Filiform corrosion is a specialized form of corrosion that occurs under a thin coating of randomly distributed threads like filaments. Filiform corrosion is also known as subfilm corrosion, filiform corrosion or worm corrosion. In this article, we look at what causes filiform corrosion, where it usually occurs, how it develops, how to detect it and how to prevent it from happening.
What is Filiform Corrosion?
Filiform corrosion occurs on metal surfaces coated with organic films, typically 0.05 to 0.1 mm (2 to 4 mils) thick, when exposed to a warm, moist atmosphere. Filiform corrosion always starts with coating defects such as scratches and weak points such as whiskers, cut edges and holes.
How Filiform Corrosion Occurs
In many respects, filiform corrosion of aluminum (Al) and magnesium (Mg) is similar to corrosion of steel. Filiform corrosion is driven by the formation of different aeration cells at defect sites on the coated substrate.
Consisting of an active head and a tail, filamentous cells receive oxygen and condensed water vapor through cracks in the coating. If the head is very acidic, the head may be filled with alumina gel and air bubbles in the aluminum. In magnesium, the head appears blackened by magnesium etching, but the caustic fluid is transparent when the head breaks. The filamentous tails of aluminum and magnesium have a white appearance. The corrosion products are hydroxides and oxides of aluminum and magnesium, respectively. The anode reaction generates Al 3+ or Mg 2+ ions, which react to form insoluble precipitates, while the hydroxyl ions generated in the oxygen reduction reaction mainly occur in the tail.
The initiation and activation mechanisms for aluminum and magnesium are essentially the same as for coated steel. The head of the acidification is the mobile pool of electrolyte, while the tail is the region where aluminum ions are transported and gradually react with hydroxide ions. The final corrosion product is partially hydrated and fully expanded in the porous tail. The head and middle of the tail are the corresponding positions in the aqueous medium for various initial reactant ions and intermediate products that corrode aluminum.
Compared with steel, aluminum and magnesium are more prone to bubble formation in acidic media, and hydrogen gas is produced in the cathodic reaction in the head area. The corrosion products at the tail were either aluminum hydroxide Al(OH) 3 , a white gel-like precipitate, or magnesium hydroxide Mg(OH) 2 , a white precipitate.
Factors Affecting Filiform Corrosion
Many factors can affect the onset of filiform corrosion, including:
The nature of the coating
Filiform corrosion occurs with all types of paints: acrylic paints, epoxy polyamides, epoxy amines and polyurethanes, whether liquid paints or electrostatic powders, regardless of the classic mode of application. This doesn't happen under seal coats like electrical tape.
surface preparation
This is an important factor. Filiform corrosion occurs on metals that have not been surface prepared, have been poorly prepared, or where the surface has been contaminated prior to painting.
properties of the alloy
The nature of the alloy is not an important factor as filiform corrosion can affect all aluminum alloys. A recent collaborative study by three European companies, Alusuisse, Hydro Aluminum and Pechiney, showed that for aluminum alloys 6060 and 6063 commonly used in the construction industry, the alloy composition has no effect unless the copper concentration exceeds 0.1%.
Where filiform corrosion is prone to occur
Typically, filiform corrosion is severe in warm coastal and tropical regions subject to salinity decline or heavily polluted industrial areas. Rough surfaces also experience more severe filiform corrosion. Aluminum alloys typically undergo filiform corrosion at humidity levels between 75% - 90% and temperatures in the range 20°C - 40°C (68°F - 104°F), and at 85% relative humidity. Accelerated growth (RH) levels. The relative humidity of the atmosphere is a key factor in initiating filiform corrosion. (Related reading: 5 Factors of Atmospheric Corrosion.)
Other main parameters controlling filiform corrosion are alloy composition, ingot and billet peeling, heat treatment, condition of metal surface layers, temperature, grinding, pickling and preliminary surface preparation. While organic coating thickness and temperature play a minor role in initiating filiform corrosion, increasing temperature increases filament growth if relative humidity is maintained within a critical range.
How to Detect Filiform Corrosion
Filiform corrosion can be identified visually without the use of a microscope. This phenomenon has been observed on steel, aluminum, and magnesium coated with thin coatings of tin, gold, silver, phosphate, enamel, or lacquer.
