Abstract
Filiform corrosion is a thread-like type of corrosion which develops under protective coatings on certain metals, usually in humid atmospheres. It is characterized by directional growth and definite structure. Controlling factors are the physical nature of the coating and the exposure conditions. Normally each growing thread consists of an active head of an unstable corrosion product and an inactive body of a stable corrosion product. Filiform corrosion usually appears in an apparently haphazard or disorderly pattern. However, its growth is uniform and orderly in direction, rate and dimensions when developed under controlled conditions. The tracks do not cross one another but deflect or join in a predictable manner.
To date no direct relationship has been found between filiform corrosion and the metallurgical pattern of the surface, the presence or absence of light, biological activity or the presence of inhibitive pigments. It has been observed under both non-metallic and metallic coatings, provided that the coatings are semi-permeable and are sufficiently elastic to yield without rupture to the volume of the oxide formed.
A theoretical explanation is offered for filiform corrosion, involving an initiating force, a driving force and a directing force. It is suggested that corrosion anodes are initiated by an electrolytic mechanism as in ordinary corrosion. The driving force is the diffusion of the corrosive atmosphere into the active head, resulting in further metallic corrosion. The directing force is explained on the basis of concentration cells.
