Towards the development of corrosion resistant aluminium alloys

Abstract

This dissertation reports some discrete developments towards more corrosion resistant Al-alloys by controlling alloy chemistry and microstructure. Al-alloys are generally passive and corrosion resistant due to the presence of a protective aluminium oxide film. However, on exposure to a corrosive environment (i.e. solution containing halide ions), Al-alloys are prone to localised attack, particularly pitting. The pits are typically initiated by the differing electrochemical interaction (and roles) of second phase particles with the surrounding Al-matrix, which can also contribute to pit propagation. The corrosion response of the alloys herein was evaluated with a combination of immersion and electrochemical tests, including potentiodynamic polarisation (PDP) and electrochemical impedance spectroscopy (EIS). Corrosion assessment was supplemented by surface analysis with scanning electron microscopy (SEM) and optical profilometry. Development towards a more corrosion resistant Al-alloy was approached in stages, wherein the first stage involved characterising the corrosion response of commercial Al-alloys for the construction (i.e. revelation) of primitive property space (hardness vs. corrosion rate). This highlighted the opportunities in minimising corrosion whilst being aware of mechanical strength (albeit that hardness was used in this project as proxy to mechanical strength). A consolidated presentation of the role of chemistry on both the corrosion kinetics and hardness suggested that reducing/eliminating Cu and limiting microstructural heterogeneity (i.e. additional phases) are effective in minimising corrosion. It was empirically revealed that AA5083 (Al-4.4Mg-0.4Mn), which has medium strength and low corrosion rates, was the ‘best compromise’ (commercial) alloy and will be further explored in this project. The next stage was to investigate the effect of alloying elements to the Al-4.0Mg-0.4Mn (similar to the AA5083 commercial alloy composition) system. This study was divided into two parts. The first part aimed to understand the effect of microalloying additions to the microstructure of Al-4.0Mg-0.4Mn in relation to the electrochemical response and subsequent corrosion morphology. The effect of microalloying upon the microstructure of the base alloy was demonstrated by the formation of additional intermetallic particles (be it dispersoids or constituent particles). The second part of the study was to investigate the effect of microalloying additions on the intergranular corrosion susceptibility. In the sensitised condition, Al-4.0Mg-0.4Mn alloys are prone to intergranular attack, as the electrochemically active β-phase (Mg2Al3) tends to precipitate at grain boundaries. A consolidated presentation of the outcomes from a number of corrosion measurement methods employed, revealed the alloying additions that can improve corrosion resistance of the Al-4.0Mg-0.4Mn alloy. For mass loss, Zn addition yields better resistance to corrosion, whereas PDP test indicates Si, Zr and Sr have lower icorr than Al-4.0Mg-0.4Mn alloy. On the other hand, addition of Ti, Si and Sr reduce the susceptibility to intergranular corrosion. Sr was particularly effective due to its ability to modify the β-phase characteristics. The corrosion response measured on the Al-4.0Mg-0.4Mn system, microalloyed with different elements, revealed that Sr and Nd imparted a good compromise between corrosion resistance and hardness. Therefore new Al-Mg alloys were studied from more systematic in-house production with different concentrations of Sr and Nd. For alloys containing Sr, the localised corrosion was not associated with the presence of the Al4Sr intermetallic particles but rather the Fe-containing constituent particles (which are present in all commercial Al-alloys). The presence of Sr greatly retarded the precipitation of β-phase. The addition of Nd revealed a similar trend, where the presence of fine Al11Nd3 intermetallics did not have a significant influence on localised corrosion. Transmission electron microscopy (TEM) analysis revealed fine Al11Nd3 intermetallics, formed at the grain boundaries along with β-phase, may serve to increase the intergranular corrosion resistance. The information from this project is intended to contribute to certain knowledge gaps in the literature on the background, basis, and development of more corrosion resistant Al-alloys; bearing in mind that the new alloys must retain or improve the mechanical properties.