Anodic undermining, corrosion

Anodic undermining has not been studied as extensively as cathodic delamination because there do not appear to be any mysteries. Galvanic effects and principles which apply to crevice corrosion provide a suitable explanation for observed cases of anodic undermining. 

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. 

Aluminum substrates are particularly prone to anodic undermining, whereas on steel this form of corrosion usually occurs under an applied potential [88]. Under conditions of free corrosion potential. 

In the advanced state of delamination, two zones of different activity can be located underneath the organic coating. The shift of the corrosion potential close to the rather anodic potential of the intact interface marks the front of the advancing cathode. Behind this cathodic area the steep slope marks the front of anodic undermining. Fiirbeth and Stratmann proved this interpretation of the SKP data with cross-sectional and surface analysis of the delaminated area [86]. 

This form of corrosion is more prevalent under organic coatings on aluminum than on other metallic surfaces, being a special form of anodic undermining. 

Anodic undermining is mainly reported on aluminum, which forms only Al + ions and porous corrosion products. Especially in the presence of chlorides, the natural oxide is easily attacked. Similarly to cathodic delamination, in this case also the anions and cations are separated leading to the formation of an HCl solution at the delamination front. 

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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