Adsorption mechanism and performance analysis of alkaloids as green corrosion inhibitors on mild steel

Adsorption mechanism and inhibitive performances of alkaloids on mild steel are studied using density functional theory (DFT). Global reactivity parameters such as global hardness ( ɳ ), fraction of electron transfer from molecule to metal ( ∆N ), and dipole moment ( μ ) are calculated to determine corrosion protection efficiency. Periodic DFT calculations show bonding interactions between heteroatoms of alkaloid and Fe atom of Fe(110) surface. Fe–-H non-classical bond formations are also observed. The order of adsorption energy ( E ads ) is quinine (86.6 kcal/mol) > cytisine (67.6 kcal/mol) ≈ mescaline > ephedrine > morphine ≈ atropine > nicotine > caffeine > tryptamine > codeine (50.2 kcal/mol). Highest E ads value of quinine is interpreted with its very favourable orbital energies that help to form chemical bonds via electron donation and back donation. These high E ads values confirmed chemisorption process and interpreted strong inhibition efficiencies of morphine and caffeine on mild steel as reported in experimental studies.