The standard test for identifying filiform corrosion resistance in the United States is ASTM D 2803, "Guide for Testing Filiform Corrosion Resistance of Metal-Organic Coatings." According to this test, coated metal coupons are scored onto bare metal and placed in a salt spray atmosphere for up to 24 hours, rinsed in distilled water, and then placed wet at 25°C (77°F) and 85 % relative humidity in a closed cabinet. Exposure times typically range from 100 to 1000 hours. The test results indicate whether the coating material produces filiform corrosion.
Industries Highly Affected by Filiform Corrosion
Aircraft structural components are fastened with bolts and rivets. These fasteners and other sharp skin edges are common starting points for filiform corrosion. Aircraft operating in warm marine environments have been reported to suffer considerable corrosion damage, especially with 2024 and 7000 aluminum alloys coated with polyurethane and other coatings.
Humidity is a critical variable for corrosion propagation, as it is necessary to dissolve salt ions.
Corrosion usually begins where there are defects in the substrate and coating. Defects can come from scratches or stone chips that weaken the bond between the substrate and coating.
Corrosion starts from this location, forming the head of the corrosion defect. Corrosion often appears as distinct thread-like filaments, such as worm tracks, that appear below the surface of the coating.
The damage to the aluminum wasn't extensive, but it was visually offensive, especially if the track was long and white in color.
This filiform corrosion can damage all types of aluminum products such as wheels, car bodies and aircraft. To repair damage, it needs to be sanded and a new coat applied. To prevent filiform corrosion, proper surface pretreatment is required.
Filiform corrosion is more severe when the concentration of chlorides on the metal is higher, mainly when aircraft are frequently flown over oceans or are located in coastal airport hangars.
Aluminum is widely used in cans and other types of packaging. Aluminum foil is often laminated to paper or cardboard to form a moisture or vapor barrier. If the aluminum foil is filiformly corroded, the product may become contaminated or dry out due to a compromised vapor barrier. Foil-laminated paperboard may degrade during its production or during subsequent storage in humid environments.
In the automotive industry, forged, unique light-alloy wheels with two-tone surfaces (polished parts) and/or polished surfaces show an increased tendency for filiform corrosion.
How to Prevent Filiform Corrosion
Typically, filiform corrosion can be prevented by reducing the relative humidity to below 60%. Unfortunately, it is not practical to directly reduce the humidity of moving objects such as airplanes and cars. However, humidity levels for components kept in long-term storage facilities can be easily controlled by adding a drying fan and humidistat, or adding a desiccant to the plastic packaging.
Components coated with a two-coat epoxy coating system and two coats of polyurethane coating resisted filiform corrosion better than a single-coat system.
When the steel substrate is galvanized, the chance of filiform corrosion is reduced. Zinc-rich primer and chromated and phosphated primer with a tough, slow-curing polyurethane and epoxy resin intermediate coat to reduce filamentary susceptibility on steel substrates. Zinc chromate primers, chromate anodizing, and chromate or chromate-phosphate conversion coatings provide varying degrees of filiform corrosion mitigation in aluminum alloys. (Another option is discussed in Advances in Liquid Nylon Polymer Coatings for the Transportation and Renewable Energy Industries.)
Multi-layer coatings on metal surfaces slow the diffusion of moisture and have fewer penetrations and defect sites than single-coat paint systems. The multi-coat system resists penetration from mechanical abrasion and has fewer hills and valleys. Thicker coatings obtained through layer buildup and slower curing have significantly improved resistance to filiform corrosion through reduced oxygen and moisture penetration, reduced solvent entrapment, and fewer initiation sites. Powder coating systems are also beneficial because they are hot melt, resulting in a tough coating with better resistance to moisture penetration. Smooth, well-prepared primed metal surfaces generally have better electrical resistance than rough surfaces.
Steel, aluminum and magnesium are all chemically reactive. Their alloys contain intermetallic compounds that disperse, precipitate and agglomerate during hot rolling and annealing. Although these alloys generally have improved mechanical properties, recent work has shown that their heterogeneity (mixing) and the presence of surface-active layers increase their susceptibility to filiform corrosion.
