The in uence of three polyindole samples on the corrosion rate of mild steel (MS) in absence and presence of different amounts of acetonitrile was investigated in 1 M HClO 4 . All samples were characterized by Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDX), Fourier-transform infrared spectroscopy (FTIR), UV-vis spectroscopy, X-ray Powder Diffraction (XRD), and contact angle measurements. Through measurements, it was determined that the polymers differ from each other on the basis of morphology, conjugation, molecular mass, and crystallinity. Also, it was found that all samples include FeCl 3 used as an oxidant. Among the samples, PIn1 has the highest anticorrosive property as it includes the highest amount of Fe and the highest density of hydrophobic group such as ClO 4 -. According to the results, it was found that PIn1 inhibited the cathodic corrosion reaction of MS in 3.5% NaCl and showed 98% protection e ciency.