INHIBITING EFFECTS OF L-TRYPTOPHAN ON THE CARBON STEEL CORROSION IN ALKALINE SALINE SOLUTION

The inhibiting effect of L-tryptophan, that considered as an eco-friendly compound on the corrosion of carbon steel in saline solution ( 3.5% NaCl) at pH 11 was investigated potentiostatically at five temperatures in the range 293313 K. Five concentrations of inhibitor were tested ranging from 5×10 to 1×10 mole L. The addition of Ltryptophan to alkaline sodium chloride solution lowered the corrosion feasibility of the alloy. On the other hand, the corrosion feasibility increased with the rise of temperature. Carbon steel corrosion rate in the saline solution was decreased on increasing L-tryptophan concentration reflecting the inhibiting effect of L-tryptophan when it is present at high concentration in the saline solution. The efficiency of inhibition (%IE) of L-tryptophan increased with increasing the concentration of the inhibitor attaining a maximum value of 71.72 % when the L-tryptophan concentration in the saline solution became 1×10 mole L, and decrease with the rise of temperature. Adsorption of L-tryptophan on the surface of carbon steel was considered to obey Langmuir isotherm. Gibbs free energy of adsorption of L-tryptophan on steel was -29.07 kJ mol indicates a physisorption of the inhibitor on the surface of carbon steel alloy. Quantum mechanical study using DFT method with B3LYP/6-31G basis set was applied to achieve good relation between the inhibition efficiency and the chemical structure of L-tryptophan.