Final answer:
Filiform corrosion on aircraft structures appears as thread-like filaments under coatings, caused by breaches in protective layers that lead to electrochemical reactions and rust formation. This process can severely compromise structural integrity with little visible warning.
Step-by-step explanation:
Filiform corrosion typically manifests as thread-like filaments under a painted or coated surface on an aircraft structure. When a protective coating such as paint is breached, small scratches can expose the underlying metal to environmental factors like oxygen and moisture. This leads to an electrochemical reaction where oxygen is reduced at the surface with a greater exposure to air, creating the cathodic site, and metallic iron (or aluminum in the case of aircraft) is oxidized at the less exposed site (the anode). Over time, a galvanic cell is created that accelerates the corrosion process, potentially leading to rust formation and the development of large pits. These pits can form underneath the coating with little visible warning, jeopardizing the structural integrity of the aircraft. The widespread implications of corrosion, such as the infamous collapse of the Silver Bridge due to corrosion of a suspension chain, underscore the importance of regular inspection and maintenance to ensure safety.