Chrysanthemum indicum extract as a sustainable inhibitor for acid-induced corrosion of API 5L X70 steel

The extract of Chrysanthemum indicum ( C. indicum ) was evaluated as a green corrosion inhibitor for API 5L X70 steel (CS) in 0.5 M H₂SO₄ solution. Its inhibitory performance was assessed using multiple techniques, including weight-loss (WL), potentiodynamic polarization (PDP), electrochemical impedance spectroscopy (EIS), and surface characterization methods such as Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDX), and atomic force microscopy (AFM). The inhibition efficiency (% IE) increased with increasing extract concentration, reaching a maximum value of approximately 91.53% at 400 ppm and 25 °C. However, the % IE decreased with increasing temperature reaching 78.28% at 45 °C at the same dose. The adsorption behavior was best described by the Langmuir adsorption isotherm, while the calculated thermodynamic parameters provided further insight into the nature of the inhibitor–metal interaction. The polarization curves indicated that the extract affected both the anodic dissolution of CS and the cathodic hydrogen-evolution reaction. FTIR analysis revealed that several functional groups present in the extract were adsorbed onto the steel surface, suggesting the formation of a thin organic protective film upon immersion of the metal in the inhibited acidic medium. Overall, the chemical and electrochemical findings were in good agreement, providing strong evidence that C. indicum extract forms a protective layer on the steel surface, thereby reducing the corrosion rate of carbon steel in sulfuric acid solution.