A new green corrosion inhibitor for P11 steel in acid medium was developed using Opuntia ficus indica plant mucilage (MOFI) as an economical and ecological source. Electrochemical frequency modulation, electrochemical impedance spectroscopy, potentiodynamic polarization, and resistance to polarization were used to determine the efficacy of MOFI inhibition. When taking into account all four methods, the maximum efficiency of inhibition was 72.7% at 65 °C and 1 g L-1 MOFI. At higher temperatures, both the efficiency of inhibition and the adsorption equilibrium constant were found to rise. With increasing MOFI concentration, the standard enthalpy of adsorption was positive and the apparent activation energy decreased. All of these measurements point to the chemical adsorption of MOFI onto the surface of P11 steel. In addition, the results show that MOFI has a high capacity to reduce acid c...