Figure 25.4 Figure showing anodic and cathodic currents.
Anodic reaction Fe
Fe2+ + 2e
Cathodic reactions: (a) 2H+ + 2e - H2 , (b) 4H + + O2 + 4e - = 2H2O
It is seen that larger the values of A, J0 and , faster the corrosion rate. Even if the emf is smaller, the process with larger J0 will occur more rapidly. E.g., the J0 for reaction (a) in the figure is 10 -6 Acm-2 and for (b) it is 10–14 A cm -2 and kinetically, the hydrogen evolution dominates the corrosion process.
Figure 25.5 Corrosion velocity diagram. In the figure,
refers to anodic current density and refers to cathodic current density. This figure is for iron being corroded by aerated water. At the corrosion potential, the total anodic current is equal to the total cathodic current.
A corrosion velocity diagram graphically indicates the regions of the corrosion potentials ( Ecorrosion ) (at which the anodic and cathodic currents are equal) and the values of corrosion currents (Icorrosion = Ianodic ).
Fe2+ + 2e
H2 , (b) 4H + + O2 + 4e - = 2H2O 
, faster the corrosion rate. Even if the emf is smaller, the process with larger J0 will occur more rapidly. E.g., the J0 for reaction (a) in the figure is 10 -6 Acm-2 and for (b) it is 10–14 A cm -2 and kinetically, the hydrogen evolution dominates the corrosion process. 
refers to anodic current density and 
refers to cathodic current density. This figure is for iron being corroded by aerated water. At the corrosion potential, the total anodic current is equal to the total cathodic current. 
