The Equisetum arvense extract was prepared by the maceration technique and experimentally tested as a corrosion inhibitor of A36 steel in 0.5 M sulfuric acid at room temperature. The corrosion resistance was estimated by the determination of the polarization curves, linear polarization resistance (LPR) and electrochemical impedance spectroscopy (EIS). In order to study the effect of the Equisetum arvense extract on the superficial morphology of metal, scanning electronic microscopy (SEM) was used. The results indicated a decrease of about two orders of magnitude in the corrosion rate, an increase in polarization resistance and a greater efficiency of inhibition by increasing the concentration of extract. The corrosion mechanism was mainly controlled by charge transfer from metal to the environment through the double electrochemical layer. The SEM images corroborated the results obtained in the corrosion tests, at higher concentration of extract less metal surface damage was observed.