Al, Alloy 1 And Elece

1504 Words 7 Pages
Fig. 2 and Table 1 present the open circuit potential versus time curves and self-corrosion rate of Al, Alloy 1 and Alloy 2 electrodes in 2 M NaCl electrolyte, respectively. Corrosion rates were obtained by weight loss measurements in 2 M NaCl solution after 60 min. As seen in Table 1, the corrosion rate increases in the following order: Al <Alloy 1 < Alloy 2. Fig. 2 indicates that open circuit potential of Alloy 1, 2 (especially Alloy2) is more negative than that of Al. It can be seen that the potential of the Al electrode shifts in the positive direction and achieves a stable value of about -0.82 V. This is due to the formation of natural oxide layer on the Al surface which is stable in saline environments and leads to ennoblement of the …show more content…
This oxide film is Al2O3 and Al(OH)3, in neutral solution, based on Pourbaix diagram [20]. The alloy 1 presents OCP value of about -1.30 V whereas that of Alloy 2 was -1.44 V, which are more negative than that of Al in the same medium. The presence of tin and gallium in aluminum matrix as alloying elements increases the adsorption of Cl- ions at the values of negative potential which leads to the alloy dissolution [10]. The surface morphologies of Al, Alloy 1 and Alloy 2 after immersion in 2 M NaCl solution for 1 h were obtained by scanning electron microscopy (SEM) (Fig. 3). The morphology of Alloy 2 (Fig. 3C) in 2 M NaCl solution have large pits, which probably is due to the presence of larger amount of Pb film at the metal/oxide interface meanwhile Alloy 1 exhibited fairly smooth surface with large number of distributed pits which may be suggest the presence of magnesium and gallium in the alloy; with the explanation that magnesium is anodic impurity to aluminum and can cause pitting corrosion (Fig. 3B). In that conditions, the Al demonstrated a smooth surface (except the line of emery paper) without any damages (Fig. …show more content…
4 and Table 2 exhibit the tafel curves and related corrosion parameters of Al, Alloy 1 and Alloy 2 electrodes in 2 M NaCl electrolyte, respectively. For Alloy 2, the corrosion potential is negative than that of Alloy 1 and Al, and the corrosion current density (icorr) of Al is less than that of Alloy 1 and Alloy 2 in 2 M NaCl medium. Passive behavior is observed on the anodic polarization curves of Al characterized by current density plateau and Aloy1 with the difference that Al current density, ipass, is much lower than that of Alloy 1. However, Alloy 2 demonstrate no passive region as presented in Fig. 4. In case of Al and Alloy 1, the polarization curves demonstrated a passive zone, thus causing anodic control of the corrosion process. Then, the passive status is pursued by a sharp increase in the current at the pitting potential, Ep. Breakdown of the formed passive layer in chloride solution takes place by interaction between Al(OH)3 film and Cl- ions which can lead to the formation of AlCl3 compound after surface saturation with corrosion products and after enough long term [21, 22]. Afterwards AlCl3 dissolves as [AlCl4]- ions at around the pitting potential [23, 24]. In case of Alloy 2, the quick rise in current density in the anodic branch suggested that the pitting potential, Ep, was near-by Ecorr indicating the higher activity of the Alloy 2 compared to Alloy 1 and Al. The important point is that the addition of Ga and Pb to Alloy 2 shifts the polarization

Related Documents