Filiform corrosion

Filiform corrosion is a particularly insidious form of corrosion that occurs under protective coatings such as paints, enamels, or metallic coatings. This type of corrosion is characterized by the formation of thin, thread-like filaments that develop beneath the surface coating, compromising the integrity of the underlying metal and the protection provided by the coating.

Mechanism of Formation

Filiform corrosion primarily develops on surfaces covered by an organic coating, such as paint, that is not completely sealed. Favorable conditions for the development of filiform corrosion include a humid environment and the presence of chlorides or other aggressive ions. The process generally begins at a point of weakness in the coating, such as a scratch, chip, or small imperfection.

Once moisture penetrates beneath the coating, an electrochemical reaction starts. The entry point of the moisture acts as the anode, where the metal oxidizes, and the oxygen present within the humid environment serves as the cathode, where oxygen reduction occurs. This creates a potential difference that drives the propagation of corrosion in the form of filaments.

Appearance and Diagnosis

Visually, filiform corrosion appears as a series of thin, irregular lines that branch out beneath the surface coating. These filaments can be easily recognized if the coating is transparent or semi-transparent. In opaque coatings, diagnosis can be more complex and often requires removing the coating in small suspected areas to confirm the presence of filiform corrosion.

Materials and Surfaces Susceptible

The metals most susceptible to filiform corrosion include:

Aluminum: Especially when anodized or painted, aluminum is particularly vulnerable to filiform corrosion.

Galvanized steel: Although the zinc layer provides some protection against corrosion, filiform corrosion can still develop beneath the organic coating applied over the zinc.

Magnesium and its alloys: Known to be highly reactive, these are prone to filiform corrosion when coated.

Prevention

Preventing filiform corrosion requires careful design and application of coatings. Some preventive measures include:

Surface Preparation: Ensure the metal surface is clean and free of contaminants before applying the coating. This can include degreasing, sandblasting, and chemical conversion treatments.

Choice of Coating: Use high-quality coatings with good adhesion and barrier properties. Coatings with specific anti-corrosive properties can reduce the risk of filiform corrosion.

Application of Coating: Apply the coating evenly and without defects. Ensure adequate thickness and complete coverage of edges and hard-to-reach surfaces.

Environmental Control: Minimize exposure to environmental conditions that favor corrosion, such as marine or industrial environments with high humidity and the presence of chlorides.

Repair and Maintenance

Once filiform corrosion is detected, it is essential to intervene promptly to prevent further damage. Typical repair operations include:

Coating Removal: Remove the coating around the affected area to fully expose the corrosive filaments.

Surface Cleaning: Clean the corroded surface using mechanical or chemical methods to remove corrosion products and prepare the surface for a new coating.

Application of New Coating: Apply a new protective coating following best practices to ensure good adhesion and coverage.

Conclusion

Filiform corrosion represents a significant challenge for industries, especially those that rely on protective coatings to maintain the integrity of their products. Understanding the mechanisms of formation, adopting effective preventive measures, and promptly intervening in the event of occurrence are crucial steps in managing this type of corrosion. With a proactive approach, it is possible to significantly reduce the impact of filiform corrosion and extend the service life of metallic components.