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. Phosphate glass inhibitors mitigate carbon steel corrosion in acidic media, but whether crystalline phosphate offers superior corrosion inhibition efficiency and the underlying mechanisms remain underexplored. Here, the authors report KZnP3O9 crystal and glass corrosion inhibitors for carbon steel Q235 in HCl solution, showing that the KZnP3O9 crystal demonstrates superior corrosion resistance over its glass counterpart, with metaphosphate ions exhibiting a higher adsorption energy on the Q235 substrate compared to orthophosphoric acid and pyrophosphoric acid, likely due to the formation of P3O9 structures.