Chemical and Electrochemical Techniques for Evaluation of Performance of some Amino Acids as Corrosion Inhibitors for Low Alloy ASTM A213 Grade T22 Boiler Steel in HCl Solutions

In this work, chemical (weight loss) and electrochemical (Tafel extrapolation and EFM) methods were employed for investigation of the corrosion inhibition of low alloy ASTM A213 grade T22 boiler steel in aerated stagnant 0.50 M HCl solutions at different temperatures (283- 333 K) by some amino acids. The investigated amino acids were serine (Ser), therionine (Thr) and glutamine (Glu). Tafel plots showed that the three tested amino acids function mainly as cathodic-type inhibitors. The inhibition efficiency increased with increase in inhibitor concentration but decreased with temperature, suggesting the occurrence of physical adsorption. Among the tested inhibitors, Glu presented the highest inhibition performance. The inhibition process was attributed to the formation of an adsorbed film on the metal surface that protects the metal against corrosive agents. XPS examinations of the electrode surface confirmed the existence of such adsorbed film. Activation energies (E° a) have been calculated in the absence and presence of various concentrations of these amino acids by measuring the temperature dependence of the corrosion rate obtained from weight-loss and Tafel extrapolation methods. It was found that the activation energy in the presence of the tested inhibitor is higher than that in bare HCl solution. The adsorptive behaviour of the three tested amino acids followed Temkin-type isotherm. Values of the standard free energy of adsorption (ΔG° ads) of the tested inhibitors were found to be lower than -40 kJ mol^-1. These results confirmed the occurrence of physical adsorption.