Macrocyclic ligands as corrosion inhibitors: Macrocyclic effects, host-guest chemistry, coordination and complexation with metal surfaces and ions

The increasing demand for durable and efficient corrosion inhibitors has intensified interest in macrocyclic ligands as an emerging class of tunable corrosion inhibitors. Several salient features, including macrocyclic effect, host-guest recognition, preorganized multidentate coordination, extensive surface coverage, and metal-ion complexation within a single molecular framework, make them an attractive and effective class of inhibitors. These properties also make them different from traditional corrosion inhibitors. The present review provides broad coverage of phthalocyanines, crown ethers, porphyrins, azamacrocycles, and calixarenes, with particular focus on their structure-performance relationships, coordination mechanisms, adsorption, and the formation of corrosion-protective films on different substrates, including steel, iron, aluminum alloys, copper, and magnesium in acidic, alkaline, and saline environments. Macrocyclic inhibitors exhibit a broad range of inhibition efficiencies depending on their molecular structure, metallic substrate, and experimental conditions. This review also highlights the roles of donor atoms, molecular geometry, ring or cavity size, metal coordination, substituents, and solubility in the adsorption, coordination, and corrosion inhibition of macrocyclic ligands. This article also discusses their challenges related to poor solubility, synthetic complexity, scalability, environmental consideration, and limited industrial-scale validation under realistic conditions. Lastly, research gaps in macrocyclic-based corrosion inhibition have also been presented with outlines, enabling effective and sustainable design of macrocycle-based corrosion inhibitors.