Sustainable marine-based corrosion inhibitor from Padina sp. for climate-conscious protection of A36 carbon steel in acidic environments

The rising demand for environmentally sustainable industrial practice calls for green alternatives to conventional corrosion inhibitors in acidic media. This work presents a novel application of Padina sp. extract (PE), a brown macroalga rarely examined for metallic corrosion control, as a biodegradable inhibitor for A36 carbon steel in 1 M HCl. Potentiodynamic polarization and EIS, supported by SEM and FTIR, were coupled with DFT and Monte Carlo simulations to quantify performance and clarify mechanisms. PE achieved a maximum inhibition efficiency of 93.4% at 5000 ppm, attributed to formation of a stable, adherent protective film; this dosage represented the best compromise between surface coverage and molecular dispersion, whereas higher loading promoted aggregation and partial film destabilization. Adsorption followed the Langmuir isotherm, indicating uniform monolayer coverage, and thermodynamic analysis yielded ∆Gads ≈ –18.9 kJ/mol, confirming spontaneous, predominantly physisorptive adsorption with minor chemisorption suggested by FTIR band shifts. Molecular simulations showed that key constituents (e.g., undecanal) possess strong electron-donating ability and adopt favorable planar orientations on Fe (110), consistent with mixed-type inhibition observed experimentally. Frontier orbital descriptors and adsorption energies further corroborated robust inhibitor–surface interactions. Overall, introducing Padina sp., an abundant yet underutilized marine biomass, provides an effective green inhibitor and demonstrates how quantum-chemical insights can guide low-toxicity corrosion-control design, while keeping the manuscript’s primary contribution focused on inhibition performance and molecular interactions.