Point Defect Model of the Passive State

In order to illustrate the reactivity of the common metals in contact with terrestrial environments. Table 4.4.1 lists the Gibbs energy changes for the reaction of various metals with oxygen and water vapor under prototypical terrestrial conditions [T = 25°C, po, = 0.21 atm, pn,o = 0.02532 atm (RH = 80%), pn, = 6.156 x 10 atm], where the partial pressure of hydrogen has been calculated from the equilibrium H20(1) = HaCg) -I- 02(g) for the prevailing conditions. The reader wiU note that all of these metals are used in our current, metals-based civilization, either in pure form or as components of alloys. 

At higher potentials, passivity is observed to break down and the dissolution rate of the metal increases dramatically. This process is commonly due to the oxidative ejection of cations from the barrier layer for example, 

Examinahon of the hndings of a great number of studies of the phenomenon of passivity leads to the following generalizations (D. Macdonald [1999] and references therein)  

For systems where the outer layer does not form, or where the outer layer presents little impediment to the transport of species to or from the barrier layer/outer layer (bl/ol) interface, the specific impedance in the absence of redox couples is very high ( 10 -10 Qcm for NiO on Ni, for example) but in the presence of redox couple [e.g. Fe(CN)6 ] the impedance is often low. This demonstrates that the barrier layers may be good electronic conductors but, generally, are poor ionic conductors. 

Given sufficient time, steady states are observed in the barrier layer thickness and in the current. While this point is still somewhat controversial, steady states have been demonstrated unequivocally for Ni, Zn, and W, amongst other metals. 

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