The Rusting Gate and the Sacrificial Anode
A steel gate on a coastal footpath rusts badly within two years, especially along its lower bars where salt spray collects. A maintenance team proposes three options: (A) repaint the gate every year with an ordinary paint; (B) attach blocks of magnesium to the lower bars; (C) attach blocks of copper to the lower bars. The gate is also bolted to a buried galvanised steel post. The team has a limited budget and wants the option that gives the longest protection for the least maintenance. They ask you to judge the electrochemical reasoning behind each option and to recommend one.
1. Explain, in terms of electron transfer, why iron rusts and why salt spray accelerates the process.
Read the worked answer
Iron rusts by an electrochemical process. At anodic regions iron is oxidised: Fe → Fe2+ + 2e-. At cathodic regions oxygen and water are reduced: O2 + 2H2O + 4e- → 4OH-. The Fe2+ and OH- ions combine and are further oxidised to hydrated iron(III) oxide, the rust. Salt spray provides ions that make the water film a better conductor, lowering the resistance between anodic and cathodic regions, so the corrosion current is larger and rusting is faster. Salt also helps break down protective oxide films, exposing fresh iron.
2. Evaluate options A, B and C using electrode potentials and recommend the best choice.
Read the worked answer
Option A: paint is a barrier, but any scratch exposes iron and corrosion continues at the break; yearly repainting is labour-intensive. Option B: magnesium is more reactive than iron (its standard electrode potential is more negative, about -2.37 V compared with -0.44 V for iron). Magnesium therefore oxidises preferentially and supplies electrons to the iron, making the iron the cathode; this is sacrificial protection and works even where the coating is damaged. Option C: copper is less reactive than iron (about +0.34 V), so iron would oxidise preferentially and the copper would accelerate corrosion. Option B is the sound choice: magnesium blocks protect the lower bars even when scratched, although they are consumed over time and must eventually be replaced. The galvanised post also protects the gate by the same zinc sacrificial mechanism.
3. The team notices the magnesium blocks become coated in white solid. Identify this and explain whether it prevents protection.
Read the worked answer
The white solid is magnesium oxide/hydroxide, formed as Mg → Mg2+ + 2e- followed by reaction with OH- and water. This layer is not a barrier that stops the electrochemical cell: magnesium remains more reactive than iron, so as long as magnesium metal remains in contact with the gate and the electrolyte, electrons continue to flow to the iron and the iron is protected. However, once the magnesium is fully consumed the protection stops, so the blocks must be inspected and replaced periodically.