Corrosion protection of high copper aluminium alloys by surface modification

Abstract

In a project devoted to the elimination of toxic materials in methods of corrosion protection, a surface modification process has been developed to improve the pitting resistance of high copper aluminium alloys such as 2024 and 7075 without the use of chromates. This process involves immersion in boiling Ce(NO₃)₃, anodic polarisation in Na₂MoO₄, and immersion in boiling CeCl₃. Very corrosion resistant surfaces were produced on 7075–T6 after applying a pretreatment step which removes copper from the outer surface layers. A chemical and an electrochemical copper removal process have been developed with the actual procedure depending on alloy composition. During exposure in 0·5M NaCl for 30 days no measurable corrosion was indicated by continuous monitoring with electrochemical impedance spectroscopy or by visual observation. For 2024, significant improvement in the resistance to localised corrosion was also obtained. Surface analysis showed increased levels of cerium and molybdenum at locations which might have been initiation sites for pits without treatment by the modified Ce–Mo process described in this paper.