Comprehensive Experimental, Theoretical, and Surface Characterization Studies on the Corrosion Inhibition Mechanism, Electrochemical Durability, and Long-Term Protection Ability of a 5‑Mercapto-1-Methyltetrazole Film on Mild Steel in 1 M HCl Solution.

In this study, the adsorption behavior and corrosion inhibition performance of 5-mercapto-1-methyltetrazole (MMT) on mild steel (MS) were comprehensively evaluated in 1 M HCl solution by electrochemical, surface-characterization and quantum-chemical methods. In order to clearly demonstrate its novelty and suitability for practical applications compared to conventional short-term assessments, the electrochemical durability and long-term protective ability of the inhibitor film were examined. It was shown that the inhibition efficiency depends on MMT concentration and remains exceptionally high, reaching approximately 97.3% (LPR) and 95.8% (PDP) even after 120 h of continuous immersion. MMT acts as a mixed-type corrosion inhibitor with a predominant cathodic inhibition effect at low concentrations. The corrosion reaction is charge-transfer-controlled, and the MMT molecules form a barrier by adsorbing at the metal/solution interface. Thermodynamic calculations yield an adsorption free energy (ΔG ads) of -33.2 kJ/mol, which suggests that the adsorption process involves mixed physisorption and chemisorption contributions, with a tendency toward stronger chemical interactions. SEM and EDX mapping analyses confirm a uniform distribution of the MMT film across the steel surface. Rather than degrading over time, the MMT film dynamically rearranges and compacts. Potential of zero charge (PZC) measurements indicate that the metal surface carries an excess positive surface charge. The assembled surface film is electrochemically durable under both chronoamperometric and cyclic voltammetric conditions.