Outstanding synergistic inhibition effect of Ca ₃ (PO ₄ ) ₂ and BTA on Q235 steel in NaCl solution

The synergistic effect of calcium phosphate [Ca 3 (PO 4 ) 2 ] and benzotriazole [BTA] on corrosion inhibition of Q235 steel in 3.5 wt-% NaCl solution was studied by means of electrochemical, ultra-depth of field, scanning electron microscopy-energy-dispersive spectroscopy (SEM-EDS), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR). Results indicated that the compound corrosion inhibitors of Ca 3 (PO 4 ) 2 and BTA provide excellent inhibition durability in long-term test, with the outstanding inhibition efficiency ( η ) up to 98.7% and the synergistic parameter ( s ) of 9.06 at the end of 28 days immersion. This is of crucial importance in practical application scenarios. Analysis of the corrosion products proves that this synergistic effect could be ascribed to the formation of a double-layered film on the surface of Q235 steel: the inner layer is cross-linked network structure between Ca 3 (PO 4 ) 2 and BTA with Fe 2+ through the lone pair electrons, while the outer layer is an adsorption film with strong adhesion, which well follows Langmuir adsorption isotherm and belongs to mixed adsorption. The double-layered film can effectively obstruct the infiltration of corrosive species and thus provide an outstanding anti-corrosion performance. This work provides a new strategy to achieve high anti-corrosion effect by using low-cost and mature inhibitors.