SHIB protects low carbon steel in hydrochloric acid through adsorption controlled corrosion inhibition

Sodium (E)-4-hydroxy-3-(((3-hydroxyphenazin-2-yl)imino)methyl)benzene sulfonate (SHIB) was synthesized and evaluated as a structurally distinctive phenazine salicylaldimine sulfonate inhibitor for low-carbon steel (LCS) in 0.5 mol L^-1 HCl. The study addresses the need for efficient acid-phase inhibitors that combine extended aromatic adsorption domains with heteroatom-rich donor sites and aqueous-phase compatibility. Weight-loss measurements, open-circuit potential monitoring, potentiodynamic polarization, electrochemical impedance spectroscopy, SEM, AFM, DFT calculations and Monte Carlo simulation were used to relate molecular structure to corrosion protection. SHIB showed a concentration-dependent response and reached 91.95% inhibition efficiency by weight loss, 92.25% by polarization and 92.80% by impedance at 300 ppm. The protective effect remained significant up to 328 K, although efficiency decreased at higher temperature, indicating partial thermal weakening of the adsorbed layer. Adsorption data were most consistent with a Langmuir-type relationship, and adsorption free energies in the range of - 27.76 to - 29.89 kJ mol^-1 were interpreted cautiously as evidence for spontaneous adsorption with dominant electrostatic contribution and partial donor-acceptor interaction. Polarization and impedance analyses indicate mixed-type corrosion suppression through surface blocking, increased charge-transfer resistance and decreased apparent double-layer capacitance. Surface microscopy confirmed reduced acid-induced damage, while theoretical calculations supported adsorption through the nitrogen, oxygen and aromatic π centers of SHIB. The results position SHIB as a promising laboratory-scale inhibitor candidate, while further work is required to assess wider temperature limits, long-term safety and field applicability.