Corrosion and surface treatment
When local cells form on an iron surface in neutral, aerated water and corrosion proceeds, which reaction mainly occurs at the cathode?
- AFe → Fe²⁺ + 2e⁻
- BO₂ + 2H₂O + 4e⁻ → 4OH⁻
- C2H⁺ + 2e⁻ → H₂
Show answer and explanation
Answer: B. O₂ + 2H₂O + 4e⁻ → 4OH⁻
Even on the surface of a single metal, inclusions, grain boundaries, strain and nonuniformities in the environment create regions of lower and higher potential, and with the metal itself acting as the wire, many small cells form. These are local cells. At the lower-potential anodes, iron dissolves as Fe → Fe²⁺ + 2e⁻ (the choice giving this equation is the anode reaction), and at the cathodes that accept these electrons, in neutral water, the main reaction is reduction of dissolved oxygen, O₂ + 2H₂O + 4e⁻ → 4OH⁻. The Fe²⁺ and OH⁻ produced react and are further oxidized into red rust (FeOOH and others). Hydrogen evolution, 2H⁺ + 2e⁻ → H₂, is the main reaction in acidic solutions rich in H⁺; in neutral water, H⁺ is scarce at about 10⁻⁷ mol/L, so it does not play the main role.