Question

Explain with the help of diagram wet corrosion in neutral medium.

Answer 1

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.
`1/2`O₂H₂O+2e^-→2OH^-
Fe²⁺+2OH^- combine to form Fe(OH)₂ & further precipitates
Fe(OH)₂ Ferric hydroxide.
Fe²⁺+2OH^-→Fe(OH)₂
Fe(OH)₂+`1/2` O₂H₂O →2Fe(OH)₂↓
Ferric Hydroxide