Mechanism of Corrosion:
The mechanism of corrosion in pipes could be understood along with the following example of iron pipes. While iron comes into contact with electrolyte water, it has a tendency to ionize and go into solution.
Fe + 2H+ → Fe++
Fe++ ions combine along with hydroxide ions (OH-) to form ferrous hydroxide (Fe(OH)2)
Fe++ + 2OH- → Fe(OH)2
The ferrous hydroxide so created reacts with dissolved oxygen to form ferric hydroxide (Fe(OH)3 ), that get deposited to the surface of pipe.
Deposition raise the roughness of pipe and ultimately decreases its carrying capacity.
2Fe(OH)2 + O2 + H2O → 2Fe(OH) 3