Filiform corrosion is most likely to occur under which condition?

• A. Under a dense coating of topcoat enamel such as polyurethane, caused by improperly cured primer.
• B. In engine bays.
• C. On the wheel wells.
• D. Under a dense coating of topcoat enamel such as polyurethane, caused by improperly cured primer.

Answer: (A)

Filiform corrosion is an under-film corrosion that grows beneath an intact coating, starting at defects at the coating-substrate interface and requiring moisture and oxygen to reach the metal surface. When a dense topcoat like polyurethane sits over a primer that hasn’t cured properly, moisture can diffuse through the coating and become trapped at the primer–metal interface. The poorly cured primer fails to form a proper barrier, allowing tiny galvanic cells to develop and propagate in a filamentary, wormlike pattern under the coating. That specific combination—a dense topcoat with an improperly cured primer—creates the exact conditions for filiform corrosion to initiate and spread, whereas the location (engine bay or wheel wells) isn’t the defining factor.