Corrosion inhibition driven by double-bridged-oxygen compact metaphosphate networks in KZnP₃O₉

Developing corrosion inhibitors with ultrahigh performance and elucidating the atomic-scale mechanisms underlying their corrosion resistance are critical to both engineering and scientific communities. Herein, we report KZnP₃O₉ crystal and glass corrosion inhibitors for the corrosion protection of carbon steel Q235 in HCl solution. The KZnP₃O₉ crystal demonstrates superior corrosion resistance over its glass counterpart, featured by a more compact protective corrosion product film with higher time dependence of impedance and lower areal and linear roughness. Molecular dynamics (MD) simulations reveal that the metaphosphate ion has a higher adsorption energy on the Q235 substrate than orthophosphoric acid and pyrophosphoric acid, and the Fe ion is indirectly adsorbed through its interaction with the phosphate ion. Specifically, the KZnP₃O₉ crystal demonstrates the highest electrochemical impedance (525 Ω·cm²) and corrosion inhibition efficiency (95%) among all inorganic phosphate corrosion inhibitors. This can be attributed to the double-bridging oxygen of a single PO₄ tetrahedron for the crystal, which forms an infinite P₃O₉ chain or ring with a higher adsorption energy, as opposed to the zero- or single-bridging oxygen of each PO₄ tetrahedron for the glass, which has isolated PO₄ or P₂O₇ structures with lower adsorption energy.