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Cathodic breakdown of anodic oxide film on Al and Al-Sn alloys in NaCl solution

Influence of cathodic polarization on stability of defined oxide films on Al and Al-Sn alloys (with up to 0.40%Sn) has been investigated in a 0.5 M NaCl solution using the potentiostatic pulse method. The dependence of the cathodic current on time (in the period of 1, 10 and 100 s) were recorded on Al and Al-Sn alloys when subjected to a potential pulse from EOCP to different negative values (up to -2.0 V). Anodic current responses to the returned to the EOCP were also recorded at three different time scales (1, 10 and 100 s). It has been estabished that cathodic polarization of passivated Al and Al-Sn alloys in chloride solution is characterized by two regions of potentials with distinctly different phenomena: the range of low and high cathodic potentials (LCP and HCP). In LCP range oxide film retains its properties, while in the HCP range cathodic breakdown and hydration of the oxide take place. The boundary between these two potential ranges shifts towards more negative potential values when the % of Sn in the alloy increases. The longer the time of cathodic pulse, the more positive the potentials at which the oxide film breakdown takes place. This shift is more marked with alloys containing higher % of Sn. Cathodic polarisation (duration of 100 s) activates alloys with 0.20 and 0.40 % Sn for anodic dissolution.

Alminium; Aluminium activation; Al-Sn alloy; Anodic oxide film; Cathodic polarization

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