Abstract: Ethyl-bis(octadecyl dipolyoxyethylene ammonium chloride) (E-BODAC) was a novel quaternary Gemini surfactant and was first found and identified as an excellent inhibitor with excellent surface activity and suitability. The ethoxy group enhances its hydrophilicity, and facilitates a dense adsorption film on metal surface. The inhibitive action and mechanism of E-BODAC on cold rolled steel (CRS) in HCl and H₂SO₄ solutions was studied in detail by theoretical and experimental methods. The surface activity and degree of polymerization of the surfactant are tightly related to the anti-corrosion properties. HCl has a lower critical micelle concentration (CMC) than H₂SO₄ solutions. The hydrophilic group in E-BODAC is the preferred site for donating and accepting electrons, with chemical interactions occurring among O, C, and Fe. The addition of E-BODAC obeys Langmuir isotherm, which can drastically reduce the diffusion of H₃O⁺, Cl^-/SO₄^2-. E-BODAC has superior corrosion inhibition performance and lower dosage, and the inhibition efficiency for CRS reaches 92.9 % in 1.0 M HCl with only 10 mg L^-1 of E-BODAC and 91.5 % in 0.50 M H₂SO₄. The inhibition is through physical and chemical adsorption to form a protective film that covers the reaction site, blocking the corrosion reaction. CLSM, SEM, AFM, XPS, and time-of-flight secondary ion mass spectrometer (TOF-SIMS) provide evidence of the adsorption of E-BODAC, efficiently slowing down the corrosion of CRS surface. E-BODAC always shows higher efficiency in HCl than in H₂SO₄, which is attributed to the re-dissolution of the adsorbed film in the H₂SO₄ solution. Besides, SO₄^2- ions are larger and migrate more slowly than Cl^- ions, resulting in weak adsorption on the metal surface. The assessment of E-BODAC's suitability and durability in different corrosive environments provides the opportunity to determine its optimum conditions and applicability to maximize its corrosion inhibition effect.

Key words: Theoretical calculation, Diffusion, Corrosion inhibition, Steel, Gemini inhibitor, TOF-SIMS