Potential Hade

Table 1.14 Approxiraale Hade potentials of some metals at pH 0 ...

Because the association of the expression passivation potential with the potential of the maximum leaves something to be desired as a definition of passivation, another potential, the Hade potential, is sometimes used to define the phenomenon of passivation. This potential can be described quite loosely (see Fig. 12.62) as the potential at which (while moving the potential back in the cathodic direction) the current begins to increase from the low trickle charge value. 

The precise definition of the Hade potential needs a little more description. Thus, if an electrode containing a passive layer is allowed to float free (the potentiostat is disconnected) and its potential is measured and plotted against time, it will fall (become less positive) and then attain a plateau on the potential-time graph (Fig. 12.64). 

Passivity is normally manifest as a sharp drop in the anodic current density to a constant value at a critical potential that is commonly referred to as the Hade potential (Figure 4.4.27). For many metals and alloys, the current density drops by three or more orders in magnitude leading to a corrosion rate in the passive state... 

---