Synergistic Corrosion Inhibition of Carbon Steel by Binary Mannich-Based Inhibitors in Hydrochloric Acid at Elevated Temperatures

Elevated temperature corrosion inhibition has posed significant challenges to carbon steel wellbores. This study proposes bis-Mannich bases (BM)/Mannich-based imidazoline quaternary ammonium salt (MIQ) composite inhibitors with synergistic multiple adsorption, whose performance is evaluated in 0.5 M HCl over a temperature range of 343–423 K and in simulated full-wellbore conditions. The results show that each single inhibitor loses effectiveness above 393 K. By contrast, the BM/MIQ composites act as a mixed-type inhibitor that maintains an inhibition efficiency of 99.6% even at 423 K in 0.5 M HCl and 92.6% in simulated bottom-wellbore conditions, demonstrating excellent inhibition performance under high-temperature conditions. The adsorption process is spontaneous and dominated by chemisorption at elevated temperatures (ΔG < −40 kJ/mol) and follows Langmuir isotherm kinetics. Quantum chemical calculations reveal a synergistic electron transfer interaction between the highest occupied molecular orbital (HOMO) of BM and the lowest unoccupied molecular orbital (LUMO) of MIQ, which plays a key role in enhancing the adsorption stability of the composite system.