Role of alloying elements tin and aluminium in anodic behaviour of homogeneous copper based alloys in 3-5 wt-%NaCl solution

Abstract

The anodic behaviour of homogeneous Cu-Sn and Cll-Al alloys in aqueous 3·5 wt-%NaCl solution has been investigated by potentiodynamic polarisation, cyclic voltammetry, and potentiostatic current-time experiments and compared with the behaviour of pure Cu. Specimens were subjected to various anodic potentials and subsequent analyses by scanning electron microscopy supplemented with energy dispersive spectroscopy. At active potentials, the alloys and the pure Cu either dissolve directly or through an adsorbed metastable CuCl layer by transport of CuCl; ions. The active-passive transition at higher applied anodic potential resulted in the formation of a thicker and more stable CuCl salt layer. Withfurther increases in anodic potential, the compact CuCl salt layer on the Cu-Sn alloys impedes the formation of CuO or Cu(OH)₂, thus maintaining perfect passivity. In contrast, on pure Cu and Cu-Al oxide or hydroxide is formed just beneath a less compact CuCl layer and so provides imperfect passivation. This difference in anodic behaviour between the homogeneous Cu Sn and Cu Al alloys is discussed, with reference to the pure Cu specimen, in terms of the difference between the CuCl layers incorporated with more protective SnOHCl.H₂O (or less protective AlCl or Al(OH)₂Cl) and those CuCl layers which are undoped.