A Study of the Influence of Alloying Additions on the Passivity of Aluminum

Abstract : The susceptibility of aluminum and aluminum alloys to pitting in aqueous chloride-containing environments is well known. Typical aluminum alloying constituents such as copper, silicon, zinc, and magnesium are added to improve mechanical properties, and not to provide corrosion resistance. Passivity enhancing species, such as Cr, Mo, Ta and W, typically exhibit very low solubility limits in aluminum, well below 1 at .%, and at these concentrations exert little influence on corrosion behavior. The solubility limits for these elements in aluminum can be exceeded and corrosion performance enhanced if the alloys are produced using a rapid solidification method such as vapor deposition. In this fourth year of the program the corrosion resistance of three binary alloys Al-V, Al-Nb, and Al-W and one physically vapor deposited ternary alloy are evaluated. The Al-W alloys exhibit the best localized corrosion resistance observed to date with increases in Ep up to 2600 mV with the addition of 10.3 at .% W to Al. Additionally, the corrosion performance of Al-8 at .% W specimens that were heat treated to produce a two-phase alloy are comparable to those found earlier for single-phase Al-8 at.% Mo alloys. Characterization of the physically vapor deposited ternary alloy reveals that several atomic percent of passivity enhancing species can be retained in solid solution with this process and therefore this technique is very promising for the production of nonequilibrium corrosion-resistance aluminum alloys.