Experimental and theoretical studies on some amino acids and their potential activity as inhibitors for the corrosion of mild steel, part 2

Substituent constants and quantum chemical parameters were calculated from PM6, PM3, AM1, RM1 and MNDO. Hamiltonians were used to predict the corrosion inhibition potential of nine amino acids grouped under three skeletons. Skeleton I consisted of cysteine (CYS), serine (SER) and amino butyric acid (ABU). Those in skeleton II included threonine (THR), alanine (ALA) and valine (VAL) while those in skeleton III are aromatic amino acids, which included phenylalanine (PHE), tryptophan (TRP) and tyrosine (TYR). Trends obtained from substituent constants were not entirely useful in predicting the corrosion inhibition potentials of the studied amino acids. However, the results obtained from quantum chemical parameters indicated that the trends for the variation of corrosion inhibition potentials of the studied amino acids in skeletons I, II and III are CYS > SER > ABU, THR > ALA > VAL and TRP > TYR > PHE, respectively. Highest values of inhibition efficiency were obtained for inhibitors in skeleton III and are attributed to the presence of aromatic ring in the molecule while the corrosion inhibition potential of inhibitors in skeletons I and II are attributed to the presence of –SH and –OH functional groups, respectively.