Compact vs. Bulky: How Molecular Structure Dictates the Corrosion Inhibition Performance of Nitrogen‐Containing Heterocyclic Amino Acids on AZ31B Magnesium Alloy

ABSTRACT This study systematically investigated the corrosion inhibition performance of three nitrogen‐containing heterocyclic amino acids‐ l ‐proline ( l ‐Pro), l ‐histidine ( l ‐His), and l ‐tryptophan ( l ‐Trp)‐in a 0.9 wt.% NaCl solution on AZ31B magnesium alloy. Electrochemical impedance spectroscopy determined the best efficiency after 36 h: l ‐Pro 71.08% (0.8 mM), l ‐His 48.78% (0.4 mM), l ‐Trp 46.80% (0.8 mM). PDP and hydrogen evolution tests further confirmed the inhibition efficacy ranking as l ‐Pro > l ‐His > l ‐Trp. Theoretical calculations indicate that the adsorption of l ‐Pro on the magnesium alloy surface relies primarily on the carboxyl group; the adsorption of l ‐His is attributed to the heterocycle and the carboxyl group; whereas the adsorption of l ‐Try is mediated mainly by the heterocyclic structure. Owing to its compact structure, l ‐Pro forms a dense adsorption layer, delivering optimal anti‐corrosion performance. Conversely, l ‐Trp's bulky ring structure induces steric hindrance, reducing coverage. This study demonstrates that molecules possessing compact, rigid structures capable of forming high‐coverage dense films exhibit greater potential as highly efficient green anti‐corrosion agents.