Choosing an Electrode for a Corroding Pipeline
A buried mild-steel pipeline is protected by a sacrificial anode. Two candidate anodes are proposed: zinc (E° Zn²⁺/Zn = −0.76 V) and magnesium (E° Mg²⁺/Mg = −2.37 V). The soil is slightly acidic and contains chloride ions. Standard reduction potentials: Fe²⁺/Fe = −0.44 V, O2 + 2H2O + 4e⁻ → 4OH⁻ E° = +0.40 V, 2H⁺ + 2e⁻ → H2 E° = 0.00 V. A technician observes that the magnesium anode is consumed very rapidly and that hydrogen bubbles appear at the steel surface. A second technician proposes switching to zinc, arguing it is 'safer because it is less reactive'.
1. Calculate the cell potential for each anode protecting iron and explain the rapid consumption of magnesium.
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In cathodic protection the anode metal is oxidised; iron is the cathode.
Zn anode: E°cell = E°cathode − E°anode = (−0.44) − (−0.76) = +0.32 V (spontaneous).
Mg anode: E°cell = (−0.44) − (−2.37) = +1.93 V (strongly spontaneous).
The much larger driving force for Mg means a higher corrosion current, so the Mg anode is consumed much faster per unit mass. Magnesium also reacts directly with acidic soil (Mg + 2H⁺ → Mg²⁺ + H2), which explains the hydrogen bubbles and accelerates its loss. This is a real trade-off: Mg protects better but has a short service life.
2. Assess the second technician's claim that zinc is 'safer because less reactive', using the potentials and the acidic, chloride-containing soil.
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The claim is partly right but misleading. Zinc still gives a spontaneous protection potential (+0.32 V), so it will protect the steel, and it is consumed more slowly - longer life, less frequent replacement. However:
- Zn is less effective at protecting exposed steel in high-chloride, acidic soil where the steel potential may drift; the protection margin is smaller.
- Zinc is amphoteric and dissolves faster in acidic conditions (Zn + 2H⁺ → Zn²⁺ + H2), so 'less reactive' is relative.
- Zinc is a heavy-metal environmental concern in soil.
A balanced judgment: Mg for aggressive/high-current-demand conditions with frequent replacement, Zn for longer-life, lower-current service. The choice depends on soil resistivity and chloride level, not simply on 'reactivity'. The hydrogen at the steel surface indicates the acidic soil is also driving direct reduction of H⁺, which consumes protection current regardless of anode.