To protect an iron pipeline buried in acidic soil

CHEMISTRY · Multi-part · AP · Chemistry · Thermodynamics and Electrochemistry · Thermodynamics and Electrochemistry

Problem

To protect an iron pipeline buried in acidic soil, engineers consider attaching a sacrificial metal anode. Corrosion occurs because iron is oxidized while oxygen dissolved in soil moisture is reduced. Relevant half-reactions (298 K): Fe2+(aq)+2e−→Fe(s)\mathrm{Fe}^{2+}(aq) + 2e^{-} \rightarrow \mathrm{Fe}(s) E∘=−0.44 VE^\circ = -0.44\,\text{V} Mg2+(aq)+2e−→Mg(s)\mathrm{Mg}^{2+}(aq) + 2e^{-} \rightarrow \mathrm{Mg}(s) E∘=−2.37 VE^\circ = -2.37\,\text{V} Al3+(aq)+3e−→Al(s)\mathrm{Al}^{3+}(aq) + 3e^{-} \rightarrow \mathrm{Al}(s) E∘=−1.66 VE^\circ = -1.66\,\text{V} Zn2+(aq)+2e−→Zn(s)\mathrm{Zn}^{2+}(aq) + 2e^{-} \rightarrow \mathrm{Zn}(s) E∘=−0.76 VE^\circ = -0.76\,\text{V} O2(g)+4H+(aq)+4e−→2 H2O(l)\mathrm{O}_{2}(g) + 4\mathrm{H}^{+}(aq) + 4e^{-} \rightarrow 2\,\mathrm{H}_{2}\mathrm{O}(l) E∘=+1.23 VE^\circ = +1.23\,\text{V} Assume activities of all aqueous species are 1 and the partial pressure of O2(g)\mathrm{O}_{2}(g) is 1 bar. (a) Construct the balanced overall equation (including physical states) for the corrosion of iron in acidic conditions when O2(g)\mathrm{O}_{2}(g) serves as the cathode reactant. Calculate Ecell∘E^\circ_{\text{cell}} for this corrosion process. (b) For each candidate sacrificial metal (Mg, Al, Zn), write the overall reaction that would occur when that metal is attached to iron and calculate the corresponding Ecell∘E^\circ_{\text{cell}} under standard conditions. (c) Based on your calculations in part (b), determine which sacrificial metal provides the greatest thermodynamic driving force for protecting the iron pipeline. Explain your reasoning. (d) Discuss two practical factors other than Ecell∘E^\circ_{\text{cell}} that engineers should consider when selecting a sacrificial anode material for long-term cathodic protection in the described environment.

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