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Difficulty: MediumCorrosion of Metals and Prevention Methods

Food containers are commonly manufactured from iron sheets coated with a thin protective layer of tin. If the protective tin coating is deeply scratched to expose both metals to damp atmosphere, which of the following electrochemical outcomes will occur?

  1. Iron will rust more rapidly than un-plated iron because iron is more electropositive than tin and acts as the anode.Answer
  2. B
    Tin will act as a sacrificial anode and dissolve preferentially to protect the exposed iron core.
  3. C
    Rusting is completely prevented because tin forms an amphoteric oxide layer over the exposed region.
  4. D
    Iron will gain electrons from tin and undergo reduction, preventing oxidation at the metal junction.

Answer

Iron will rust more rapidly than un-plated iron because iron is more electropositive than tin and acts as the anode.
In the reactivity series, iron is placed above tin. When a tin-plated iron surface is scratched, an electrochemical cell is formed in the presence of atmospheric water and oxygen. Because iron is more electropositive than tin, iron preferentially loses electrons (undergoes oxidation) and acts as the anode. This leads to accelerated rusting of the iron substrate compared to un-plated iron.

Step-by-Step Solution

1
Compare the standard electrode potentials (reactivity) of iron (FeFe) and tin (SnSn).
FeFe has a more negative standard reduction potential (E∘=−0.44 VE^\circ = -0.44\text{ V}) than SnSn (E∘=−0.14 VE^\circ = -0.14\text{ V}), meaning iron is more electropositive (more reactive).
The more electropositive metal in electrical contact acts as the anode in the presence of an electrolyte.
2
Identify the anodic and cathodic reactions taking place at the scratched junction.
Iron oxidizes at the anode (Fe(s)→Fe(aq)2++2e−Fe_{(s)} \rightarrow Fe^{2+}_{(aq)} + 2e^-) while oxygen and water are reduced at the tin cathode surface (O2(g)+2H2O(l)+4e−→4OH(aq)−O_{2(g)} + 2H_2O_{(l)} + 4e^- \rightarrow 4OH^-_{(aq)}).
Since iron is the anode, its oxidation rate is accelerated due to the large cathodic area of tin relative to the small exposed iron anode area.

Key Concept

Tinning vs Galvanizing in Corrosion Prevention
Estimated Time:1m 0s
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Topics

Corrosion of Metals and Prevention Methods

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