Corrosion Inhibitors for Metals in Acidic Environments: Recent Developments, Mechanistic Understanding, and Future Perspectives.

Corrosion of metallic materials in acidic environments remains a critical economic and safety concern in industries such as petroleum refining, acid pickling, chemical cleaning, and descaling. This review critically summarizes recent advances in corrosion inhibition strategies for mild steel and other industrial metals exposed to aggressive acidic media, with emphasis on organic, inorganic, polymeric, and environmentally friendly inhibitors. Organic and polymeric inhibitors predominantly function through adsorption-driven mechanisms involving heteroatoms, π-electron systems, and functional groups, forming protective interfacial films through physisorption, chemisorption, or mixed adsorption processes. In contrast, inorganic inhibitors mitigate corrosion through surface passivation and formation of stable oxide layers. Increasing environmental regulations have accelerated the transition from toxic chromate and phosphate-based inhibitors toward sustainable green alternative derived from plant extracts, biopolymers, pharmaceuticals, and biobased resources. Comparative evaluation reveals inhibition efficiencies frequently validated by electrochemical impedance techniques such as potentiodynamic polarization and electrochemical impedance spectroscopy, complemented by surface characterization using SEM, AFM, and XPS. Despite promising laboratory-scale performance, challenges persist, including thermal instability, long-term durability, limited industrial scalability, and insufficient structure-property correlations. Future developments are expected to focus on bioderived polymers, ionic liquids, nanocomposite inhibitors, and machine-learning-assisted molecular design, offering pathways toward high-performance and environmentally sustainable corrosion protection systems.