Prevent Filiform Corrosion Under Coatings

Filiform corrosion is a specific form of corrosion that occurs underneath thin coatings in threads with random distribution such as filaments. Filiform corrosion is often referred to as under film corrosion as well as filamentary corrosion or worm track corrosion. In this article, we will look at the reasons behind filiform corrosion in the area where it normally is seen, how it occurs and how to recognize it and the best way to stop it from happening.

So What Is filiform corrosion?

Filiform corrosion is a problem that occurs on metallic surfaces that have been coated with a thin organic layer that is typically 0.05 to 0.1 millimeters (2 to 4 millimeters) thick when exposed to humid and warm air. The process of forming corrosion begins with coating imperfections such as scratches as well as weak points like beards or cut edges, as well as holes. 

The filiform character in the filiform corrosion (left). Filiform corrosion tunnels from underneath a coating .

Figure 1. Filiform corrosion (left). Filiform corrosion tunnels are formed underneath the coating (right).

How Does Filiform Corrosion Arise?

In many ways, the corrosion of aluminum (Al) or magnesium (Mg) is identical to corrosion that occurs on stainless steel. It is believed that filiform corrosion occurs because it is driven by the development of aeration cells with differential aeration at the sites of defects on substrates coated.

The filiform cell comprises the active head, and also a tail which is supplied with oxygen and condensed vapor via cracks within the applied coating. The head could contain alumina gel or gas bubbles of aluminum if the head is extremely acidic. In magnesium, the head is black due to the magnesium etching. However, the corrosive fluid appears clear once the head has been broken. The tails of aluminum and magnesium are white in appearance. These corrosion products are oxides and hydroxides of magnesium and aluminum respectively. Anodic reactions generate Al3+ and Mg2+ ions that create insoluble precipitates the hydroxyl Ions produced by the reaction of oxygen reduction that is predominant at the end of the tail.

The mechanism for initiating and activation in both aluminum and magnesium is essentially identical to that of coated steel. Acidified heads are an eddy pool of electrolytes. the tail is an area where aluminum ions get transported, and a gradual reaction with hydroxyl ions takes place. The end corrosion product is partially hydrated, and then fully expanded within the tail’s porous. In the tail’s middle and head segments are the same locations for the various reactant ions in the initial reaction as well as the intermediate products of the corroding of aluminum in aqueous media. (To know more, please read Aluminum Corrosion: 5 Incredible facts you should know.)

Contrary to steel, magnesium and aluminum exhibit a greater propensity to develop blisters in acidic media in which hydrogen gas develops through a cathodic reaction within the head area. Tail’s corrosion products are aluminum trihydroxide Al(OH)3 which is a white gelatinous precipitate as well as magnesium hydroxide Mg(OH)2 which is a white precipitate.

Factors that affect the filiform corrosion process

Many factors influence the onset of filiform corrosion There are a variety of factors that can trigger the onset of filiform corrosion,

The nature of the coating

The filiform corrosion is present in all kinds of paints, including acrylic lacquers, epoxy-polyamides polyurethanes, and epoxy-amines, and regardless of the standard technique of application that is the liquid paint as well as electrostatic powdering. It is not a problem under sealed coatings like an electrician’s tape.

Surface preparation

This is a major element. Filiform corrosion is a result of metal that has not received surface preparation, or poor preparation, or on metal that is contaminated prior to coating.

The nature of the alloy

The nature of the alloy isn’t an essential element since filiform corrosion could be a problem for aluminum alloys in general. A recent study that was conducted by 3 European firms, Alusuisse, Hydro Aluminium, and Pechiney found that for the most frequently employed alloys within the construction industry 6060 and 6063 composition of the alloy is not a factor in the event that the copper content is greater than 0.1 percent.

The areas where Filiform Corrosion is the most likely to develop

Typically filiform corrosion is particularly severe in tropical and warm coastal regions where salt falls occur or in highly polluted industrial zones. The rougher surfaces are also subject to a higher degree of filiform corrosion. The filiform corrosion is typically seen with aluminum-based alloys when the humidity of 75% and 90%, and within temperatures ranging from 20degC to 40degC (68degF up to 104degF), and the growth rate increases when the relative humidity is at 85% (RH) at 85. The relative humidity of the air is the most important element that triggers filiform corrosion. (Related study: Five Factors in Atmospheric Corrosion.)

Other important parameters that govern filiform corrosion are compositions of alloys, scalping of billets and ingots and heat treatments, condition of the surface of the metal layer and temperature, grinding pre-treatment of the surface and pickling. The amount of organic coating and temperature play an important part in triggering filiform corrosion as well, raising the temperature can increase filament’s growth, provided that the relative humidity is within the range of acceptable.

