What type of corrosion appears "worm-like" under paint?

The appearance of corrosion that resembles "worm-like" patterns under paint is characteristic of filiform corrosion. This type of corrosion occurs when moisture gets trapped under a painted or coated surface, creating an ideal environment for corrosion to develop. The corrosion progresses in a localized manner, often starting from a point where there is a defect in the coating, such as a flaw, scratch, or pinhole, and it spreads out beneath the surface in a filamentous, thread-like manner.

The unique, elongated paths of corrosion that resemble worms are why it is specifically identified as filiform. The conditions that promote this type of corrosion are typically high humidity and the presence of chlorides, making it more common in marine environments. Understanding the appearance and behavior of filiform corrosion is essential for maintenance personnel in order to prevent premature failure of paint systems and the underlying metal structure.

Intergranular corrosion, by contrast, affects the grain boundaries of the metal, leading to a different pattern and type of damage. Fretting corrosion involves the removal of surface material due to repeated motion, which appears as abrasive wear rather than worm-like structures. Uniform surface corrosion, as the name suggests, leads to a general, even deterioration across the entire surface, lacking the localized effects seen in