An expired antibiotic drug Kynurenic acid (KYNA) was investigated for its adsorption and corrosion inhibition performance on mild steel (MS) in 1.0 M HNO 3 solution. Chemical methods like weight loss measurement, electrochemical methods like impedance spectroscopy, potentiodynamic polarization and linear polarization measurements were used to identify the adsorption behavior of KYNA in HNO 3 solution. The inhibition efficiency improved with an increase in KYNA concentration in nitric acid medium. The adsorption process of KYNA was spontaneous and favored Langmuir adsorption isotherm. Potentiodynamic polarization measurements indicate that the evaluated inhibitor acts as mixed type. The values of thermodynamic and activation parameters (R 2 and ΔG ads 0 ) were calculated and discussed. The surface morphology of MS was characterized by using scanning electron microscopy, atomic force microscopy, Fourier transforms infrared spectroscopy and theoretical study. From both the experimental and theoretical studies, the investigated inhibitor molecule exhibited surface active properties and performed as a good inhibitor for corrosion control of MS in nitric acid medium. The experimental results corroborated with results obtained from theoretical molecular dynamics simulation with density functional theory studies.

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