How do anodic and cathodic corrosion inhibitors slow down corrosion?

In one line

Corrosion inhibitors are substances added in small amounts to the corrosive medium that reduce the rate of corrosion by blocking either the anodic or the cathodic reaction.

Explained simply

A corrosion cell has two halves: the anode, where metal dissolves, and the cathode, where H⁺ or O₂ takes electrons. Stop either half and the whole cell stops, like cutting one wire of a circuit. Anodic inhibitors block the anode; cathodic inhibitors block the cathode.

How it works

1. Anodic inhibitors

• Examples: chromates, phosphates, tungstates, nitrites, molybdates (anions of transition elements with high oxygen content).
• They react with metal ions formed at the anode to give a sparingly soluble compound that forms a protective film on the anodic area, or they promote passivation.
• Caution: if added in insufficient amount, some anodic area stays unprotected; a small anode with a large cathode then leads to severe pitting. So they must be used in adequate concentration.

2. Cathodic inhibitors

• In acidic medium (cathode reaction is H₂ evolution):
  • Organic inhibitors such as amines, mercaptans, thiourea, heterocyclic nitrogen compounds and heavy-metal soaps adsorb on the metal surface and slow the diffusion of H⁺ to the cathode.
  • Salts of As and Sb deposit on cathodic areas and raise hydrogen overvoltage, slowing H₂ evolution.
• In neutral medium (cathode reaction is O₂ reduction to OH⁻):
  • Oxygen scavengers remove dissolved O₂: Na₂SO₃ (2Na₂SO₃ + O₂ → 2Na₂SO₄) or hydrazine (N₂H₄ + O₂ → N₂ + 2H₂O).
  • Mg, Zn or Ni salts react with OH⁻ at the cathode to form insoluble hydroxides that block the cathodic area and restrict O₂ diffusion.

Key formula

No numerical formula. Key reactions: \( \mathrm{2Na_2SO_3 + O_2 \rightarrow 2Na_2SO_4} \); \( \mathrm{N_2H_4 + O_2 \rightarrow N_2 + 2H_2O} \).

Quick example

Car radiator coolants contain inhibitors (such as nitrites, phosphates or organic inhibitors) to protect the metal parts. Boiler feed water is treated with sodium sulphite or hydrazine to remove dissolved oxygen. Acid used for pickling steel contains organic inhibitors so that the acid removes rust but not the steel.

Common mistakes

• Calling Na₂SO₃ an anodic inhibitor. It removes O₂ (cathodic reactant), so it is cathodic.
• Not mentioning the pitting risk with too little anodic inhibitor.
• Thinking inhibitors are coatings applied to the metal; they are added to the medium.

How MU asks it

"What are corrosion inhibitors? Explain anodic and cathodic inhibitors with examples" (5 marks). For full marks: definition, mechanism of each type, at least two examples each, the acidic vs neutral split for cathodic inhibitors with reactions, and the pitting caution.