yellow circles actually positive metal ions).
2. Shows what happens when the metal is stressed by a strong force.
   • The layers of atoms can slide over each other (called the slip effect) and the bonding is maintained as the mobile electrons keep in contact with ions of the giant lattice, so the metal object remains intact BUT the metal is physically a different shape.
   • This is why most metals are so malleable (easily bent or hammered into shape) and ductile (can be drawn out into a thin wire without breaking).

   • For the same reason, most metals (pure or alloyed), are not brittle because as the layers of atoms slide over each other, no bonds between the metal ions in the giant lattice and the delocalised electrons are permanently broken.

3. Shows an alloy mixture.
   • Alloys are not usually considered as compounds (despite the fact that all the atoms are chemically bonded together), but described as a physical mixing of a metal plus at least one other material (shown by red circle).
   • The other material can be another metal e.g. nickel or manganese added to iron in steel, or a non–metal e.g. carbon, and it can be bigger or smaller than the original metal atoms.
   • Many alloys are produced like this to give a stronger metal.
   • The presence of the other alloying atoms, smaller (purple) or bigger (blue) disrupts the symmetry of the layers of the metal lattice and this distortion reduces the 'slip ability' of one layer to slide over another layer of metal atoms.
     • Iron has an atomic metallic radius of 0.126 nm.
     • Carbon is a smaller atom, C radius 0.077 nm, silicon Si radius 0.117 nm, both found in steel.
       • Both have a smaller radius than iron and both increase the hardness of steel.
       • Atoms smaller than iron (usually a non-metal) can fit into holes in the metallic lattice, whereas the atoms larger than iron to replace the iron atom (see diagram 3. above).
     • Other metals in steels have a similar metallic radii/nm compared to iron
       • e.g. chromium Cr 0.125, nickel Ni 0.124 and manganese Mn 0.129.
       • BUT, they can have other positive effects like Cr increasing the steels anti-corrosion properties.
     • Tungsten W, has an metallic atomic radius of 0.137 nm, larger than iron atoms, and so increases the hardness and strength of steel.
   • Sometimes the added element or compound chemically bonds strongly to the surrounding metal atoms of the original metal atoms.
     • You don't need to know this, but the compound 'cementite', Fe₃C, is found in steel - an alloy of iron and carbon.
     • Fe₃C is an example of an iron carbide compound and their crystals make 'softish' iron much stronger.
   • The result is a stronger harder less malleable metal, but one better suited to most uses of steels.
4. The main point about using alloys is that you can make up, and try out, all sorts of different compositions until you find the one that best suits the required purpose in terms of tensile/compression strength, malleability, electrical conductivity or corrosion resistance etc.
   • Most metals in everyday use in the home or industry are alloys.
   • This is because pure metals such as copper, gold, iron and aluminium are too soft for most uses and so are mixed with other metals to make far more useful harder alloys.
   • The are hundreds of alloys of steel made by alloying iron with other metals to increase the strength or anti-corrosion properties of the metal.
     • Steel is used in building and bridge construction, car bodies, railway lines and countless other objects that need to have a high tensile strength.
   • Pure metals can be either too soft (e.g. like copper or tin) or too brittle (e.g. like zinc) to be used directly and are therefore often alloyed to make superior metals like brass or bronze.
   • The properties of metals are readily matched to a particular use e.g.
     • Aluminium alloys are strong and light (relatively low density for a metal), they do not corrode easily and so are used in aircraft construction, greenhouse frames and not as expensive as titanium alloys.
     • Cooking pans made of stainless steel are good conductors of heat, strong with good anti-corrosion properties and steel has a high melting point.
     • Copper is malleable and ductile, easily drawn out into wire, an excellent conductor of electricity, and so is widely used in electrical circuitry.

   • 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,

   • For more on specific metals and alloys see my detailed notes on Transition Metals.

 Doc Brown's Chemistry 

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