Structures Cathodically or Anodically Polarized

In the case of steel that exchanges a current I with the concrete from an external source, either anodic or cathodic, the relation f, = 4 shown in Section 7.1, which expresses equality between the electrons produced and consumed at the surface of the steel in the absence of exchanged current, should be modified. In fact, electrons extracted or provided by the external current also have to be taken into consideration. If the current is exchanged anodically the following condition occurs  

If the current exchanged passes from the concrete to the steel or vice versa, the above equations are verified for a potential, respectively, more negative or more positive than the corrosion potential, so that the steel is polarized either cathodically or anodically. 

F re 7.n Schematic representation of the influence of external anodic or cathodic polarization 

Variations of the rate of anodic and cathodic processes, as well as variations of potential, due to the external current, depend on the direction and magnitude of the current and on the anodic and cathodic polarization curves. This will be dealt with in the following chapters. 

Anodic and cathodic processes may take place preferentially on separate areas of the surface of the reinforcement, leading to a macrocell. This can be established, for instance, between active and passive areas of the reinforcement. Current circulating between the former, which are less noble and thus function as anodes, and the latter, which are more noble and thus function as cathodes, accelerates the corrosion attack on active surfaces while further stabilising the protective state of passive ones. The magnitude of this current, known as the macrocell current, increases as the difference in the free corrosion potential between passive and active rebars increases, and decreases as the dissipation produced by the current itself at the anodic and cathodic sites and within the concrete increases. 

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