Definition - What does Filiform Corrosion mean?
Filiform, or underfilm corrosion is a type of corrosion that is commonly known as "localized" and is normally linked to magnesium and aluminum alloys that utilize an organic form of coating. 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 has a tendency of taking place in conditions with a high level of humidity. Nitrates, sulfates, carbonates and condensates that contain halides have been associated with filiform corrosion.
Corrosionpedia explains Filiform Corrosion
Filiform corrosion can occur in places with conditions slightly above room temperature and at a humidity level of 75%. In places where the corrosion has taken place, there is a thread-like filament appearance that forms under the coating.
The coating will bulge and have an appearance like that of a lawn riddled by the tunnels of a mole, due to the effect of the corrosion. The filament will then continue to form up to the points where the coating is no longer continuous. There are possibilities of filiform corrosion forming with a number of coating systems.
Normally, the damage to the metal is not to a large extent. However, there is the detrimental effect to the appearance of metal that has been corroded. For instance, the filiform corrosion of magnesium or aluminum can form a precipitate that is white in color, resulting in a tail. On the other hand, the filiform corrosion of iron tends to form a head that has a fluid that is green in color and a tail of a red precipitate.
A number of approaches have been known to reduce the effect of filiform corrosion. One of them is the application of a number of layers of coating. Use of a chromate that conatins a primer on aluminum or conversion coating has also been known to reduce filiform corrosion. In addition to that, zinc that has a primer on steel can be used.