Effects of structure of the ionic head of cationic surfactant on its inhibition of acid corrosion of mild steel

Two n-alkyl-quaternary ammonium compounds were studied as corrosion inhibitors for acid corrosion of mild steel using electrochemical and weight loss methods. The two compounds are hexadecylpyridinium bromide (HDPB) and hexadecyltrimethyl ammonium bromide (HDTB). The influence of the structure of the ionic head on the inhibition action of the two cationic surfactants was studied by analyzing the data at different concentrations and temperatures. The inhibition efficiency increases with the concentration. It increases with temperature in the case of HDPB but decreases in the case of HDTB. The apparent activation energy, E _a of corrosion in the presence of HDPB was found to be lower than in blank (0.5 m H₂SO₄). In the case of HDTB, E _a was larger than that of the blank. A larger extent of adsorption for HDPB on the metal surface was evident from the larger negative values of the free energy of adsorption. The results yielded the extent and mode of adsorption of the inhibitors on mild steel. The stronger adsorption of HDPB was attributed to the differences in the molecular structures of the inhibitors.