Corrosion resistance of a new Al 6013-20 SiC(P) in salt spray chamber

Aluminum 6013 alloy (0.82Si, 0.95Mg, and 0.35Mn) is finding increasing usage in new aircraft designs, automotives, and structural applications due to its good stretch forming character in T4 temper (solution heat treated and naturally aged to a substantially stable conditions) compared to alloy 2024 (4.4Cu, 0.6Mn, 1.5Mg, and balance Al) and Al 6061 (Si0.51 to 0.71, Fe0.35, Cu0.15, Mn0.85, Mg0.15, 0.25Cr, 0.15Zn, and balanced Al). The newly developed Al 6013 reinforced with 20 vol.% SiC(P) has a higher strength than its unreinforced counterpart. Whereas the corrosion behavior of Al 6013 has been reported in literature, there is no previous data on Al 6013 reinforced with SiC(P). A knowledge of the corrosion behavior of this alloy is crucial to its applications in aerospace, structural, and automotive industry. The first results of corrosion study of this alloy in 3.5 wt.% Na Cl in a salt spray chamber are presented. Three tempers F (as fabricated), O (annealed), and T4 (age hardened and stabilized at room temperature) of the alloy Al 6013-30 SiC(P) were exposed to environmental chamber in accordance with ASTM recommended practice. The corrosion rate of the alloy showed a decrease with increased exposure period and after 800 h of exposure no appreciable change in the rate of corrosion was observed. The lowest rate of corrosion (4.83 mdd) was shown by temper T4 followed by tempers F and O after 1200 h of exposure in the increasing order of corrosion rate. Fluctuations in the corrosion rate with time are related to the kinetics of growth and dissolution of Al(OH)₃ film, which was detected by fourier transformation infrared (FTIR) spectroscopy (FTIS). The film was composed of an inner compact layer and outer bulk layer dependent on the refreshment rate from the bulk solution. Micrograph examination by scanning electron microscopy (SEM) showed the presence of pits covered by aluminum hydroxide gel, which isolates the pit from the bulk solution. The acidic conditions of pits also cause intergranular attack. Pitting was observed to be less pronounced in temper T4 compared to F and O tempers, the former also showing the highest resistance to corrosion. The data generated predict promising application potential of this alloy in environment where resistance to corrosion is also a major consideration.