Corrosion inhibition of mild steel in 5% HCl by a biodegradable gemini surfactant and its synergistic enhancement with sodium benzoate

The combined inhibition performance of a novel biodegradable gemini surfactant with a carbonate spacer (GS12) and sodium benzoate (NaBenz) toward mild steel corrosion in 5% HCl was evaluated using gravimetric measurements, electrochemical techniques such as Potentiodynamic Polarization (PDP) and Electrochemical Impedance Spectroscopy (EIS), and surface characterization methods (XPS, AFM, SEM–EDX, and contact angle analysis). Weight loss results demonstrate that GS12 exhibits excellent inhibition efficiency at remarkably low concentrations over a wide temperature range, achieving a maximum efficiency of 98.2% with corrosion rate 162.66 ± 0.54 mpy at 1 × 10−6 M and 80 °C. Synergism parameters (Sθ > 1) confirm a positive interaction between GS12 and NaBenz, leading to enhanced corrosion protection. Polarization studies reveal that GS12 acts as a mixed-type inhibitor, irrespective of the presence of NaBenz. Surface analyses show a substantial reduction in surface damage and the formation of a more uniform and hydrophobic protective layer in the presence of GS12, which is further improved upon the addition of NaBenz. These results demonstrate that the GS12 + NaBenz system offers an effective and environmentally benign strategy for high-temperature corrosion mitigation of mild steel in acidic media.