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Fig. 4 shows typical potentiodynamic polarisation curves for the samples of AA6061, obtained in solutions of concentration 3.5% NaCl at pH values of (a) 2, (b) 6, and (c) 12, respectively. A similar shape is observed in two polarisation curves which were obtained in the slightly neutral and acidic solution, with a little difference on their cathodic branch controlling the corrosion process, indeed the current density in the slightly neutral solution is the lowest. The onset of pitting is not visible in these two cases since the pitting
potentials Epit coincided with the corrosion potentials Ecor and the anodic Tafel slopes of two curves are practically zero. From these results, it can be concluded that the main degradation mechanism in these solutions of pH values 2 and 6 is pitting corrosion and the pitting potential is independent of pH over this pH range. We can observe this on the micrographic photos shown in Figs. 11 and 12.
Fig. 5 shows an example of positive hysteresis, with pitting potential located at the same position of that of corrosion potential, repassivation potential Erp less than the Ecor and large area of the hysteresis loop, suggesting the nucleation and growth of pitting during the reverse scan [41,42]. In general, the larger the area of the hysteresis loop, the greater the susceptibility of the material to pitting corrosion.
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