How to Prevent Filiform Corrosion Getting Underneath Your Coatings

Filiform corrosion is a distinct form of corrosion that occurs under thin coatings in randomly distributed threads like filaments. Filiform corrosion is also identified as under film corrosion, filamentary corrosion, or worm track corrosion. This article explores the causes of filiform corrosion, where it typically appears, how it develops, how to identify it, and how to stop it from occurring.

What is filiform corrosion?

Filiform corrosion occurs on metallic surfaces covered with a thin organic film, typically 0.05 to 0.1 mm (2 to 4 mils) thick when revealed to warm, humid atmospheric air. Filiform corrosion always begins at coating defects such as scratches and weak features such as beards, cut edges, and holes. 

How Filiform Corrosion Occurs

In many regards, filiform corrosion on aluminum (Al) and magnesium (Mg) is similar to corrosion on steel. Filiform corrosion is driven by the formation of differential aeration cells at defect sites on coated substrates.

The filiform cell consists of an active head and a tail that receives oxygen and condensed water vapor through cracks in the applied coating. The head may be filled with alumina gel and gas bubbles in aluminum if the head is very acidic. In magnesium, the head looks blackish because of the magnesium etching, but the corrosive liquid is clear when the head is cracked. Filiform ends in aluminum and magnesium are whitish in appearance. The corrosion products are hydroxides and oxides of aluminum and magnesium. Anodic results produce Al3+ or Mg2+ ions, which react to form insoluble precipitates with the hydroxyl ions generated in the oxygen-reduction reaction transpiring predominately in the tail.

The mechanism of initiation and activation in aluminum and magnesium are the same as for coated steel. The acidified head is a moving pool of electrolytes, but the tail is a region where aluminum ions are transported, and a continuous reaction with hydroxyl ions occurs. The final corrosion products are partially hydrated and fully expanded in the porous tail. The tail's head and middle sections are similar locations for the various initial reactant ions and the intermediate products of corroding aluminum in aqueous media. 

In contrast to steel, aluminum, and magnesium display a greater tendency to form blisters in acidic media, with hydrogen gas evolved in cathodic reactions in the head region. The tail's corrosion products are either aluminum trihydroxide Al(OH)3, a whitish gelatinous precipitate, or magnesium hydroxide Mg(OH)2, a whitish residue.

Factors Affecting Filiform Corrosion

Various factors affect the initiation of filiform corrosion, including:

The nature of the coating

Filiform corrosion occurs with all types of paints: acrylic lacquersepoxy-polyamidesepoxy-amines, and polyurethanes, and with whatever the classic mode of application, whether it be liquid paint or electrostatic powdering. It does not occur under sealed coatings such as electrician's tape.