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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) |
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| Electroplating G onto M: | Gm+ + m e- |  |
G | depends on metal |
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| 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 | ||
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O2 + 2 H2O + 2 e- | |
H2O2 + 2 OH- | -0.146 Volt | ||
Reaction Occurrences
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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. |
