Filiform
Corrosion
Filiform corrosion is a rather peculiar form of localized corrosion that affects painted
metals. The corrosion pattern consists of thread-like filaments (“worm tracks”) that meander on a metal surface under the coating. Examples are shown in Figures 1 and 2. Filiform
is common on steel, aluminum, tin, and magnesium, but does not occur on zinc or zinc-coated steel. Filiform used to be a serious problem on auto bodies, where filaments often
massed to form scab corrosion (as in the center area of Figure 1). The problem disappeared
almost completely when manufacturers changed from cold-rolled steel to galvanized steel.
However, a few automobiles have aluminum bodies as do most fire trucks and emergency
vehicles, so filiform still can be a problem on some vehicles. Filiform also occurs on food
and beverage cans, aluminum wheels, and coil-coated metal.
Filiform tends to occur over metal that is contaminated with salts or where the coated metal
is exposed to salt water often and/or for long periods of time. Filiform often occurs at cut edges and where paint has been damaged
(scratches, chips, edge damage). The problem is worse at high humidity (60–5% RH) and high temperature (75–100 F) and is more
likely to occur where adhesion is poor. The filaments are thin (0. 2–2.0 mm) and shallow, but can be quite long. Growth rates of
0.01 mm/day to nearly 1 mm/day have been observed. Filaments often move randomly, but will follow characteristic machining or
extrusion directions when they exist. Filaments rarely cross one another, but rather turn or stop growing when faced with another
filament. The head of a filament is an active corrosion cell that travels over the surface of the metal leaving behind a tail consisting
of corrosion products such as oxides and hydroxides. The movement is very worm-like, which explains the colloquial name, “worm
tracks.” Multiple filaments or worms are common.
Filiform corrosion begins with electrolyte going through or under the coating or else with water penetrating and encountering
a soluble salt on the surface of the metal. Either way, electrolyte is in contact with the metal. A small amount of metal dissolves,
ions form, and osmotic action pulls in more water. A small corrosion cell is operating. Under the right conditions (particularly
the presence of chlorides and high humidity), differentials in oxygen concentration and electrical potential develop across the
cell. This modified cell forms the head of a filament. The area with little or no oxygen becomes the anode and the oxygen-rich
part of the cell is the cathode. The filament moves by anodic attack of the metal surface. The corrosion is driven by the potential
difference across the head. As the head slowly moves, oxygen diffuses through the tail to the cathode at the back of the head and
maintains a high concentration of dissolved oxygen there. The anodic region at the front of the head is acidic, has a high concentration of soluble salts and dissolves very little oxygen. As the metal corrodes, the coating above it loses adhesion and is pushed
up by the corrosion products, which leads to the characteristic appearance of the filament.
A variety of actions can help in the prevention of filiform corrosion. Probably the best strategy is to make certain that there
is excellent adhesion of the coating over a salt-free metal substrate. This can be achieved by rigorous cleaning of the metal sub-
strate, use of an effective pretreatment (historically, a chromate, but now more likely to be a zirconium oxide) and a highly
adherent primer. In addition, using multiple coating layers that have low water and electrolyte permeability, sealing of cut or
drilled edges, adequate coverage of sharp corners and edges, and prevention of surface defects such as pinholes, cratering, and
dewetting all help. Zinc-rich primers reduce the tendency for filiform to occur over steel.
Testing for filiform corrosion resistance usually involves contamination of scribed-painted panels by exposure to salt spray or a salt solution (hydrochloric acid and hydrogen chloride gas also have been used) followed by placement of the specimens in a
humidity chamber for a period of time. Ideally, the test panels will have been produced
on the customer’s line. Filiform corrosion test standards include ASTM D 2803, “Standard
Guide for Testing Filiform Corrosion Resistance of Organic Coatings on Metal” and ISO
4623, “Paints and varnishes—Determination of resistance to filiform corrosion—Part 1:
Steel substrates; Part 2: Aluminum substrates.”
An excellent reference with details on the history, mechanism, testing, and prevention
of filiform corrosion is Bautista, A., Prog. Org. Coat., 28, 49-58 (1996).
Figure 2—The appearance of a coating
where filiform corrosion has occurred.
Figure 1—Filiform and scab corrosion
after the coating has been removed.
“Coatings Clinic” is intended to provide a better understanding of the many defects and failures that affect the appearance and performance of coatings.
We invite you to send your questions, comments, experiences, and/or photos of coatings defects to Cliff Schoff, c/o “Coatings Clinic,” Coatings Tech,
527 Plymouth Rd., Ste. 415, Plymouth Meeting, PA 19462; or email publications@coatingstech.org.
