Corrosion of Metals

What is corrosion? §

• Oxidation of Metal
• Rust
  • Iron Oxide produced from the rusting/oxidation of Fe (iron)
• Corrosion is usually defined as the degradation of metals due to an electrocehmical process. (except for few metals such as gold, platinum)
• most familiar example of corrosion is the formation of rust on iron. iron will rust when exposed to oxygen & water.
• Corrosion is enhanced by presence of:
  • impurities
  • air and moisture
  • electrolytes
  • strains in metal like dent, scratches etc.

Dry Corrosion §

• no water, direct reaction of oxygen with metal, producing metal oxide.
• direct chemical attack of atmospheric gases like O2, SO2, CO2, H2S

wet or electrochemical corrosion §

• Occurs due to setting up large numbers of galvanic cells in metal in presence of impurity as well as in the presence of impurity as well as presence of moisture.
  • e.g. rusting of iron in moist atmosphere.
• 2Fe + O2 + 2H2O -> 2Fe(OH)2 (s)
• rusted nail looks bumpy and flaky vs oxidised zinc, zinc oxidised protects zinc: e.g. a barrier of protection
• when rust is formed, it lifts and flakes, exposing more ion underneath.
• rust forms flaky iron.
• chromium non reactive metal.
• putting polymer, painting paint and non-protective metal}: they all want a protection called barrier protection.
• there are many methods of protecting metal against corrosion:
  • barrier protection.
  • cathodic protection (sacrificial reaction), iccp (impressed corrosive cathodic protection).
  • alloy formation.
• ms pilling wont talk about cathodic protection!

barrier protection §

• thin barrier developed between reductant and oxidant.
  • painting of the surface.
  • coating surface with a thin film of some non corrosive metal like nickel, chromium etc.

coating the metal: (SAVE IRON ) §

• galvanising (thing coating of Zn on iron)
• zn oxidises easily and forms an additional hard protective layer of zinc oxide. If the zinc coating is scratched, the zinc will act as an anode and will still corrode before the steel or the iron.
• electroplating (coating of Cu, Ni or Cr on iron with aid of direct current)
• Tin plating (coating of tin on iron)
• ms pilling has three plates
  • iron covered with zinc
  • iron covered with Sn
  • iron (controlled plate)
• calculate rate of corrosion
• independent variable: material that iron is covered with (or lack of)
• dependent variable: rate of corrosion
• controlled variables:
  • temperature
  • surface area
  • humidity (?)
• ms pilling makes scratch /cross section deep enough in all three plates, labelling them as A, B and C.
• which will corrode the most?
• FOR ZINC (A)
  • zinc (stronger reducing agent) is sacrificial anode - because it is corroded instead of the iron it covered.
    • e.g. if you have galvanised roof and it oxidised, there is no need to rush to protect it.
  • protected corrosion
  • sacrificial protection
• FOR TIN (B)
  • oxidised more readily for iron coated in tin - because it has lower SRP meaning it is more likely to oxidise than tin.
  • iron is cathode, tin is anode.

applications of sacrificial protection §

• “oil rig in the ocean” - ms pilling
• to protect underground pipes, a sacrificial anode is added.
• water pipe is turned into cathode and active metal is used as the anode.
• often magnesium is used as the sacrificial anode (at least forfor iron):
  • $M{g}^{2+}+2e\to Mg$ Ered = -2.37V