Definition
A form of electrochemical corrosion that occurs on a metal surface where two areas are in contact with the same electrolyte (such as water or moisture) but have different concentrations of dissolved metal ions. The area with the lower metal-ion concentration becomes anodic and corrodes, while the area with the higher concentration becomes cathodic and is protected. This commonly forms in tight crevices, under faying surfaces of riveted or bolted joints, and beneath debris where trapped moisture cannot freely circulate.
Plain English
A type of corrosion that happens when moisture sits between two parts of the same metal, and the trapped area has fewer dissolved metal particles in it than the open area. The trapped spot eats away while the open spot stays intact. It is most often found in tight gaps, under rivet heads, and where two metal surfaces press together.
Context Anchor
Seen in aircraft corrosion-control and maintenance discussions, especially around trapped moisture, joints, seams, deposits, and areas where liquid can sit without moving.
Derivation
Concentration-cell refers to a small electrochemical cell created by a difference in concentration of a substance, in this case metal ions dissolved in trapped moisture. The term comes straight from chemistry: a cell driven by uneven concentration rather than by two different metals.
Why Pilots Care
Unchecked, it weakens airframe components and can lead to structural failure if not detected during routine inspections.
Analogy
Think of two spots on the same metal part sitting under slightly different puddles. If one puddle has less dissolved metal in it, that spot tends to supply more metal to the liquid, so that area is eaten away first.
Grounding Statement
Picture moisture trapped in a seam on an aircraft skin: the chemistry inside the trapped moisture can differ from the chemistry outside it, and that difference can make one area corrode.
Intuition Check
Do not assume this requires two different metals touching each other. Metal-ion concentration-cell corrosion can happen on one piece of metal when the liquid touching it has uneven dissolved metal content.
Example Sentence 1
During the annual inspection, the mechanic found metal-ion concentration-cell corrosion under a row of rivets along the wing's lower lap seam.
Example Sentence 2
Preventing metal-ion concentration-cell corrosion requires keeping faying surfaces dry and properly sealed.