What is Filiform Corrosion?
Filiform corrosion is a type of localized corrosion that occurs on metal surfaces that are coated with an organic coating such as paint. It is also known as “underfilm,” “localized,” or “filamentary” corrosion. This type of corrosion is commonly observed on aluminum and magnesium alloys that use an organic coating that is typically 0.05 to 0.1 millimeters thick.
Filiform corrosion appears as thin, worm-like filaments that spread under the coating on the metal surface. The filaments are usually white or gray and have a diameter of 0.1 to 1.0 millimeters. The corrosion appears to originate at a small point on the surface of the metal and then spreads in a radial pattern. The corrosion products are usually white or gray and can be seen under the coating.
Corrosion occurs when moisture or a corrosive solution penetrates through a defect in the coating and comes into contact with the metal surface. The defect can be a pinhole, scratch, or other imperfection in the coating. Once the metal surface is exposed, an electrochemical reaction occurs between the metal and the environment, leading to the formation of corrosion products. The corrosion products then accumulate under the coating, causing it to lift and peel away from the metal surface.
Filiform corrosion is a common problem in the automotive, aerospace, and marine industries, where aluminum and magnesium alloys are widely used. It can cause significant damage to the metal surface and lead to the failure of the coating. To prevent corrosion, it is essential to use high-quality coatings that are free from defects and to ensure that the metal surface is properly prepared before applying the coating. Regular inspection and maintenance of the coating are also necessary to detect and repair any defects before they can lead to corrosion.
Causes of Filiform Corrosion
Filiform corrosion is a type of localized corrosion that occurs 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. The following are some of the most common causes of filiform corrosion:
- Defects in the coating: Corrosion often starts at small defects in the coating such as pinholes, scratches, or cracks. These defects allow moisture and other corrosive substances to penetrate the coating and reach the metal surface. Once the metal surface is exposed, corrosion can begin.
- Humidity: High humidity levels can cause corrosion to occur. When the relative humidity is high, moisture can penetrate the coating and reach the metal surface, which can lead to corrosion.
- Temperature: High temperatures can also contribute to the development of corrosion. When the temperature is high, the rate of corrosion increases, which can lead to the formation of corrosion.
- Contaminants: Contaminants such as salts, acids, and other corrosive substances can also contribute to the development of corrosion. These contaminants can penetrate the coating and reach the metal surface, where they can cause corrosion to occur.
- Mechanical damage: Mechanical damage to the coating can also contribute to the development of corrosion. When the coating is damaged, it can allow moisture and other corrosive substances to penetrate the coating and reach the metal surface, which can lead to corrosion.
In summary, filiform corrosion is a type of localized corrosion that occurs commonly within magnesium and aluminum alloys that use an organic form of coating. The most common causes of filiform corrosion include defects in the coating, humidity, temperature, contaminants, and mechanical damage.
Effects of Filiform Corrosion
Filiform corrosion is a type of localized corrosion that can occur on metal surfaces that are coated with an organic film. This type of corrosion can occur on a variety of metals, including aluminum and magnesium alloys. The effects of corrosion can be severe, leading to the degradation of the metal surface and potentially compromising the integrity of the entire structure.
One of the primary effects of filiform corrosion is the formation of filaments or “worms” on the metal surface. These filaments can be several millimeters long and can cause the coating to lift or peel away from the metal surface. As the filaments grow, they can create channels for moisture and other corrosive agents to penetrate the metal surface, leading to further corrosion and degradation.
Filiform corrosion can also cause discoloration and staining on the metal surface. This can be particularly problematic for applications where aesthetics are important, such as in the automotive or aerospace industries. In addition to the visual effects, corrosion can also lead to the formation of pits and other surface defects, which can compromise the strength and durability of the metal.
Another potential effect of filiform corrosion is the release of toxic gases or chemicals. This can occur when the organic film on the metal surface is degraded, releasing volatile organic compounds (VOCs) or other harmful substances into the environment. This can be a particular concern in industrial or manufacturing settings where large quantities of metal are used and exposed to the elements.
