Sacrificial protection versus tin coating on a coastal bridge
A coastal footbridge has steel bolts. One set is galvanised with zinc; another is electroplated with tin. During maintenance, a worker scratches through the coating on one bolt of each type, exposing the iron beneath. The bridge is in a salty, damp environment. Standard reduction potentials are: Zn2+ + 2e− ⇌ Zn, E° = −0.76 V; Fe2+ + 2e− ⇌ Fe, E° = −0.44 V; Sn2+ + 2e− ⇌ Sn, E° = −0.14 V; O2 + 2H2O + 4e− ⇌ 4OH−, E° = +0.40 V. One worker claims both coatings protect the iron equally because both are barrier coatings. A second worker says the tin-coated bolt will rust faster than bare iron once scratched. Evaluate both claims.
1. Use the potentials to decide which metal is oxidised in each scratched bolt. Support your answer with half-equations.
Read the worked answer
Zinc is more negative (−0.76 V) than iron (−0.44 V), so zinc is more easily oxidised. In a scratched galvanised bolt, zinc is the anode: Zn(s) → Zn2+(aq) + 2e−. Iron is protected and does not oxidise while zinc remains. Tin is less negative (−0.14 V) than iron, so iron is more easily oxidised. In a scratched tin-coated bolt, iron is the anode: Fe(s) → Fe2+(aq) + 2e−. Tin acts as the cathode. Therefore the first worker is wrong: the coatings do not behave equally once scratched.
2. Write the overall cell reaction for the corroding tin-coated bolt in oxygenated seawater and explain why the tin coating makes rusting faster than on bare iron.
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Anode: Fe(s) → Fe2+(aq) + 2e−. Cathode: O2(g) + 2H2O(l) + 4e− → 4OH−(aq). Overall: 2Fe(s) + O2(g) + 2H2O(l) → 2Fe(OH)2(s), which is further oxidised to rust. Tin is cathodic to iron. The scratch exposes a small area of iron, while the surrounding tin is a large cathode. This small-anode/large-cathode geometry concentrates the corrosion current at the scratch, so iron dissolves faster than on bare iron, where anodic and cathodic sites are microscopic and distributed.
3. Recommend a coating for the bolts and justify your choice using the potentials and practical constraints.
Read the worked answer
Use zinc galvanising. Zinc is more negative than iron, so it is oxidised preferentially and acts as a sacrificial anode even when the coating is scratched. The Zn/Fe potential difference is large, giving a strong driving force for zinc corrosion rather than iron corrosion. Tin is unsuitable in salt water because, once scratched, it makes iron the anode and can accelerate rusting. A practical limitation is that zinc is consumed over time and may need replacing, but it protects the iron while present.