How to detect the presence of filiform corrosion?

The appearance of filiform corrosion can be observed visually without the use of a microscope. It’s been seen on coated aluminum, steel and magnesium, with an extremely thin coating of silver, gold, tin, and phosphate. It can also be observed on enamel or lacquer.

The most commonly used test to determine resistance to corrosion caused by filiform corrosion that is used in the United States is ASTM D 2803, Guide for Testing Filiform Corrosion Resistance of Organic Coatings on Metal. According to this test, the coated specimens of metal are traced to the metal surface and then exposed to a fog of salt that lasts for 24 to 48 hours. They are then rinsed with distilled water and then placed into closed cabinets at 25°C (77degF) as well as 85 percent RH. This time of exposure is typically between 100 and 1000 hours. The test results will show if the coating material is prone to filiform corrosion.

Industries that are most affected by filiform Corrosion

Aircraft structural parts are secured using rivets and bolts. These fasteners, along with other skin edges that are sharp are typical initiation points that can lead to the development of filiform corrosion. It’s been reported the fact that planes that operate in warmer marine environments suffer significant corrosion damage, especially in the aluminum alloys 2024 and 7000 that have been coated in polyurethane as well as other coatings.

Humidity is the primary element for corrosion to develop since it is required for the dissolution of salt Ions.

Corrosion usually begins when there is a defect in the substrate and coating layer. The defect can occur by a scratch or stone chip which weakens the bond of adhesive to the substrate and the coating.

The corrosion begins at this location and creates the head of the corrosion defect. The corrosion usually is seen as an individual thread-like filament that appears similar to a worm’s track which appears beneath the coating surface.

The damage isn’t too severe to the aluminum, but it can be aesthetically objectionable, particularly where the track runs long, and is white in the color.

This kind of filiform corrosion could cause harm to all kinds of aluminum items like wheels, automobile bodies, and aircraft. Repairing the damage, requires the application of a new coating. coating. To avoid filiform corrosion, proper surface preparation is necessary.

This form of corrosion became more severe in the instances when the chloride levels on the metal were excessive, particularly when aircraft often flew over the ocean or were situated in hangers at airfields located along the coast.

Aluminum is extensively used in cans as well as other kinds of packaging. Aluminum foil is often coated with paper or paperboard to create a moisture or vapor barrier. In the event that the foil is consumed by filiform corrosion, it is possible for the product to be contaminated or dry out due to the fact that the vapor barrier has been broken. Degradation of foiled laminated paperboard can be triggered during the process of production or storage in a humid environment.

For the auto industry forging, distinct light alloys with twin-tone areas (polished sections) or polished surfaces have an increased likelihood of filiform corrosion.

How to Prevent the Filiform Corrosion

Usually, filiform corrosion is prevented by decreasing the relative humidity to below 60 percent. However, it’s not feasible to cut down on the humidity of moving objects like automobiles and aircraft. However, the level of humidity of the components stored in storage facilities for a long time is easily managed by adding drying fans and humidistats or using desiccants in plastic containers.

Components that are primed using 2 layers of epoxy coating systems and two polyurethane coats are more able to be resistant to filiform corrosion than systems that are coated in a single layer.

The risk to develop filiform corrosion is decreased by the time the substrate gets galvanized. Zinc-rich primers as well as those that are phosphatized and chromated have tough slow curing intermediate coats made of polyurethane and epoxy which have decreased susceptibility to filiform corrosion for steel substrates. Zinc primers chromate as well as chromic acid anodizing and chromate, or chromate-phosphate conversion coatings provide different levels of protection against filiform corrosion in aluminum alloys. (Another option is covered in the article titled Advances on Liquid Nylon Multipolymer coatings for in the Transportation and Renewable Energy Industries.)

Multiple coats applied to metal surfaces reduce the spread of moisture. They also have fewer areas of penetration and defects than one-coat paint systems. Multi-coat paint systems block penetration through mechanical abrasion and also have fewer valleys and hills. Thicker coatings created by layer building and slow curing have shown significantly better resistance towards filiform corrosion due to a decrease in the penetration of moisture and oxygen as well as reducing solvent entrapment and fewer initiation sites. Powder coating systems also benefit since they are thermally fused which results in hard coatings with greater resistance to water permeability. Well-made, smooth primed metal surfaces typically have superior resistance against rougher surfaces.

The elements magnesium, aluminum, and steel can all be chemically active. Their alloys are composed of intermetallic compounds dispersed, precipitated, or formed throughout hot rolling and an annealing process. While these alloys typically have enhanced mechanical properties, However, recent research shows they are heterogeneous (intermixture) along with the inclusion of active layers on their surface make them more susceptible to filiform corrosion.