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Influence of Zinc on Surface Treatments of Aluminium-Zinc Alloys

Gentile, Marialuisa

[Thesis]. Manchester, UK: The University of Manchester; 2010.

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Abstract

This research work studies the influence of zinc on surface treatments such as mechanical polishing, anodic alkaline etching, alkaline etching and electropolishing. Solid-solution binary alloys containing 0.6, 1.0 and 1.9at.%Zn were investigated using scanning and transmission electron microscopies and ion beam analysis. Initially, the near-surface composition of the surface pretreated aluminium alloys were determined using Rutherford backscattering spectroscopy (RBS), medium energy ion scattering (MEIS) and glow discharge optical spectroscopy (GDOES). Subsequently, the surface morphologies of the pretreated specimens were characterized by secondary electron microscopy (SEM) and atomic force microscopy (AFM). Further related work was undertaken on the effect of the employed surface treatments on the anodic behaviour of aluminium-zinc alloys. A final analysis was carried out on the influence of grain orientation on zinc enrichment and surface morphologies of aluminium-zinc alloys developed during alkaline etching.The results reveal that surface pretreatments of aluminium-zinc alloys lead to zinc enrichment at the alloy/film interface. The number of zinc atoms contained in the enriched layer depends upon the employed surface pretreatment and the alloy composition. Surface pretreatments influence the topography of Al-Zn alloys. In particular, for electropolishing, alkaline etching and anodic alkaline etching, the resultant surface morphologies were associated with the oxidation-dissolution-precipitation mechanisms occurring at the alloy/film interface and at the film/solution interface.Anodizing of Al-Zn alloys shows that the anodic films growth on Al-Zn alloys in rolled condition and after surface treatment becomes detached from the substrate independently of the surface treatment employed, although detachment of the anodic film occur earlier on surface pre-treated Al-Zn alloys that developed zinc enrichment. The final studies correlated the mechanisms of oxidation and dissolution with the grain orientation. The result revealed that the (111) grain dissolves faster than other grains, while the higher number of zinc enriched atoms were measured on the (001) grain.