engleski

Electrochemical properties of Al-Sn Alloys in sodium chloride solution

The treatment of aluminium with small quantities of elements such as In, Hg, Sn, Ga and Bi activates the process of Al anodic dissolution. Polarization of high-purity aluminium in the cathodic direction in neutral electrolytes is characterized by increase of hydrogen evolution rate, past a certain negative potential limit, and also by increasing rate of metal dissolution. In this case a number of phenomena could be expected to appear: (a) formation of hydride, (b) incorporation of alkali metals following cation reduction or (c) electron tunelling through a very thin oxide leading to hydrogen evolution at the oxide / electrolyte interface. The present investigations were directed towards finding some indication of the existance of some of these processes in the case of Al-Sn alloy in two regions of potentials, before and after the occurrence of hydration respectively. The experiments were performed with two Al-Sn binary alloys (0.02% and 0.4% Sn) - Alcan International Ltd., which had been prepared on super pure (5N) Al base. A study of behaviour of these alloys at cathodic polarization in 2M NaCl solution by means of electrochemical methods complemented by SEM and EDAX analysis provided and insight into the effect of addition of tin on the electrochemical behaviour of aluminium. Electrochemical measurements were performed with a potentiostat (PAR M273) driven by a computer (PC 386 SX). Potential sweeps (0.5 mVs^-1) were applied to the Al-Sn electrodes starting from pitting potential in the negative direction down to -0.2 V vs. SCE and back. The current response was converted to a logarithmic scale and the resulting Tafel plots were obtained. When the potential returns in the positive direction, a new corrosion potential appears, Ec (II), which is much more negative then Ec (I). This means that a substances has formed on the surface which cannot oxidate easily, e. g. aluminium hydride. However, in that region of high cathodic potential and pH, formation of SnH_4 is also possible. The stability region of SnH_4 corresponds to the region of "hyperactivity" of aluminium. The hyperactivity is provoked by released individual tin atoms on the reacting surface after dissolving from the solid solution. The tin atoms are than very quickly reduced to hydrides, so that tin leaves the metal surface. The EDAX analysis of crevices formed confirms that there is no tin in those places. In the case of the alloy Al - 0.4% Sn, a change of the potential from -1300 mV in the cathodic direction leads to the "superactivation". A double corrosion potential appears when the potential returns from the cathodic in the anodic direction. The potential Ec (II)_2 is due to the corrosion of the separated tin phase in the alloy microstructure. The aluminium superactivity is caused by the presence of tin in the solid solution. The solid solution elements exist in aluminium as separate atoms, which, as tin has a low solidification point in comparison to aluminium, remain on the reacting surface separately or in small groups at the beginning, being more mobile. The accumulation of mobile species under the protective film leads to local weakening of the film in some places, where already developed pits can be observed. The EDAX analysis of their interior indicates tin agglomerations. Hyperactive behaviour has not been observed in the case of Al - 0.4% Sn alloy. This is due to the phenomenon of superactivity which occured when the potential changed in the negative direction, it having been established that in binary alloys hyperactivity never occurs once superactivity has occured, even at potentials below the Sn/SnH_4 reversible potential.

Al-Sn alloys; hyperactivity; superactivity

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