The pitting corrosion of copper in borate buffer containing sodium chloride is studied by using potentiostatic and potentiodynamic techniques complemented mth scanning electron microscopy and EDAX. The breakdown potential shifts towards more negative values as the sodium chloride concentration increases. During pitting both soluble Cu(I) and Cu(I1) species are detected. The first stage of pitting is explained through the competition between the passive layer formation and the nucleation and growth of the CuCl layer in equilibrium with Cu(I)-chloride complexes in solution. When salt nuclei reach the metal surface, pit growth under charge-transfer control is found. In the following stage the kinetics of pit growth changes to a diffusion controlled process when the thick CuCl layer is completed. Secondary breakdown of the salt layer results in copper dissolution through Cu(II) soluble species. The corresponding overall process is discussed in terms of a sum of nucleation and growth processes. The reaction model reproduces the potentiostatic current transients of copper in weakly alkaline borate buffer containing sodium chloride.

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The effect of the hydrogen diffusion on the hydrogen electrode reaction has been studied and rigorous kinetic expressions for the Tafel-Heyrovsky-Volmer mechanism has been derived. The analysis of the dependences of the current density ( j) on overpotential (Z), particularly oriented to the hydrogen oxidation reaction (hor), leads to the following main results: (i) in the Tafel-Volmer (TV) route, the current density reaches a maximum value ( j max ) less or equal to the limiting diffusion current density (j L ); (ii) j max is always equal to j L for the HeyrovskyVolmer (HV) route; (iii) in the simultaneous occurrence of both routes (THV), the current density always reaches the j L value, although in the range of overpotentials of applied interest (0 r Z/V r 0.6) the j max value, characteristic of the TV route, can also be obtained.The Levich-Koutecky plots have also been analysed and it has been demonstrated that the j(Z) dependence for the hor under activated control cannot always be obtained from these plots.