Definition
A form of electrochemical corrosion that occurs when a film of electrolyte (such as water) on a metal surface contains different concentrations of dissolved metal ions in different areas. The region with the lower metal ion concentration becomes anodic and corrodes, while the region with the higher metal ion concentration becomes cathodic and is protected. The result is localized metal loss in the area where ion concentration is lowest.
Plain English
When water sits on a metal part and some areas of that water have more dissolved metal in them than others, the metal under the cleaner water gets eaten away. It is a type of corrosion driven by uneven chemistry in the moisture on the surface.
Context Anchor
Seen in aircraft corrosion discussions, especially during inspection of seams, joints, lap areas, crevices, and places where moisture or corrosion products can stay trapped against metal.
Derivation
The name describes the mechanism directly: a corrosion 'cell' (a small electrical circuit on the metal surface) driven by a high concentration of metal ions in one part of the electrolyte compared to another. 'Concentration cell' is a standard electrochemistry term meaning a battery-like cell powered by a difference in concentration rather than by different metals.
Why Pilots Care
Left untreated, this corrosion weakens airframe components and can lead to structural failure or costly repairs.
Analogy
It is like two wet spots on the same metal part having different chemical “strengths.” That difference can make one spot give up metal while the other spot does not.
Grounding Statement
Picture moisture trapped in a seam: as metal dissolves into that moisture unevenly, the chemistry becomes different from one spot to the next, and corrosion can start at the weaker spot.
Intuition Check
Do not assume the area with the higher metal-ion concentration is always the area being eaten away. In this corrosion cell, the lower metal-ion area is the part that usually corrodes.
Example Sentence 1
During the inspection, the mechanic found high metal ion concentration cell corrosion under a wing skin lap joint where moisture had been trapped for months.
Example Sentence 2
High metal ion concentration cell corrosion appears most often at lap joints and fastener lines where electrolyte concentration differs from one side of the joint to the other.