and aluminum or any of their alloys and coating 12 can be any type of coating applied to metal or metal alloys including acrylic coated metal objects are highly susceptible to underfilm corrosion underfilm corrosion begins when the metal substrate is exposed to moisture and oxygen this can occur because of imperfections in the coating or because of the diffusion of oxygen and water through the coating fig2 illustrates typical filiform corrosion on a coated metal object rust filaments 16 follow paths that approximately radiate from a point of origin as filiforms grow or propagate they occasionally have an opportunity to interact those skilled in the art know that an active filiform head will not cross an inactive tail of a rust filament instead filiform heads u201c reflect u201d from the tail and can become entrapped as the space available for the filiform to grow diminishes fig3 shows a detail view of the self entrapment of rust filament 16 the arrow in fig3 indicates the direction of propagation of filiform 16 as rust filament 16 propagates it reflects off both the tails of other filiform and its own tail after several reflections rust filament 16 creates an inactive perimeter which it cannot cross and its growth is thereby limited to the region within the inactive perimeter the general concept of.