Failure Analysis

Filiform Corrosion

Filiform corrosion is a form of under-film corrosion that spreads in thin, worm-like threads beneath a coating. It is common on coated aluminum and steel in humid conditions.

Coatingpedia Editorial TeamUpdated October 29, 2026 4 min read

Filiform corrosion is a type of corrosion that grows beneath organic coatings in the form of thin, randomly directed threads or filaments. The name comes from the Latin filum, meaning thread. Each filament has an active head that advances and a tail of corrosion products behind it. Filiform corrosion most often affects coated aluminum, magnesium and steel in humid environments, and is a significant concern in architectural aluminum, aircraft, automotive parts and packaging. It is related to, but distinct from, undercutting corrosion at a scribe or defect.

Appearance

Fine, worm-like threads visible through the coating, typically a few tenths of a millimeter to a few millimeters wide.
Threads often start at edges, cut edges, scratches, fastener holes or pinholes.
On aluminum, threads appear as white or gray lines under the film; on steel, as reddish-brown lines.
Threads rarely cross each other and may bend or reflect when they approach another filament.
The coating above the filaments is usually raised slightly.

Mechanism

Filiform corrosion is an electrochemical process (see corrosion science basics) that typically needs:

A coating defect or edge where moisture and contaminants can reach the metal.
High relative humidity, often above about 60 to 65 percent, with the most active growth at higher humidity.
An initiating contaminant, often chlorides from salt or other ionic species.
Oxygen, which diffuses through the coating to the tail of the filament.

The head of the filament is acidic and oxygen-poor, acting as the anode where metal dissolves. The tail, where oxygen reaches the metal through the porous corrosion products, acts as the cathode. This separation drives the filament forward. The process is a form of differential aeration, similar to crevice corrosion (see pitting and crevice corrosion).

Where it occurs

Architectural aluminum such as window frames, curtain walls and storefronts, especially in coastal areas.
Aircraft skins and painted aluminum components (see aerospace coatings).
Automotive wheels and trim, particularly at chips and edges.
Coated steel cans and packaging.
Coil-coated sheet at cut edges (see coil coating).

Factors that increase risk

Inadequate pretreatment of aluminum, such as poor cleaning or missing conversion coatings.
Residual contaminants such as salts from processing or handling.
Thin or porous films at edges where coatings pull away during cure (see edge corrosion).
Mechanical damage that exposes metal.
Certain alloys with higher copper content, which are more susceptible.
Coastal or de-icing salt exposure combined with humidity.

Testing

Laboratory tests deliberately initiate filiform corrosion so that pretreatments and coatings can be compared:

ASTM D2803, a guide for filiform corrosion resistance of organic coatings on metal.
ISO 4623, parts 1 and 2, for steel and aluminum substrates.
Automotive and aerospace specifications with their own procedures.

A typical test involves scribing the coated panel, exposing the scribe briefly to an acid or salt vapor to initiate corrosion, then storing the panel at controlled temperature and high humidity for a set period. Filament length and density are then measured. Results are used for ranking rather than predicting exact service life (see salt spray and cyclic corrosion testing).

Prevention

Pretreatment

Proper cleaning, deoxidizing and conversion treatment of aluminum is the most important factor. Anodizing, chromate and non-chromate conversion coatings, and other pretreatments improve resistance (see anodizing).

Coating system

Use primers with corrosion-inhibiting pigments where appropriate.
Apply adequate film thickness, especially at edges.
Select coatings with low water vapor permeability (see water vapor transmission of coating films).

Design and fabrication

Round or seal sharp edges.
Avoid cutting coated parts after finishing, or seal cut edges.
Minimize crevices where moisture collects.

Maintenance

Clean surfaces to remove salts in coastal and urban environments.
Repair chips and scratches promptly.

Repair

Once filiform corrosion is established, the affected coating usually must be removed, the corrosion products cleaned off, the metal pretreated again, and a new coating system applied. Painting over active filaments does not stop them.

In practice

In practice, filiform corrosion is most often found on painted aluminum in coastal or humid areas, such as window frames, trailers, aircraft parts and architectural panels. Inspectors look first at cut edges, fastener holes and scratches, where filaments usually start. Prevention focuses on proper pretreatment, sealed edges and good film thickness. Once filaments are present, repair normally means removing the affected coating, cleaning and pretreating the metal again, and recoating.

Key terms

Filament. The thread-like corrosion track that travels beneath the coating.
Active head. The leading end of the filament, where oxygen depletion and acidity drive corrosion.
Tail. The trailing section of the filament filled with corrosion products.
Scribe. A cut through the coating that serves as the starting point in laboratory filiform tests.
Pretreatment quality. The single biggest factor in resistance, especially on aluminum.

Frequently asked questions

Does filiform corrosion weaken aluminum?

It is usually a shallow, cosmetic form of corrosion at first, but it damages appearance and coating integrity and can lead to more serious corrosion if ignored.

Can powder coatings suffer filiform corrosion?

Yes. Powder coatings on poorly pretreated aluminum are commonly affected. Pretreatment quality is the main defense (see powder coatings).

Why does it happen in humid conditions but not in rain?

Filiform growth depends on a specific humidity range that keeps the filament head moist. Liquid water often leads to blistering or general corrosion instead.

Educational reference. Coating performance varies by formulation. Always follow the manufacturer’s product data sheet, safety data sheet and your project specification.