Abstract: The synergistic inhibition effect of novel Mannich-base imidazoline (PPLA) mixed with thiourea (TU) or its derivatives allylthiourea (ATU) and phenylthiourea (PTU) for mild steel corrosion in sulfuric acid solution has been systematically investigated by both experimental and theoretical methods. It was discovered that the addition of small-molecule thiourea-type corrosion inhibitors can significantly improve the adsorption performance of PPLA molecules on the steel surface, particularly at elevated temperatures. The corrosion inhibition efficiencies of PPLA+ TU/ATU/PTU mixed system can achieve 99.6 %, 99.7 %, and 99.8 % even at 343 K, respectively. Theoretical calculations reveal that TU/ATU/PTU, in addition to simply filling the voids in the corrosion inhibitor film, can also synergistically enhance the adsorption stability of inhibitors through intermolecular interactions with PPLA. These cooperative effects effectively impede the penetration and diffusion of corrosive species, thereby achieving superior corrosion inhibition performance.

Key words: Mild steel, Sulfuric acid, Corrosion inhibitors, Synergism, Theoretical calculations