Anodic undermining filiform corrosion

Filiform corrosion is characterized by formation of interconnecting filaments of corrosion under a paint film upon exposure to a humid environment. Filiform corrosion typically occurs only when the relative humidity exceeds about 65%. The mechanism is complicated and has been the subject of considerable discussion in the literature (, 1A, 2 1, 2, 24). Basically, a localized corrosion mechanism is responsible (Figure 8). The head of the growing filament is anodic, and as a result the filiform corrosion process has been termed a specialized form of anodic undermining... 

After the initiation step, blisters can grow by different mechanisms, known as cathodic delamination, anodic undermining or filiform corrosion (FFC). The first two shall be briefly described, whereas FFC, due to its specific characteristics, will be left for Sect. 5.4.3.3. 

A specific t) e of delamination that is also driven by a differential aeration cell is filiform corrosion [5,6,62,63]. In most cases it is related to anodic undermining, especially because it occurs mainly on aluminum alloys. Filiform corrosion is a specific t5 e of delamination that occurs only under atmospheric condition with relative humidity between qjproximately 50 and 90%. Small threadlike delamination tracks are formed that in general follow irregularities on the surface (either rolling or grinding marks). New interest in this special form of corrosion was stimulated by many practical cases along the coastal areas in Europe. It may be expected that new mechanistic information will become available within the next 2 years. 

By using the SKP (see Sect. 5.4.2.5), it was possible to measure the local potentials underneath an organic coating in situ without the deterioration of the corroding system [178]. The mechanism of FFC consisting of an anodic reaction at the corrosion front is reflected in rather different electrode potentials around the filament s head. Whereas, for cathodic undermining, the delamination front is positively polarized with respect to the already delaminated zone and the head of the filiform filament shows a negative potential with respect to the tail (Fig. 35) [178]. Therefore, the tip can be identified as the local anode and the local cathode is situated behind the anode within the tail. 

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