corrosion of metals

• Corrosion


  Degradation of metals in the environment
• Most metals react with substances when exposed to the environment for prolonged periods
• Corrosion occurs at the surface of the metal only
• Rusting


  A chemical reaction between iron, water and oxygen to form the compound iron(III) oxide or hydrated iron(III) oxide
• Oxygen and water must be present for rusting to occur
• Rusting is a redox process and it occurs faster in salty water since the presence of sodium chloride catalyses the process
• Rusting of iron


  4Fe (s) + 3O (g) + xH O (l) → 2Fe O .xH O (s)
• Rust is a soft solid substance that flakes off the surface of iron easily, exposing fresh iron below which then undergoes rusting
• Over time all of the iron rusts and its structure becomes weakened
• Aluminium reacts with oxygen to produce aluminium oxide, Al2O3
• The aluminium oxide forms a tough protective layer that covers the aluminium, preventing further corrosion
• Corrosion and rusting are not the same process. Corrosion is the general term used to describe the degradation of metal surfaces whereas rusting is the specific type of corrosion that happens to iron.
• Investigating conditions for rusting

  1. Set up apparatus with control test tubes

  2. Observe results
• The results show that both air and water must be present for rusting to occur
• Rust prevention methods


  • Barrier methods
  • Galvanising / Sacrificial protection
• Galvanising


  Process where iron is coated with a layer of zinc
• If the coating is damaged or scratched, the iron is still protected from rusting because zinc preferentially corrodes as it is higher up the reactivity series than iron
• Sacrificial corrosion


  When a more reactive metal is intentionally allowed to corrode to protect a less reactive metal
• Alloys are mixtures of metals where the metals are not chemically combined
• Alloys often have properties that can be very different from the metals they contain, for example they can have more strength, hardness or resistance to corrosion or extreme temperatures
• Alloys contain atoms of different sizes, which distorts the regular arrangements of atoms
• This makes it more difficult for the layers to slide over each other, so they are usually much harder and stronger than the pure metal
• Copper alloys


  • Bronze
  • Brass
• Aluminium alloys


  • Aluminium with copper, manganese and silicon
  • Magnalium (aluminium and magnesium)
• Gold alloys


  • Gold mixed with copper, zinc and silver
• Steel alloys


  • Iron with added metals like chromium, manganese or nickel
• The composition of alloys can be carefully controlled to ensure the finished alloys have desired characteristics for particular uses.
• Ceramics


  Materials with high melting points and thermal resistance, used widely as construction materials and in domestic appliances
• Glass ceramics


  • Soda-lime glass
  • Borosilicate glass
• Clay ceramics


  Hardened materials that resist compressive forces, used to make bricks
• Polymers


  Poor conductors of heat and electricity, good thermal and electrical insulators
• Composites


  Made from two components: reinforcement and matrix
• Composites


  • Fibreglass
  • Steel reinforced concrete
  • Wood
• The properties of composites depend on the reinforcement and matrix used so composites can be tailor engineered to meet specific needs
• Don't get confused between an alloy and a composite: Alloys are uniform mixtures of metals whereas composites have two or more distinguishable materials.