Electrolyte
concentration |
Consider a metal bathed in an electrolyte containing its own ions. The basic corrosion reaction where a metal atom losses an electron and enters the electrolyte as an ion can proceed both forward and backwards, and will eventually reach equilibrium.
If a region of the electrolyte (adjacent to the metal) were to exhibit a decreased concentration of metal ions, this region would become anodic to the other portions of the metal surface. As a result, this portion of the metal would corrode faster in order to increase the local ion concentration.
The net affect is that local corrosion rates are modulated in order to homogenize reduction ion concentrations within the electrolyte.
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Oxidation
concentration |
Perhaps the most common concentration cell affecting engineered structures is that of oxygen gas. When oxygen has access to a moist metal surface, corrosion is promoted. However, it is promoted the most where the oxygen concentration is the least (for the reasons described in the above box).
As a result, sections of a metal that are covered by dirt or scale will often corrode faster, since the flow of oxygen to these sections is restricted. An increased corrosion rate will lead to increased residue, further restricting the oxygen flow to worsen the situation. Pitting often results from this "runaway" reaction.
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