Overall, the effects of filiform corrosion can be significant and can compromise the integrity and safety of metal structures. It is important to take steps to prevent and mitigate the effects of corrosion, including regular inspection and maintenance of metal surfaces, the use of corrosion-resistant coatings and materials, and the implementation of effective corrosion control strategies.
Prevention and Mitigation Strategies
Filiform corrosion is a type of corrosion that affects metallic surfaces that are coated with a thin organic film. The corrosion occurs when the surface is exposed to warm, humid atmospheric air. The corrosion typically starts at coating defects such as scratches, and weak points such as beards, cut edges, and holes. Here are some prevention and mitigation strategies to consider:
Coating Selection
The type of coating used on a metallic surface is an important consideration in preventing filiform corrosion. It is important to select a coating that is resistant to moisture and can withstand the environmental conditions to which it will be exposed. Coatings that have a high resistance to water and can form a tight bond with the metallic surface are preferred.
Surface Preparation
Proper surface preparation is essential in preventing filiform corrosion. The surface should be thoroughly cleaned, degreased, and dried before the coating is applied. Any rust or other contaminants should be removed using a wire brush or sandpaper. The surface should be free of any defects, such as scratches or dents, before the coating is applied.
Humidity Control
Humidity control is an important factor in preventing filiform corrosion. The relative humidity in the environment should be kept below 60% to prevent the formation of water droplets on the surface of the coating. If the humidity cannot be controlled, a coating that is resistant to moisture should be used.
Inspection and Maintenance
Regular inspection and maintenance of the coated surfaces can help prevent filiform corrosion. Any defects, such as scratches or dents, should be repaired immediately to prevent the corrosion from spreading. The coating should be inspected for signs of damage, such as cracking or peeling, and repaired as needed.
Corrosion Inhibitors
Corrosion inhibitors can be used to prevent filiform corrosion. These inhibitors work by forming a protective layer on the surface of the metallic substrate, which prevents the corrosion from occurring. Inhibitors can be applied as a coating or added to the coating formulation.
Cathodic Protection
Cathodic protection is a technique used to prevent corrosion by making the metallic substrate the cathode in a corrosion cell. This is achieved by applying a small electric current to the metallic substrate, which reduces the potential difference between the anode and cathode. Cathodic protection can be used in conjunction with other corrosion prevention techniques to provide additional protection against filiform corrosion.
In summary, filiform corrosion can be prevented and mitigated by selecting the right coating, properly preparing the surface, controlling humidity, inspecting and maintaining the coated surfaces, using corrosion inhibitors, and applying cathodic protection. By following these strategies, the risk of filiform corrosion can be minimized, and the life of the coated surface can be extended.
Frequently Asked Questions
What are the characteristics of filiform corrosion?
Filiform corrosion is a type of localized corrosion that appears as thin, worm-like filaments underneath a coating on a metal surface. It is often seen in areas where the coating has been breached, allowing moisture to enter and create an electrochemical cell. The filaments can vary in length and thickness and may be accompanied by a powdery or flaky residue.
What materials are susceptible to filiform corrosion?
Filiform corrosion is commonly found in aluminum and magnesium alloys that have been coated with an organic coating. The thickness of the coating can also play a role in susceptibility, with thinner coatings being more prone to corrosion. Other metals that can be affected include steel, zinc, and copper.
How is filiform corrosion detected and diagnosed?
Filiform corrosion can be detected visually by the appearance of the worm-like filaments and associated residue. It can also be identified using various testing methods, such as salt spray testing, humidity testing, and electrochemical impedance spectroscopy. A thorough visual inspection and testing can help diagnose the extent of the corrosion and determine the appropriate treatment.
What are the treatment options for filiform corrosion?
Treatment options for filiform corrosion depend on the severity of the corrosion and the type of metal and coating involved. In some cases, simply removing the affected coating and applying a new one can be sufficient. More severe cases may require blasting or chemical stripping to remove the corrosion and then applying a new coating. In extreme cases, the metal may need to be replaced entirely. Preventative measures such as proper coating selection, surface preparation, and maintenance can also help prevent corrosion from occurring.