Question

Corrosion of iron is essentially an electrochemical phenomenon where the cell reaction are

• A. $Fe$ is oxidised to $Fe ^{2+}$ and dissolved oxygen in water is reduced to $OH ^{-}$
• B. $\ce{Fe}$ is oxidised to $\ce{Fe^{3+}}$ and $\ce{H_2O}$ is reduced to $\ce{O_2^{2-}}$
• C. $\ce{Fe}$ is oxidised to $\ce{Fe^{2+}}$ and $\ce{H_2O}$ is reduced to $\ce{O_2^-}$
• D. $\ce{Fe}$ is oxidised to $\ce{Fe^{2+}}$ and $\ce{H_2O}$ is reduced to $\ce{O_2}$.

Answer 1

Answer: (A)

$Fe \longrightarrow Fe ^{2+}+2 e^{-}$ (anode reaction)

$O _{2}+2 H _{2} O +4 e^{-} \longrightarrow 3 OH$ (cathode reaction)

The overall reaction is

$2 Fe + O _{2}+2 H _{2} O \longrightarrow 2 Fe ( OH )_{2}$

$Fe ( OH )_{2}$ may be dehydrated to iron oxide $FeO$, or further oxidised to $Fe ( OH )_{3}$ and then dehydrated to iron rust, $Fe _{2} O _{3}$