GATE 2024 MT – Question 42
A quenched steel is tempered for the same time at $T_1<T_2<T_3$, giving the shown microstructures. Black circles are cementite in ferrite; $\bar r_1,\bar r_2,\bar r_3$ are mean cementite radii and $V_1,V_2,V_3$ its volume fractions. If cementite is more noble than ferrite, which microstructure has the highest corrosion rate in aqueous 3.5 wt.% NaCl?
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Correct answer: (B) Microstructure at $T_2$
Explanation
Nobler cementite acts cathodically while ferrite dissolves anodically.
T1 has a smaller cementite fraction; T2 and T3 have the same larger fraction, but T2 has finer particles and more distributed galvanic interfaces.
The combination gives the highest expected corrosion rate at **T2, B**, for the depicted microstructures.