Strontium and barium 4-carboxylphenylphosphonates: Hybrid anti-corrosion coatings for magnesium alloy

This paper reports two alkaline-earth metal phosphonates with formulae M(4-cppH2)2 [M = Sr (1), Ba (2); 4-cppH3 = 4-carboxylphenylphosphonic acid]. Compound 1 shows a chain structure made up of edge-sharing {SrO8} polyhedra and {PO3C} tetrahedra. While in compound 2, the edge-sharing {BaO8} polyhedra are connected by the {PO3C} tetrahedra to form a two-dimensional inorganic layer. Neighboring chains in 1 or layers in 2 are cross-linked by hydrogen bond interactions between the protonated carboxylate groups, resulting in three-dimensional supramolecular structures. The magnesium alloys coated with 1 or 2 films show significantly improved anti-corrosion behaviors compared to the bare substrate.