Coatings Clinic: 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.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

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 substrate, 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).

"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," CoatingsTech, 527 Plymouth Rd., Ste. 415, Plymouth Meeting, PA 19462; or email [e-mail].

Schoff Associates

By Clifford K. Schoff