Explain with the help of diagram wet corrosion in neutral medium.
Electrochemical corrosion occurs:
i)When conducting liquid is in contact with metal.
ii)Two dissimilar metals or alloys are dipped in electrolyte.
- The corrosion can take place by H2 evolution mechanism or O2 absorption, mechanism.
- The wet corrosion in neutral medium takes place by O2 absorption mechanism.
Take an example of Fe in contact with water.
- Fe covered with oxide film acts as cathode and a crack in coating acts as an anode.
- At room temperature the water consists of 8ppm of O2.
- Being larger area of cathode,protected layer of Fe2+ ions.
- At crack,anode Fe sheds e- & goes into the water as Fe2+ ions.
Fe → Fe2++2e-
O2 in water accepts e- and OH- is formed.
Fe2++2OH- combine to form Fe(OH)2 & further precipitates
Fe(OH)2 Ferric hydroxide.
Fe(OH)2+`1/2` O2H2O →2Fe(OH)2↓