GATE 2021 MT – Question 62
At 25 °C, iron corrodes in deaerated pH-3 acid with corrosion current density 4 μA/cm$^2$. Its corrosion potential is ______ V (two decimal places). Given cathodic Tafel slope $\beta_c=0.1$ V per decade, hydrogen exchange-current density on iron $10^{-9}$ A/cm$^2$, $R=8.314$ J mol$^{-1}$ K$^{-1}$, $F=96500$ C/mol. Potentials are relative to the standard hydrogen electrode.
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Correct answer: -0.60 to -0.50
Explanation
At pH 3 the equilibrium hydrogen potential is −0.05916×3=−0.1775 V. Cathodic overpotential is −0.1log10(4×10⁻⁶/10⁻⁹)=−0.3602 V. Sum gives corrosion potential **−0.54 V** to two decimals.