Filiform corrosion—also known as "underfilm," "localized" or "filamentary" corrosion—is a type of corrosion occurring commonly within magnesium and aluminum alloys that use an organic form of coating (typically, organic coatings 0.05 to 0.1 millimeters thick) when exposed to warm and humid air. However, it can also occur on other coated metals such as steel, iron and zinc.
The mechanism for corrosion allows water and oxygen to migrate. The dissolved oxygen has its highest concentration at the back of the head. When the oxygen is reduced in the tail region, the metal ion dissolution and formation proceeds to the head. This type of corrosion tends to take place in high-humidity conditions. Nitrates, sulfates, carbonates and condensates that contain halides have been associated with filiform corrosion.
Filiform corrosion affects surface appearance with no consequence for the substrate’s mechanical resistance. This type of corrosion develops on defects or weak points of lacquered surfaces—and surfaces similar to lacquered surfaces—and spreads in the form of filaments.
Filaments are made up of a head and a tail; these, in turn, make up the mobile electrochemical cell on the surface of a substrate. These make up the two different sites:
- The filament head, which is the anode.
- The filament tail, which is the cathode.
Filiform corrosion can occur in conditions slightly above room temperature and at a humidity level of 75%.
In places where filiform corrosion has occurred, a thread-like filament forms under the coating. The coating will bulge and have an appearance like that of a lawn riddled by mole tunnels. The filament will then continue to form until the coating is no longer continuous. Filiform corrosion can form with forming with a number of coating systems.
Filiform corrosion's unique appearance resembles fine filaments that look like worm-like threads and that seem to emanate from one or more defects in a slightly random directions. Normally, it doesn't extensively damage the metal. However, it has a detrimental effect on the corroded metal's appearance.
For instance, filiform corrosion within magnesium or aluminum can form a white precipitate, resulting in a tail. On the other hand, filiform corrosion in iron tends to form a head with green fluid and a tail of red precipitate. Filiform corrosion often starts off as coating defects like scratches and weak points such as beards and holes.
A number of approaches have been known to reduce filiform corrosion. One of them is applying several coating layers. One might also use a chromate containing a primer on aluminum or a conversion coating. Zinc with a primer on steel can also be used as a treatment.