Filiform Corrosion Testing
Filiform corrosion is the term used to describe threadlike lines of corrosion that form on painted metal surfaces. The corrosion starts at a paint defect where the metal is exposed and is driven by osmotic pressure. Soluble metal salts form at the leading edge of the corrosion threads and draw water in from the inactive tail of the threads. The water reacts with the metal to form more metal salts and the system is self propagating. Filiform corrosion pushes up the coating directly above it, giving the appearance of thin raised fingers.
The standard filiform corrosion test used by the UDRI coating group is based on the protocol described in National Aerospace Standard (NAS) 1534, Primer Coating: Low Volatile Organic Compound, Chemical and Solvent Resistant. The test is included in the scope of our ISO 17025 certification, ensuring customers of high quality evaluations.
Scribed, painted samples are placed in a corrosion chamber which has been saturated with hydrochloric acid (HCl). After one hour of exposure, samples are removed from the acidic atmosphere and placed in an environmental chamber that is set to a specific temperature and humidity. Precise environmental control is important, since temperature and humidity play major roles in filiform propagation. UDRI measures and records this information on a daily basis to confirm that conditions are maintained. We also routinely run a set of control panels along with the samples to verify validity of the test. Standard exposure time in the environmental chamber is 1000 hours.
After samples have been removed from the environmental chamber, they are thoroughly rinsed with deionized water and then dried. An initial visual examination is made and film defects are noted. The paint is then chemically stripped off half the sample so that accurate measurements of the filiform threads can be made. Corrosion threads longer than 1/8th inch are considered significant and the number and length of these are recorded.
UDRI has found that sample preparation, especially surface treatment prior to painting, greatly affects the filiform corrosion rate. When samples are prepared it is important that the pretreatment conditions emulate conditions which will be used in the field. The UDRI process line can be used to obtain these conditions.
If you would like a quote on how much it would cost to test your samples, please contact UDRI's Bill Culhane or call 937-255-0955.
Other Corrosion Testing Available:
ASTM B 117: Salt Spray (Fog)
GM 9540P: Accelerated Corrosion
ASTM G 85-A5: Standard Practice for Modified Salt Spray (Fog) Testing (Prohesion)