Passivity breakdown mechanism applied potential

Analysis of passivation transients on an initially active surface either by applying a steep potential jump into the passive range or by creating fresh surfaces at constant applied potential by nonelectrochemical depassivation (chemical passivity breakdown mechanical scratching, ultrasonic waves, etc. radiative laser beam impact [112,113]). These techniques have proved to be of outstanding importance for the investigation of the mechanism of localized corrosion associated with passivity breakdown [114,115]. 

Once local breakdown has occurred at these defects by local dissolution of the passive layer or by local accumulation of voids and mechanical collapse, halides from the electrolyte have direct access to the bare metal surface. They compete with OH and it is a matter of the electrode potential and the bulk concentration of the halides whether these nuclei will repassivate or continue their growth. The total potential drop concentrates at the metal-electrolyte interface at these pit nuclei. If the applied potential is large enough a related intense local metal dissolution leads to an accumulation of corrosion products and thus of halides that prevents their repassivation, i.e., the nucleus will grow to a pit. 

In the discussion of E the vs pH diagram for iron in water depicted in Figure 1.70, we noted that, with application of high positive potentials, the system moves into a region of passivity and results in a reduced corrosion rate. The passive film formed should be coherent and insulating to withstand corrosion and mechanical breakdown. Upon formation of the passive state the corrosion rate is reduced. Thus by polarization and applying more positive potentials than corrosion potentials the metal attains passivity and is protected. This is the principle of anodic protection. It is necessary that the potential of passivation be maintained at all times, since deviations outside the range would result in severe corrosion. 

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