Evaluating the influence of silicon on phase formation, hardness and corrosion in Al–6.5% Mg alloy

Al–Mg alloys are widely investigated as candidate materials for sacrificial anodes because of their favourable electrochemical activity and resistance to passivation in chloride environments. In cathodic protection systems, aluminium, zinc, and magnesium alloys are commonly used, with most commercial aluminium anodes based on Al–Zn–In or Al–Zn–Mg systems. However, the influence of silicon additions on the phase evolution, mechanical properties, and corrosion behaviour of Al–6.5%Mg alloys remains insufficiently understood. In this study, Al–6.5%Mg alloys containing 1.5%, 3%, 4.5%, and 6% Si were produced using conventional melting and casting. The alloys were characterised using optical microscopy, SEM/EDS, X-ray diffraction, hardness testing, potentiodynamic polarisation, and 8-week immersion tests in 3.5 wt.% NaCl solution. Results show that increasing Si content modifies the morphology and distribution of Mg₂Si phases from fine dispersed particles to semi-continuous networks, affecting both hardness and corrosion behaviour. Hardness ranged from 67 to 83 BHN, with the highest values at 1.5% and 6% Si. Corrosion resistance decreased when Si exceeded 1.5% owing to intensified micro-galvanic interactions. The Al–6.5%Mg–1.5%Si alloy exhibited the most favourable balance of corrosion resistance and controlled anodic dissolution for sacrificial anode applications in chloride environments.