Corrosion of Metals

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• Corrosion is usually defined as the degradation of metals due to an electrochemical process
  • Except few metals such ass gold and platinum
• Perhaps the most familiar example of corrosion is the formation of rust on iron
  • Iron will rust when it is exposed to oxygen and water
  • Water acts as an electrolyte and a catalyst
  • “Eats” the metal, and eventually the metal will disintegrate
• Corrosion is enhanced by the presence of
  • Impurities
  • Air and moisture
  • Electrolytes
  • Strains in dent, scratches, etc
• Types of corrosion
  • Dry or chemical corrosion
  • Wet or electrochemical corrosion

Dry Corrosion

$2Zn+O_2\to 2ZnO$

It involves direct chemical attack of atmospheric gases like $O_2$, $S{O}_2$, $C{O}_2$, $H_{2}S$

Example: tarnishing of magnesium ribbon or silverware

$2Mg+O_2\to 2MgO$

Wet or Electrochemical Corrosion

It occurs due to setting up of a large number of tiny galvanic cells in metal in presence of impurity as well as in presence of moisture

Example: Rusting of iron in moist atmosphere

Prevention of corrosion

There are many methods of protecting metals against corrosion

• Barrier protection
  • I.e. coating iron with oil
• Cathodic protection
  • Sacrificial protection (spontaneous reaction)
  • ICCP (impressed current cathodic protection)
• Alloy formation

Barrier protection

Here, a thin barrier is developed between the surface of iron (metal) and atmosphere by one of the following methods

1. Painting of the surface
2. Coating the surface with a thin film of some non-corrosive metal such as nickel, chromium, etc

Coating the metal:

• Galvanising (thin coating of Zn on iron)

Zn oxidise easily and forms an additional hard protective layer of zinc oxide. If thhe zinc coating is scratched, the zinc will at as an anode and will still corrode before the steel or iron. Zinc is sacrificial anode

• Electroplating (coating of Cu, Ni, or Cr on iron with aid of direct current)

• Tin plating (coating of tin on iron)

Sacrificial Protection

• To protect underground pipes, a sacrificial anode is added
  • Sacrificial anode: strong reducing agent (more reactive) metal that will oxidise preferentially to iron (or other metals) to protect it (from oxidising)
• The water pipe is turned into the cathode and an active metal is used as the anode
• Often, magnesium is used as the sacrificial anode

Fe^{2+} + 2e \to Fe \text{, E}_{red}=-0.44 \text{ V} \end{align}$$ ### Cathodic protection - ICCP, impressed current cathodic protection systems are used to provide cathodic protection for pipelines, ship hulls, offshore production platforms, water treatment equipment, etc - The principal advantage of ICCP is its greater output capacity as compared to sacrificial anode systems - Thus, when protection is desired for large, poorly coated and bare structures, ICCP is often the choice - It requires eternal DC power source that is energised by standard AC current