Synthetic Corrosion Inhibitors for Mild Steel in Hydrochloric Acid Medium: A Comprehensive Review of Advances

Abstract

Mild steel undergoes aggressive dissolution when exposed to hydrochloric acid solutions, a problem of enormous practical consequence in industrial operations ranging from acid descaling and pickling to oil-well stimulation. Over the fifteen-year span from 2010 to 2025, a remarkable body of experimental and computational work has accumulated on the development, characterization, and mechanistic understanding of synthetic organic corrosion inhibitors for this metal-medium combination. The present review examines that body of work, organized around the principal experimental and theoretical tools employed by researchers. Beginning with the effect of inhibitor concentration on protection efficiency, the discussion proceeds through the thermodynamic activation parameters derived from temperature-dependent gravimetric studies, the adsorption isotherms and associated free-energy parameters that characterize inhibitor-surface binding, and the electrochemical methods—Tafel polarization and electrochemical impedance spectroscopy— that reveal the mode and kinetics of inhibitor action at the electrode-electrolyte interface. Density functional theory calculations and their principal descriptors, including frontier orbital energies, energy gap, electron-transfer fraction, and Fukui functions, are reviewed in the context of structure-property correlations. Surface characterization data from scanning electron microscopy, atomic force microscopy, FT-IR spectroscopy, contact angle measurements, and UV-visible spectrophotometry are assessed as independent confirmation of film formation. Throughout, special attention is given to the contributions of Kalkhambkar and Rajappa from D.M.S. Mandal's Bhaurao Kakatkar College, Belgaum, whose multi-technique investigations of quinazoline Schiff bases and amino acidderived compounds in hydrochloric acid medium constitute notable contributions to the field. Four consolidated data tables covering 28 inhibitor systems are provided, enabling direct comparison of inhibition efficiencies, activation parameters, DFT descriptors, and adsorption thermodynamics across structurally diverse compound classes.