Mechanisms of Filiform Corrosion

Filiform corrosion or ‘underfilm’ corrosion is a special type of corrosion characterized by thread like filaments of corrosion products that may have considerable length but little width or depth. This type of corrosion occurs beneath various organic films on aluminum, steel and magnesium surfaces. One of the most important factors in causing filiform corrosion is relative humidity. Sixty to ninety-five percent relative humidity is needed to produce filiform corrosion and at higher humidities the filaments become broader and may form blisters. Filiform growth is inhibited or may not form at all if the film is impermeable to moisture1. In addition, filiform generally does not appear under films which are fully permeable such as those formulated above their critical pigment volume concentrations2.