Abstract:The role of Mg 2 Si in the electrochemical behavior of Al-Si-Mg aluminum alloys has been studied using four experimental aluminum alloys with variations in the Si/Mg molar ratio (A1-0.12, A2-0.49, A3-0.89, A4-1.05), in neutral aerated 0.1M NaCl solutions. Accordingly, the corrosion potential in open circuit (E oc ) and polarization resistance (Rp) were measured. Results show that the augment in Si/Mg molar ratio increases the presence of Mg 2 Si intermetallic phase. During immersion tests in neutral aerated chloride solutions the anodic activity of the Mg 2 Si intermetallic decreased rapidly, as indicated by a fast ennoblement of open circuit potential. After the immersion period (7 days), higher Rp values for alloys A3 and A4 (21 and 26 K cm 2 , respectively) as compared to those for alloys A1 and A2 (5 and 10 K cm 2 , respectively), suggest a greater corrosion resistance in Cl -containing environments for alloys A3 and A4.

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The corrosion resistance of Al-Mg-Si/SiC p composites produced by the pressureless infiltration method [using SiC p preforms with 50% porosity containing rice hull ash (RHA) and four custom-made alloys with varying Si/Mg molar ratio] was evaluated in neutral 0.1 M NaCl solutions. The deleterious phase Al 4 C 3 was successfully suppressed in composites with Si/Mg molar ratios of 0.89 and 1.05, but not in those with lower Si/Mg molar ratios (0.12 and 0.49). Results of cyclic polarizations in deareated 0.1 M NaCl solutions showed that with increasing Si/Mg molar ratio, passive current density increased but pitting susceptibility decreased both for reinforced and unreinforced alloys. Immersion tests in aerated 0.1 M NaCl showed that for composites with Si/Mg molar ratios of 0.12 and 0.49 chemical degradation by hydrolysis of Al 4 C 3 was followed by intense anodic dissolution at the matrix-reinforcement interface, while composites corresponding to Si/Mg molar ratios of 0.89 and 1.05 did not exhibit intense localized attack. Possible reasons for the improvement in resistance to localized corrosion are discussed.

The degradation characteristics of Al/SiC p /spinel composites prepared with fly ash (FA) and rice hull ash (RHA) under environmental conditions were investigated. Composite specimens were prepared with recycled aluminum via reactive infiltration in the temperature range 1050-1150 8C for 50-70 min and, in argon atmosphere at a pressure slightly above that of the atmospheric pressure. Results reveal that although both FA and RHA help in preventing SiC p dissolution and the subsequent chemical degradation of the composites, due to the interaction of native carbon in FA with liquid aluminum, FA-composites are susceptible to corrosion via Al 4 C 3 . Moreover, this phase accelerates the degradation process and increases the damage severity. The primary corrosion mechanism in both types of composites is attributed to microgalvanic coupling between the intermetallic Mg 2 Si and the matrix. Accordingly, an appropriate control of the Si/(Si þ Mg) molar ratio in the aluminum alloy hinders the Mg 2 Si corrosion mechanism in both types of composites and a proper FA calcination prevents chemical degradation in FA composites.

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