Development of “stainless” aluminum alloys by surface modification

Abstract

In a research project devoted to the elimination of toxic materials in methods of corrosion protection, surface modification processes have been developed to improve the pitting resistance of aluminum alloys without the use of chromates. These processes use chemical treatments in various cerium salt solutions and an electrochemical treatment in a molybdate solution. The details of the surface modification method depend on alloy composition. For Al 6061 and Al 6013 treated by immersion in hot Ce(NO3)3 and CeCl3 followed by anodic polarization in Na2MoO4, pitting did not occur during immersion in 0.5 M NaCl for 60 days. Surface modification of Al 7075-T6 and Al 2024-T3 involves immersion in boiling Ce(NO3)3, anodic polarization in Na2MoO4 and immersion in boiling CeCl3. Very corrosion-resistant surfaces were produced on Al 7075-T6 after applying an electrochemical pretreatment step which removes copper from the outer surface layers. During exposure of Al 7075 in 0.5 M NaCl for 30 days no measurable corrosion was indicated by continuous monitoring with electrochemical impedance spectroscopy and by visual observation. For Al 2024, significant improvements in the resistance to localized corrosion were also obtained with these processes. A further modification of the CeMo process which is very effective for Al 2024 consists of immersion in hot cerium acetate, anodic polarization in Na2MoO4 and immesion in hot Ce(NO3)3. Impedance data collected in 0.5 M NaCl remained capacitive for 30 days, which is indicative of lack of localized corrosion. Surface anlaysis showed increased levels of Ce and Mo at sites where Cu-containing compounds are located. Apparently local cathodes are eliminated during the surface modification process, thereby reducing the driving force for pitting.