What is the hydrogen evolution mechanism of wet corrosion in an acidic medium?

In one line

In an acidic, oxygen-poor environment, the metal dissolves at anodic areas and the released electrons reduce H⁺ ions to hydrogen gas at cathodic areas.

Explained simply

Wet corrosion works like a tiny battery formed on the metal surface. One region acts as the anode and dissolves; another acts as the cathode where electrons are used up. The acid solution is the electrolyte that completes the circuit. In acids, the electron-eater at the cathode is H⁺, so hydrogen bubbles come off. That is why it is called hydrogen evolution.

How it works

1. Conditions: acidic medium (industrial waste water, acid solutions, acidic soil), little or no dissolved oxygen.
2. Anodic and cathodic areas form because of impurities, stress or contact with a less active metal. Usually the anodic area is large and cathodic areas are small (impurity sites).
3. At anode (oxidation): Fe → Fe²⁺ + 2e⁻. Iron dissolves.
4. Electrons flow through the metal from anode to cathode.
5. At cathode (reduction): 2H⁺ + 2e⁻ → H₂↑
6. Overall: Fe + 2H⁺ → Fe²⁺ + H₂↑ (e.g., Fe + 2HCl → FeCl₂ + H₂)

All metals placed above hydrogen in the electrochemical series tend to dissolve in acidic solution with evolution of hydrogen. The result is uniform displacement of H₂ and loss of metal, often as general thinning.

Key formula

Anode: \( \mathrm{Fe \rightarrow Fe^{2+} + 2e^-} \)

Cathode: \( \mathrm{2H^+ + 2e^- \rightarrow H_2} \)

Quick example

Steel tanks storing acidic industrial effluent, or iron pipes in acidic soil, corrode by this mechanism. A simple lab demonstration: an iron nail in dilute HCl gives hydrogen bubbles while the nail slowly thins.

Contrast with the neutral medium: in neutral water containing dissolved oxygen, the cathode reaction becomes O₂ absorption instead of H₂ evolution.

Common mistakes

• Writing the cathode reaction as Fe²⁺ + 2e⁻ → Fe. The metal does not deposit; H⁺ is reduced.
• Showing electrons flowing through the solution. Electrons move through the metal; ions move in the solution.
• Forgetting to state the condition (acidic, absence of oxygen).

How MU asks it

"Explain the mechanism of wet corrosion by hydrogen evolution" (5 marks), often paired with oxygen absorption as "Explain electrochemical corrosion in acidic and neutral media" (6 marks). For full marks give conditions, a labelled diagram (anode, cathode, electron flow, H₂ bubbles, electrolyte), the anode, cathode and overall reactions, and one example.