Microstructure and corrosion behaviour of the extruded high-strength aluminium alloys AA6000 series

The microstructure and corrosion behaviour of extruded high strength AlMgSi(Cu) alloys with and without Cr, Zr and Mn dispersoid-forming element additions have been investigated using electron backscatter diffraction (EBSD), immersion corrosion testing BS ISO 11846, electrochemical testing, scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDS) and transmission electron microscopy (TEM) techniques. EBSD showed the alloy had coarse grains near the surface and fibrous grains in the interior, which is characteristics of extruded aluminium alloys containing dispersoid-forming elements. The beneficial effects of Zr, Mn and Cr dispersoid-forming additions can be observed as they promoted a thinner peripheral coarse grain (PCG) layer and higher fraction of fibre texture in the fibrous region especially when they added in combined. Recrystallised grains below the PCG layer produced cube texture as the dominant texture component while fibrous region mainly formed fibre texture with the <111> directions aligned along the extrusion direction. The electrochemical testing results showed that additions of high Cu content (0.8 wt%) in alloys has shifted the corrosion potential to a more noble value in 3.5 wt% NaCl solution. The polarisation curves showed an active corrosion behaviour with no passivation throughout the experiment. Immersion corrosion testing results showed that additions of Cu of more than 0.1 wt% promoted localised corrosion with inter granular corrosion (IGC) and pitting as the predominant corrosion modes. Lowest Cu content (0.04 wt%) significantly lessens the aggressive IGC attack. The presence of the characteristic Q-phase (Al-Mg-Si-Cu type precipitate) is believed to increase the overall susceptibility to the IGC of the Cu-rich alloys due to the micro-galvanic coupling between the noble Q-phase particles and the adjacent matrix aluminium. Results also showed Fe-containing intermetallic particles which contained substantial amount of Fe, Mn and Si could lead to pitting corrosion as well as trenching of the matrix aluminium. Along with the high Cu content, a thicker PCG layer significantly reduced the resistance to corrosion by increasing the depth of IGC attack and in some cases caused exfoliation corrosion. The corrosion penetration depth in an industrial Cr-free alloy with a thick PCG layer reached approximately 600 µm after 24 h immersion time and showed exfoliation corrosion after 5 h. Whereas, the Cr-containing variant of the alloy with a thinner PCG layer had a corrosion depth of approximately 255 µm after 24 h immersion time. Heat treatment was equally important to the corrosion behaviour as the peak-aged T6 condition seemed to aggravate corrosion attack most likely due to the formation of PFZs, while in the overaged T7 condition there was a remarkably reduced IGC attack in the alloy with a high Cu (0.73 wt%) content and increased localised pitting in the alloy with a lowest Cu (0.04 wt%) content.