The results presented in this paper were obtained with two Al-Sn binary alloys (0.02 and 0.4 wt% Sn) which had been prepared on super pure (99.999%) Al base. A study of behaviour of these alloys at cathodic polarization in 2 M NaCl solution by means of electrochemical methods complemented by SEM and EDAX analysis provided an insight into the effect of addition of tin on the electrochemical behaviour of aluminium. It has been observed that a low tin content causes aluminium to behave hyperactively at high negative potential values, which leads to crevice formation and conditions favourable for further dissolution. The result of the change of potential in the negative direction with the alloy containing 0.4 wt% tin is superactivation of aluminium as consequence of de-filming processes.