Corrosion and Polarization Reactions

Assume to immerse a zinc anode in a hydrochloric acid solution. The anodic reaction is that already seen in Eq. (13.16) 

The rate at which Zn is dissolved as Zn and, therefore, corrodes depends on the cathodic reaction (3) in Eq. (13.17) by which hydrogen is reduced taking electrons available from reaction (13.22). Note that in the case considered, hydrogen is coming in large quantities from hydrochloric acid. It must be observed 

At this point the dissolution rate of Zn is equal to that of hydrogen ions production. The corresponding current density icorr is called corrosion current of zinc in hydrochloric acid. Therefore, on the contact surface zinc-acidic solution microscopic galvanic cells are formed of the type shown in Fig. 13.3 in which the electrons made available by zinc that oxidizes reduce the acid determining a new equilibrium point in which anode and cathode have the same potential Econ The variation of the electrode potentials with respect to their equilibrium values that leads to the common value E orr is called polarization. The thermodynamic driving force that sustains corrosion is given by the difference 

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