Thin oxide film formation, metal values

Figure 16. Activation barrier A for the formation of a breakthrough pore in a thin surface oxide film on metal as a function of electrode potential at two different surface tensions, om, of the metal/electrolyte interface.7The solid lines indicate the values of A b against Aand the dotted lines correspond to die critical potentials for the pore formation. ACd= 1 F m-2, a = 0.01 J m-2, h = 2 x 10-9 m, a, am = 0.41 J m 2 b, am 0.21 J m 2 (From N. Sato, J. Electmchem. Soc. 129, 255, 1982, Fig. 3. Reproduced by permission of The Electrochemical Society, Inc.)...

Figure 18. Dependence of activation barrier A f for the nucleation of a thin oxide film on the metal surface as a function of electrode potential. Ey is the equilibrium potential of anodic oxide formation.7 The solid line represents the value of A against and the dotted line corresponds to the critical potential for the film formation. AE = 0.2 V, Cd= -1Fm-2, am = 0.411 m 2, a -0.01 J m-2,...

The major flaw of the presented theory of friction is that it does not take into account the role of the environment. In fact, pure metal surfaces cannot exist in contact with the atmosphere, they immediately adsorb gas molecules and oxidize (Chap. 3). The presence of adsorbed species and thin oxide films prevents the formation of metal-metal bonds between the contacting surfaces. In air, the shear stress of the asperity junctions corresponds to that of oxidized surfaces rather than metal-metal contacts. The value of the shear strength in equation (10.6), and hence also that of the friction coefficient, depends therefore strongly on the chemical state of the contacting surfaces. 

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