Corrosion Inhibition of Zinc in Hydrochloric Acid by Ocimum sanctum: A Critical Review of Adsorption, Thermodynamic, Electrochemical and Surface-Characterization Evidence

Abstract

Zinc is widely employed in galvanizing, sacrificial protection, batteries and other industrial applications because of its favourable electrochemical and protective characteristics. Nevertheless, its relatively high reactivity in acidic environments, particularly hydrochloric acid (HCl), can result in rapid anodic dissolution and significant material loss. The use of corrosion inhibitors is therefore essential in industrial operations involving acid pickling, descaling and cleaning. Conventional inorganic and synthetic organic inhibitors can provide effective protection but are increasingly constrained by concerns regarding toxicity, persistence and environmental impact. Consequently, plant-derived corrosion inhibitors have received considerable attention as potentially sustainable alternatives. This review critically examines the reported corrosion-inhibition performance of Ocimum sanctum (Holy basil/Tulsi) extracts for zinc exposed to hydrochloric acid, with particular emphasis on adsorption behaviour, thermodynamic and kinetic parameters, electrochemical response, and surface characterization. Available studies indicate that O. sanctum contains diverse potentially active constituents, including eugenol, phenolic compounds, flavonoids, tannins, saponins and other oxygen- and nitrogen-containing molecules capable of interacting with metal surfaces. Adsorption-isotherm analyses predominantly support Langmuir-type behaviour, although Freundlich and Temkin models provide complementary evidence of surface heterogeneity and adsorbate–adsorbate interactions. Reported negative Gibbs free-energy values indicate spontaneous adsorption, while the variation of enthalpy, entropy and activation-energy parameters suggests that the inhibition process can involve both physical and chemical interactions, depending on extract composition and experimental conditions. Electrochemical investigations generally demonstrate increased charge-transfer resistance and reduced corrosion activity in the presence of the extract, while polarization measurements indicate suppression of both anodic zinc dissolution and cathodic hydrogen evolution. SEM, EDX and FTIR observations further support the formation of an adsorbed organic protective layer on the zinc surface. However, substantial differences in extraction procedures, phytochemical composition, inhibitor concentration, acid concentration, temperature and electrochemical protocols make direct comparison among published results difficult. Future research should therefore prioritize extract standardization, identification of the dominant active constituents, mechanistic studies combining electrochemical measurements with molecular modelling, long-term stability assessment, and techno-economic evaluation of scale-up. Overall, O. sanctum represents a promising plant-derived corrosion inhibitor, but its transition from laboratory investigation to industrial application requires greater experimental standardization and mechanistic validation.