Definition
A severe form of intergranular corrosion in which corrosion progresses along the grain boundaries of a metal parallel to the surface, producing layered, flake-like swelling that lifts and separates the metal in thin sheets. It is most commonly seen in extruded aluminum alloy parts, where the grain structure is elongated in the direction of extrusion.
Plain English
A type of corrosion that eats sideways through the metal in flat layers, causing it to puff up and peel apart like the pages of a wet book.
Context Anchor
Seen during aircraft inspection and maintenance, especially on aluminum skins, fittings, and structural parts that have been exposed to moisture or trapped contaminants.
Derivation
From the Latin 'exfoliare,' meaning 'to strip off leaves' (ex- 'off' + folium 'leaf'). The name fits because the corroded metal lifts away in thin, leaf-like layers.
Why Pilots Care
If undetected, it can reduce the strength of load-bearing aluminum parts and lead to structural failure.
Analogy
It is like the pages of a damp book swelling and separating. The outside may show only roughness at first, but the layers inside are being pushed apart.
Intuition Check
Do not think of exfoliation corrosion as ordinary surface rust. The important point is that the metal is separating in layers, which can reduce strength even when the damaged area looks small.
Example Sentence 1
The mechanic found exfoliation corrosion along the lower wing skin where the aluminum had begun to swell and flake into thin layers.
Example Sentence 2
Areas showing exfoliation corrosion were treated and reinforced before the aircraft returned to service.