Where is filiform corrosion most likely to occur on an aircraft?

• A. Under a dense coating of topcoat enamel such as polyurethane, caused by improperly cured primer.
• B. Under a thin coat of paint.
• C. In exterior aluminum joints.
• D. In non-painted areas.

Answer: (A)

Filiform corrosion is a form of interfacial corrosion that grows beneath a painted coating on aluminum, driven by moisture trapped at the coating-adhesion interface. It is most likely when a dense topcoat is applied over a primer that hasn’t cured properly. If the primer hasn’t set fully, solvents and moisture can remain in the layer and under the coating, and a thick, hard topcoat can trap those vapors and moisture. This creates tiny electrochemical cells at the metal-coating interface, allowing corrosion to advance under the film in thread-like, branching filaments from defects or edges.

Why this scenario fits best is that the dense topcoat provides the confinement needed for moisture to stay in contact with the metal surface, while improper primer curing leaves the interface more susceptible to moisture ingress and poor adhesion. In contrast, a very thin paint layer won’t trap moisture as effectively; corrosion under paint is more typical of issues at joints or in non-painted areas where there isn’t a protective film to foster the same under-film growth. Non-painted areas don’t have a coating interface for filiform growth to occur beneath, so this mechanism wouldn’t apply there.

So, the condition that promotes filiform corrosion is a dense topcoat over a primer that hasn’t cured properly, because it traps moisture and creates the environment for under-film, filiform growth.