Corrosion Technician Practice Exam

Filiform corrosion happens under coatings when moisture and oxygen create an electrochemical cell beneath the coating. Defects or pinholes in the coating let oxygen diffuse into and under the film, while the metal surface directly beneath the coating becomes anodic and dissolves. The surrounding areas, where oxygen is more available, act as cathodes. This sets up an oxygen concentration cell that drives metal loss in a thread-like, wormy front that propagates under the coating from the defect outward. That distinctive under-coating, filamentary growth is the hallmark of filiform corrosion.

This is different from uniform corrosion, which would show even thinning across exposed areas; from pitting corrosion, which forms isolated, small holes; and from galvanic corrosion, which requires two different metals in electrical contact in a conductive environment. Filiform specifically describes corrosion that travels beneath a coating due to an oxygen concentration cell mechanism.