Answer: In both cases iron is the anode, \(\text{Fe}\to\text{Fe}^{2+}+2e^-\). (a) In acid, H⁺ is reduced to H₂ at the cathode. (b) In neutral aerated water, O₂ is reduced to OH⁻, Fe(OH)₂ forms and is oxidised to rust, \(\text{Fe}_2\text{O}_3\cdot x\text{H}_2\text{O}\).
Basis: wet corrosion occurs when a metal is in contact with a conducting liquid. Separate anodic and cathodic areas form on the metal; the anode is oxidised and electrons flow through the metal to the cathode.
(a) Hydrogen evolution type (acidic medium, little oxygen). Example: iron in an industrial waste-water tank with dilute acid.
Anode: \(\text{Fe}\to\text{Fe}^{2+}+2e^-\) (iron dissolves)
Cathode: \(2\text{H}^++2e^-\to\text{H}_2\uparrow\)
Overall: \(\text{Fe}+2\text{H}^+\to\text{Fe}^{2+}+\text{H}_2\). Large cathodic and small anodic areas are typical, so iron is eaten away and hydrogen is evolved.
(b) Oxygen absorption type (neutral or slightly alkaline, aerated medium). Example: rusting of iron in moist air or water. The surface usually has a crack in the oxide film; the bare metal there acts as the anode and the film-covered area as the cathode.
Anode: \(\text{Fe}\to\text{Fe}^{2+}+2e^-\)
Cathode: \(\frac12\text{O}_2+\text{H}_2\text{O}+2e^-\to2\text{OH}^-\)
The Fe²⁺ and OH⁻ ions diffuse and meet: \(\text{Fe}^{2+}+2\text{OH}^-\to\text{Fe(OH)}_2\downarrow\)
With enough oxygen: \(4\text{Fe(OH)}_2+\text{O}_2+2\text{H}_2\text{O}\to4\text{Fe(OH)}_3\), which loses water to give yellowish-brown rust, \(\text{Fe}_2\text{O}_3\cdot x\text{H}_2\text{O}\). With limited oxygen, black magnetite \(\text{Fe}_3\text{O}_4\) forms.
Diagram (described): an iron surface covered by a water drop. Near the edge of the drop (more oxygen) is the cathode, where O₂ is reduced to OH⁻; the centre of the drop (less oxygen) is the anode, where Fe goes into solution as Fe²⁺. Electrons flow through the iron from the centre to the edge. Rust deposits in a ring between the two regions, since Fe²⁺ and OH⁻ meet there.
Note: rust is formed away from the anode, so corrosion (loss of metal) happens at the anode but rust appears near the cathode.
