Corrosion in the past and today

One of the first uses of iron was for the production of weapons. Iron weapons had the advantage to be more resistant than the copper ones. But they had the disadvantage to rust very easily. That's the reason why a soldier had to keep his equipment in good shape with a stone and a little bit of oil.

Everyone has already seen a metallic object attacked by corrosion: in a kitchen, in a bathroom or outside: cars, statues...

Today, the massive use of metals made it necessary to find techniques to protect them against corrosion (painting and varnish on the bottom of ships, metallic covering )

 
   
Concerning iron:

Oxidation of iron at high temperatures (about 600 °C):

2 Fe + O2 --> 2 FeO (wustite)

6 FeO + O2 --> 2 Fe3O4 (magnetite)

4 Fe3O4 + O2 --> 6 Fe2O3 (hematite)

 
   
Rust:

Rust is a mixing of oxides and hydroxides of iron. In some cases, it forms a layer which protects the metal in the future by reducing or stopping the progression of the layer of oxide: this effect is called passivation.

Different factors determinate corrosion: the material, the composition in impurities (other elements), the alloys, the forms and quantities of the surfaces, the composition of the air or the liquid and the temperature.

 
   
Dry oxidation of calcium

The increasing in mass of a sample of calcium interacting with the air at room temperature:

Dm = f(t)

The increasing of the mass Dm at a time t is:

mt - mi = Dm
(mi is the mass at the beginning mt the mass at the time t)

Oxidation of calcium with dry air:

2 Ca (solid) + O2 (gas) --> 2 CaO (solid)

 
   
Wet corrosion: etching of the iron nail:

The second box "Wet corrosion: etching" contains 3 different solutions:

(1) the depassivation solution (a copper sulphate solution),

(2) the etching solution (an aqueous sodium hypochlorite solution, its commonly used for bleaching or disinfecting),

(3) and the rinsing solution (water).

At the end, the surface of the nail is damaged by the chemical reaction called "oxidoreduction"or "redox reaction".

The first solution, the depassivation solution (a copper sulphate solution), contains ions of copper (Cu2+) which react with iron:

Fe + Cu2+ --> Fe2+ + Cu

This reaction permits to accelerate the attack of the tip.

The second solution is the etching solution (an aqueous sodium hypochlorite solution; its commonly used for bleaching or disinfecting).

The anodic oxidation:

4 Fe --> 4 Fe2+ + 8e- and 4 Fe2+ --> 4 Fe3+ + 4e-

The cathodic reduction:

3 O2 + 6 H2O + 12e- --> 12 OH-

In total:

4 Fe + 3 O2 + 6 H2O --> 4 Fe3+ + 12 OH- (or 4 Fe(OH)3 )

 

 

Back to Teachers