Definition
A form of corrosion that appears as fine, thread-like filaments creeping under a painted or coated metal surface, typically aluminum. It occurs when moisture penetrates a small break in the coating and reacts with the metal beneath, producing wandering trails of corrosion that lift or blister the paint as they spread.
Plain English
Tiny worm-like lines of corrosion that grow underneath the paint on aluminum aircraft skin, lifting the paint as they spread.
Context Anchor
Seen during aircraft inspections, especially when checking painted aluminum or magnesium surfaces for corrosion under the finish.
Derivation
From the Latin filum, meaning 'thread.' The corrosion is named for its thread-like appearance — fine, branching lines visible beneath the paint.
Why Pilots Care
It can compromise the integrity of aircraft structures if allowed to progress, often requiring surface treatment and repainting.
Analogy
It is like seeing little roots growing under a layer of paint. The surface may still be covered, but something is spreading underneath it.
Intuition Check
Do not assume filiform corrosion is just a surface stain or scratched paint. It is corrosion spreading under the coating, even though the metal may not be fully exposed.
Example Sentence 1
During the preflight, the pilot noticed faint thread-like lines under the paint on the wing and reported possible filiform corrosion to the maintenance shop.
Example Sentence 2
Regular washing helps prevent filiform corrosion from developing on the wings.