Filiform Corrosion

Filiform corrosion is a type of localized corrosion that occurs on the surface of coated metals. It appears as thread-like or worm-like narrow filaments or ridges, which propagate under a coating or paint film, resulting in visible blisters or lifted coatings.

Corrosion mechanism

Filiform corrosion is a type of corrosion that occurs beneath a coating. This type of corrosion starts at imperfections or defects in the coating where corrosive species can reach the metal substrate and form corrosion products. The propagation of corrosion under the coating causes it to lift, as the corrosion products take up more space than the original metal. Filiform corrosion is electrochemical in nature, where the area at the head of the worm becomes anodic and the region at the tail cathodic. Here the head contains more metal ions and less oxygen and chloride ions than the tail. This creates a potential difference that promotes corrosion at the anode and increases the rate of corrosion, ultimately causing the delamination of the coating.

Illustration of a cross-section of a coated metal that suffers from filiform corrosion

How to recognize filiform corrosion?

Filiform corrosion can be recognized by its characteristic appearance. It usually appears as thin, worm-like, thread-like or filament-like lines, which can be either straight or irregular, that emerge from a point under the coating and spread outwards. These lines can have different colors, such as brown, black, or red. As the corrosion progresses, the coating may lift, creating blisters or bumps on the surface.

Filiform corrosion in daily life

Filiform corrosion can occur on any metal surface that is coated with a paint, enamel, or other protective layer.

It is commonly found in the automotive industry on the metal bodies of cars and trucks, particularly on areas that are exposed to moisture or salt, such as wheel wells and door frames. It can also occur on metal structures, such as bridges and pipelines, that are exposed to environmental factors such as humidity, rain, and salt spray. Additionally, filiform corrosion can occur on household items such as metal appliances or furniture, particularly those exposed to humid or damp environments.

How to prevent filiform corrosion?

There are several ways to prevent filiform corrosion. Here are some examples:

Before applying a coating, ensure that the surface is free from any contaminants such as oil, grease, or moisture. Next, a coating is applied that is known to be corrosion resistant in the application environment.

In some cases, coatings with corrosion inhibitors may be preferred over regular coatings. That’s because the corrosion inhibitors provide addition protection at the imperfections of the coatings or where the coating is locally mechanically damaged.

Early signs of filiform corrosion can be detected by regular inspection. This allows to perform repairs and prevent the propagation of the corrosion under the coating.

Illustration of an engineer performing inspection and repair