Crevice and Intergranular Corrosion

Local accumulation of dirt on a steel structure in a damp environment is enough to set up an anodic area underneath it by excluding air. Similarly, chipped paintwork results in lateral spreading of anodic areas under the paintwork, radially outward from the chips. At the chipped site, air has relatively free access to the metal, but under the paint the oxygen is excluded and anodic activity becomes intense, spreading under the paint and leaving a trail of rust behind where air has slowly diffused in to oxidize the Fe +(aq). 

Such rusting phenomena as these are distressingly familiar in marine environments or in moderately cold climates where salts are used to deice roads. Acceleration of corrosion by seawater or sea spray, or by road salt, has several origins  

Even in unstressed metals, however, the boundaries of the grains in the metal structure (Fig. 5.4) have relatively high free energy and thus tend to 

There are several ways in which weld decay can be avoided  

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