Amino acids as corrosion inhibitors in hydrochloric acid solutions

Inhibition of the corrosion of iron in hydrochloric acid solutions by amino acids was studied. Twenty-two different common amino acids and four related compounds were used. Potentiodynamic polarization curves were obtained for iron wire in deaerated 1 M hydrochloric acid solution. Intersection of the linear Tafel lines at the corrosion potential gave values of the corrosion current. At 10 mM concentration of the inhibitors, the best results were obtained with 3,5-diiodotyrosine, with an inhibition efficiency of 87%. The best common amino acid was tryptophan with an inhibition efficiency of 80%. Hydroxyproline, cystine, and cysteine acted as corrosion accelerators. Definite trends were observed which were related to the molecular structure. In general, amino acids with longer hydrocarbon chains showed greater inhibition. Additional amino groups or groups which increased electron density on the alpha amino group also increased the inhibition efficiency.