The Influence of Microstructure and Microchemistry on the Corrosion of 6xxx Series Aluminium Alloys

The 6xxx series Al-alloys, which are based on the Al-Mg-Si-(Cu) system, are of relevance to automotive and aerospace applications. They are precipitation hardenable alloys, whereby artificial aging improves their strength through the formation of nanoscale precipitates. Precipitates have a different composition and therefore different electrochemical characteristics compared to the alloy matrix. Microgalvanic coupling can occur between the alloy matrix and precipitates, leading to localised corrosion attack. Therefore, an increase in the strength of 6xxx series Al-alloys is usually associated with an increase in localised corrosion susceptibility. It is subsequently a challenge to tailor the microstructure in these alloys to simultaneously obtain optimum mechanical properties and corrosion resistance. This PhD thesis aims at identifying compositions (i.e, Si:Mg ratio and Cu content) and ageing conditions for 6xxx series Al-alloys where desirable corrosion resistance is achieved. Furthermore, this work also aims at answering fundamental questions regarding the mechanisms of intergranular corrosion (IGC) in 6xxx series Al-alloys and also, elucidating the role of Q-phase (Al_xCu_yMg_zSi_w) in the IGC behaviour of Cu-containing 6xxx series Al-alloys - focusing mainly on its electrochemical response and quasi in-situ corrosion. <br> <br> Al-alloy sheets, varying in composition (Si:Mg ratio and Cu content) were studied herein. These alloy sheets underwent four different ageing treatments; viz. naturally aged (1 week at room temperature), under aged (20 min at 185 °C), peak aged (6 h at 185 °C) and over aged (24 hrs at 185 °C), following solution treatment and water quenching. PANDAT^TM was used for the phase simulation and equilibrium phase calculation, which revealed the presence of different equilibrium phases upon slight variation in the Si:Mg ratio and Cu content. Potentiostatic tests were employed to determine the correlation between metastable pitting events and the microstructural features. It was observed that precipitate thickness was the key parameter in dictating the evolution of metastable pitting events. Cu was found to refine the thickness of the precipitates and increase their density number, which influenced the evolution of metastable pitting. Thus Cu proved to be beneficial in 6xxx series Al-alloys by improving their strength and inhibiting the corrosion initiation process, i.e., metastable pitting, up to the peak aged condition. The IGC propagation mechanisms of 6xxx series Al-alloys were investigated using the ASTM G110 standard. It was observed that Cu was detrimental to 6xxx series Al-alloys in terms of IGC upon artificial ageing at 185°C, even for a short ageing time of 20 min. IGC persisted up to the over aged condition. However, by decreasing the Si:Mg ratio, the severity of IGC was reduced. The microstructure of the pristine alloys was characterised using high resolution scanning transmission electron microscopy (STEM) and 3D atom probe tomography. It was observed that the cause of IGC is due to the formation of galvanic couple between different metastable/stable phase precipitates (such as, pure Cu-film, Q-phase, β-phase (Mg₂Si), S-phase (Al₂CuMg) and θ-phase (Al₂Cu) based on the basic alloy composition and ageing) along grain boundaries adjacent to precipitate free zones (PFZs). Further, the electrochemical and corrosion behaviour of Q-phase was found for the first time using quasi in-situ STEM, quasi in-situ scanning electron microscopy (SEM) and the electrochemical microcell technique. The Q-phase was found to be cathodic with respect to pure Al and θ-phase