MICROTUNNELLING DURING SELECTIVE ALLOY DISSOLUTION AND DURING PITTING.

The kinetics and the morphology of selective dissolution of alloys containing a noble component and of pitting of passive metals are discussed emphasizing common properties of both localized dissolution processes. Dealloying of CuAu(Pd)-alloys and pitting of aluminum and aluminum alloys are considered as typical cases. Dealloying requires the nucleation of terrace site vacancy clusters, since easily dissolvable kink sites of screw dislocations are blocked by noble metal atoms. Pitting requires the nucleation of pores in the passive oxide film, which inhibits active metal dissolution. In both cases, rapid local crystallographic attack, ranging from etch pitting to very narrow tunnelling, is involved in the nucleation process. The term tunnelling is appropriate for all these events. While the scale of tunnelling events is much larger in the case of pitting compared with the case of dealloying, the calculation of the breakthrough potential of dealloying and of pitting is basically equivalent.