Localised corrosion

Crevice Corrosion localised corrosion resulting from the presence of a crevice in a fabricated component or between two surfaces, at least one of which is a metal. 

Bad connections in terminals, due to loose screws and poor contacts in socket receptacles or fuse carriers, can lead to local sparking which, in turn, causes corrosion, localised overheating of conductors and possible ignition of anything flammable nearby. 

To summarise, unexpected corrosion failures are much more likely to occur by localised attack than by uniform attack (which can easily be detected) and although corrosion handbooks are useful for making initial choices of materials for applications where corrosion is important, critical components must be checked for life-to-fracture in closely controlled experiments resembling the actual environment as nearly as possible. 

Localised attack or fracture due to the synergistic action of a mechanical factor and corrosion... 

Finally, it should be noted that in both cases the effect of crystal defects and microstructural features must, in general, be to tend to make the corrosion less uniform and more localised. 

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In this section the interaction of a metal with its aqueous environment will be considered from the viewpoint Of thermodynamics and electrode kinetics, and in order to simplify the discussion it will be assumed that the metal is a homogeneous continuum, and no account will be taken of submicroscopic, microscopic and macroscopic heterogeneities, which are dealt with elsewhere see Sections 1.3 and 20.4). Furthermore, emphasis will be placed on uniform corrosion since localised attack is considered in Section 1.6. 

Finally, it is important to point out that although in localised corrosion the anodic and cathodic areas are physically distinguishable, it does not follow that the total geometrical areas available are actually involved in the charge transfer process. Thus in the corrosion of two dissimilar metals in contact (bimetallic corrosion) the metal of more positive potential (the predominantly cathodic area of the bimetallic couple) may have a very much larger area than that of the predominantly anodic metal, but only the area adjacent to the anode may be effective as a cathode. In fact in a solution of high resistivity the effective areas of both metals will not extend appreciably from the interface of contact. Thus the effective areas of the anodic and cathodic sites may be much smaller than their geometrical areas. 

Pourbaix, A., Characteristics of Localised Corrosion of Steel in Chloride Solutions , Corrosion, 27, 449 (1971)... 

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