Sulphonamide derivatives as the inhibitors for mild steel: The correlation of molecular structure effect and the corrosion efficiency

•Structural effect of sulphonamides on corrosion inhibition is explored.•5 mmol/L SD provides the inhibitory efficiency of 92.7% for mild steel in acidic solution.•The inhibition efficiency is in order of SD > p-TSH > STI by experimental and theoretical results. The corrosion mitigation of different functional groups of the sulphonamide derivatives, sulfadiazine (SD), p-toluenesulfonylhydrazide (p-TSH) and sulfathiazole (STI), were compared in this paper. The strong correlation result between the inhibition efficiency and molecular structure could provide a novel insight in inhibition mechanism. Electrochemical experimental results together with the Langmuir adsorption isotherm observation suggested sulphonamides could spontaneously adsorb on the surface of Q235 to form compact protection films for blocking corrosive media. By density function theory (DFT) calculations for the protonated molecules with the metal (Fe atom), we found that protonated SD molecule had stronger interaction with the metallic surface in comparison with the protonated p-TSH and STI, which fully validated the inhibition efficiency (η) on the mild steel surfaces is in order of SD (η = 92.7%) > p-TSH (η = 79.8%) > STI (η = 66.8%) by using elecetrochemical experiments. Experimental studies together with theoretical calculations have presented a good direction for exploring the mechanism of structural effect of sulphonamides on the corrosion mitigation efficiency, which is beneficial to sheding insight on the design of new organic inhibitors in the future.