engineering fundamentals Cathode Reactions
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Common Cathode Reactions
A variety of reactions can occur on the cathode of metal M to consume electrons generated by the corrosion of M at the anode.

Possibilities include (1) other metals G that are cathodic to M, (2) water formation, (3) peroxide formation, (4) hydroxyl formation, and (5) hydrogen gas formation:

Reduction Process Reaction Potential
(wrt H2)
Electroplating G onto M:    Gm+ + m e- G   depends
on metal
Water formation:    O2 + 4 H+ + 4 e- 2 H2O    1.229 Volt
Peroxide formation:    O2 + 2 H+ + 2 e- H2O2    0.695 Volt
Hydroxyl formation:    O2 + 2 H2O + 4 e- 4 OH-   0.401 Volt
Hydrogen gas formation:    2 H+ + 2 e- H2   0.000 Volt
Peroxide/
Hydroxyl
formation:
  O2 + 2 H2O + 2 e- H2O2 + 2 OH-   -0.146 Volt
Reaction Occurrences
Under normal circumstances the first reaction contributes little to corrosion. Instead, the electroplating reduction reaction is typically applied in controlled situations where electroplating is a desired effect.

The 3 reactions consuming H+ ions are also rare, unless the metal is exposed to acid containing a significant concentration of H+ (e.g. hydrochloric acid).

Perhaps the most common reduction reaction found in practice is the creation of hydroxyl ions, since the required ingredients are oxygen and water (which is why oxygen and water have such bad reputations with respect to corrosion). The created hydroxyl ions typically combine with the metal ions released from the anode to produce surface deposits.

For example, OH- ions combine with corroded iron ions to produce Fe(OH)3, which is ordinary red iron rust.

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