The corrosion behaviour of a microcrystalline aluminium (mc-Al) coating fabricated from pure cast aluminium substrate by magnetron sputtering and the pure polycrystalline aluminium (pc-Al) were studied using electrochemical impedance spectroscopy and potentiodynamic polarization techniques in aerated 0.5 M H 2 SO 4 at 30 ± 1°C. The corrosion inhibiting effect of polyacrylic acid (PAA) and synergistic effect of iodide ions was also investigated. Results obtained show that surface microcrystallization increases the corrosion susceptibility of pure cast aluminium leading to decrease in interfacial impedance and an increase in the kinetics of the anodic dissolution. Introduction of PAA into the corrosive medium was observed to retard the corrosion rates of both specimens. The inhibition mechanism was affected by the microstructure of the sample. PAA functions as a mixed inhibitor but under cathodic control for pc-Al and under anodic control for mc-Al. Addition of iodide ions synergistically increased the inhibition efficiency of PAA and this effect was more pronounced with mc-Al.