Aqueous layer metal-hydroxyl bonds

Fig. 4 Important steps during initial stages of S02-induced atmospheric corrosion formation of protons and bisulfite ions in the aqueous layer (a), ligand exchange between a surface hydroxyl group and a bisulfite ion and subsequent weakening of adjacent bonds through proton-bonded neighbors (b), detachment of metal surface center that enters the aqueous phase as a hydrated metal anion-complex (c). (Reproduced with permission from Ref [7].)...

Nevertheless, there are many instances where electrochemical corrosion mechanisms may play a primary role in affecting the service performance of bonded joints. It should be noted that such mechanisms of attack involve both the presence of (a) anodic sites, where reaction with the metallic substrate occurs and electrons are generated, and (b) cathodic sites, where the electrons are consumed. The major reaction leads to the generation of hydroxyl ions, and the liquid present at these sites will become strongly basic and so possess a relatively high pH. Thus, typically an aqueous (electrolyte) layer needs to be present, since, without such an aqueous film, no electrical current can flow from the anodic to the cathodic sites. These aspects are illustrated, for example, by the schematic electrochemical corrosion mechanism for an organic coating on a steel substrate shown in Fig. 4, which is discussed in detail in Section 2.S.2.2. 

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