An experimental study has been performed on the interrelationship between visual appearance, surface topography and light scattering on AA6063-T6 aluminium extrusions etched to a depth of up to 700μm. The topography was characterized by use of White Light Interferometry (WLI), and light scattering was measured by use of a glossmeter and photogoniometer. The main parameters for the visual appearance evaluation were gloss, directionality, streakiness and homogeneity. This combined approach of visual assessment and measurements provides valuable insight into how measurable quantities relate to the perceived visual appearance for industrially relevant aluminium surfaces.

Preferential grain etching is a problem occurring on aluminium products during alkaline etching prior to anodizing, especially if the etching bath contains small amounts of dissolved Zn. The visually uneven, "grainy" surface is caused by different etch rates for different grains, giving a terrace-like surface topography. Polished samples of an AlMgSi alloy with different amounts of Zn were etched in NaOH solution. Grain orientations in selected areas were determined by Electron BackScatter Diffraction (EBSD). The low-Zn samples did not show preferential grain etching. On the higher-Zn alloys, grains with surface planes close to {111} were found to etch at a higher rate than the other orientations. Secondary Ion Mass Spectrometry (SIMS) depth profiling and imaging, combined with White Light Interferometry (WLI) analysis, showed a clear correlation between the deep lying grains and a surface enriched in Zn, suggesting that Zn enrichment by alkaline etching occurred by selective dissolution of aluminium.

Certain 6000-series extrusions may develop susceptibility to intergranular corrosion (IGC) by improper heat treatment, especially if copper is present as an alloying element. Although occurrence of IGC in such cases is documented, the underlying mechanisms are not adequately explained. We present corrosion data for two model alloys, having different Cu content and Mg:Si ratio, showing that the susceptibility to IGC depended primarily on the Cu content and secondly on thermal processing. Low Cu samples (0.0005wt.% Cu) were essentially resistant to IGC. High Cu samples (0.12wt.% Cu), which were air cooled after extrusion, exhibited significant IGC. However, IGC susceptibility was reduced significantly as a result of artificial aging to peak strength. Water quenched high Cu samples were essentially resistant to IGC. However, slight IGC susceptibility was introduced after aging. Electron optical characterisation revealed Al4Mg8Si7Cu2 (Q-phase) grain boundary precipitates on all the variants susceptible to IGC. The susceptibility was attributed to microgalvanic coupling between Q-phase grain boundary precipitates (noble) and the adjacent depleted zone (active).

Susceptibility to intergranular corrosion (IGC) of 6000-series model alloys extruded in the laboratory was investigated as a function of Cu content, cooling rate after extrusion and artificial aging. One alloy type contained about 0.6 wt% each of Mg and Si with varying Cu content. The second type was a Cu-free alloy with higher Si content (ca. 1%). Extrusions with low Cu content (≤ 0.02 wt%) were resistant to localised corrosion, while those with high Cu content (0.17 wt%) could become susceptible to IGC. FE-SEM investigation revealed large grain boundary precipitates on air cooled samples. These precipitates were Mg2Si and Q-phase (Al5Cu2Mg8Si6) in the samples susceptible to IGC. Only Mg2Si was present in the corrosion-resistant samples. IGC susceptibility was attributed to the microgalvanic coupling between the noble Q-phase particles and the adjacent depleted zone. IGC can be prevented by proper heat treatment.

Initiation and early propagation of pitting and filiform corrosion on bare alloy AZ91 (9% Al, 1% Zn) are investigated by natural immersion corrosion tests, electrochemical measurements and microanalytical studies. Initiation sites are few. Corrosion spreads from these sites first in the form of filiform corrosion for a limited period of time and pitting which later develops into a cellular type of etching..The important factors affecting filiform corrosion are temperature, material structure and degree of polarization at the anodic sites. Filiform attack on AZ91, unlike the classical mechanisms of filiform corrosion on coated metals, is driven by hydrogen evolution reaction on the cathodic sites of the surface, occurs under significant anodic control, propagates at a high, constant speed independent of degree of polarization along preferential paths determined by compositional and crystallographic factors, and is a temporary phenomenon under open circuit conditions. Pitting corrosion is more predominant with decreasing anodic polarization.

Corrosion behavior of mold-cast Mg alloy AZ91 is investigated in as-cast (F), homogenized (T4), and artificially-aged (T6) conditions in chloride media. Heat treatment influences mainly the distribution of intermetallic beta-phase (Mg17Al12) in the alloy. Aging to T6 temper causes precipitation of the phase as a nearly continuous network of secondary particles along the grain boundaries. The alloy exhibits much better corrosion resistance in the T6 temper than it does in the other two conditions. It is observed that the beta-phase is inert to the chloride solution in comparison to the magnesium matrix and acts as a corrosion barrier. The mechanism responsible for this behavior is investigated by studying the electrochemical properties of synthetically prepared beta-phase and solid-solution alloys representative of the magnesium matrix.