The inhibition efficacy of expired sulfamethoxazole drug towards the damage of C <b>-</b> steel in hydrochloric acid solution

Sulfamethoxazole (SMA), an antibiotic, was investigated as a corrosion inhibitor for carbon steel in hydrochloric acid solutions using both chemical and electrochemical techniques. The corrosion rate decreased with increasing SMA concentration. Inhibition efficiency improved significantly, reaching up to 95.3% at a concentration of 0.005 M and a temperature of 25 °C. The inhibition mechanism is attributed to the adsorption of SMA molecules onto the surface of carbon steel. The negative values of the standard free energy of adsorption (ΔG°ads) indicate that the adsorption process is spontaneous. Depending on temperature, ΔG°ads values ranged from −38.18 to −34.34 kJ mol−¹, suggesting a mixed physisorption – chemisorption mechanism. The decrease in the adsorption equilibrium constant (Kads) with increasing temperature is attributed to desorption of SMA molecules from the steel surface into the solution. The relatively high adsorption energy confirms the effective protective performance of SMA as a good corrosion inhibitor.