Electrochemical behaviour of Al, Al-Sn, Al-Zn and Al-Zn-Sn alloys in chloride solutions containing stannous ions

The electrochemical behaviours of Al-1 (99.999%), Al-11 (99.6%). Al-Sn, Al-Zn and Al Zn-Sn alloys were investigated in 0.6 M NaCl solutions containing Sn2+ ions. The study was carried out using polarization and potentiostatic current-time techniques complemented by scanning electron microscope-energy dispersive X-ray analyzer investigations. It is found that Sn2+ ions have no activating influence on the dissolution of Al-1, Al-11 and Al-Sn alloy during I-t measurements. This is attributed to the difficulty of Sn diffusion into the surface layers, for the activation process occurs only when the activator ions diffuse into the bulk and create defects in the oxide layers. On the contrary, the addition of Sn2+ causes activation of Al-Zn and Al-Zn-Sn alloys by varying degrees depending on Sn2+ content. The presence of zinc as an alloying component in the alloys facilitates the diffusion of the deposited Sn into the surface layers revealing activation. XPS results confirm the presence of both Sn4+ and Sn2+ oxidation states in the oxide film on the surface of Al-Zn alloy as well as Sn-0 after treatment with 10(-3) M Sn2+. The effect of simultaneous addition of Sn2+ and Zn2+ on the dissolution of Al in 0.6 M NaCl was also studied. (C) 2001 Elsevier Science Ltd. All rights reserved.