Surface protection of steel in oil well acidizing fluids using L-theanine-based corrosion inhibitor formulations: Experimental and theoretical evaluation

Corrosion of X80 steel in 1 M and 15% HCl solutions and its inhibition using L-theanine (LTN), a natural white powder of green tea leaves extracts, was investigated using electrochemical frequency modulation (EFM), potentiodynamic polarization (PDP), linear polarization resistance (LPR), electrochemical impedance spectroscopy (EIS) and weight loss (WL) techniques. Increase in concentration of LTN improves its adsorption on X80 steel surface, forms thin protective film that increases charge transfer resistance and inhibition efficiency, but reduces corrosion current density and double layer capacitance. LTN acts as mixed type inhibitor with anodic predominance. Adsorption of LTN is spontaneous, exothermic and best approximated by Temkin adsorption isotherm. Composites of LTN and some synergistic compounds afford higher efficiencies (up to 89.2%).at 90 degrees C than pure LTN. Quantum chemical computations were also carried out by Conductor-like Screening Model (COSMO-GGA/PW91 of Dmol(3)) to support experimental findings. The active adsorption sites were predicted as C(5), O (12) and N(11) based on Mulliken charge distribution and Fukui electrophilic and nucleophilic functions. The energies of interaction were compared with rigid adsorption energies at studied temperatures (30 - 90 degrees C). FTIR analyses support that O and N sites were actively involved in adsorption.