EIS and potentiodynamic polarization studies on immiscible monotectic Al–In alloys

• The Al–In alloy microstructure is characterized by In droplets spread in an Al matrix.
• The scale of phases forming the microstructure affect the electrochemical behavior.
• Larger interphase spacing/droplet diameter is related to higher corrosion resistance.
• The deleterious effect in corrosion resistance is due to strains and galvanic cells.

The electrochemical behavior of monotectic Al–In alloys is experimentally investigated. Electrochemical impedance spectroscopy (EIS), potentiodynamic anodic polarization techniques and an equivalent circuit analysis were used to evaluate the corrosion response in a stagnant and naturally aerated 0.5 M NaCl solution at 25 °C. It was found that a better galvanic protection can be provided for microstructures having indium droplets of larger diameters and larger interphase spacings. From five samples extracted along the length of a directionally solidified Al–In casting, that having smallest interphase spacing (λ = 18 μm) and droplet diameter (d = 0.7 μm) had its corrosion resistance significantly decreased (about 2 and 3 times in terms of the current density and polarization resistance) when compared with that of the sample having the coarsest microstructure (λ = 60 μm and d = 2.5 μm). Such behavior is attributed to both localized strains between aluminum and indium boundaries and the corrosion potential of the indium particles.