This work reports the development of environmentally friendly coatings for the protection against corrosion of AZ91 magnesium alloy substrates. As a novelty, the synergic combination of the use of silica precursors and phosphonic acid molecules has been investigated for first time in the protection of AZ91 Mg alloy. Hybrid films have been prepared by the sol–gel synthesis using tetraethoxysilane (TEOS) and methylmethoxysilane (MTMS), as co-monomers; and 1,2-diaminoethanetetrakismethylenephosphonic acid (EDTPO) and phenylphosphonic acid (PhPA), as adhesion promoter molecules. In order to improve the homogenous deposition of the silica network, several chemical pre-treatments were approached. The highest corrosion resistance, as determined by electrochemical impedance spectroscopy (EIS), has been observed for the AZ91 Mg alloy submitted to a passivation treatment with Na3PO4/NaOH, before the sol-gel deposition. Particularly, the addition of low concentrations of EDTPO, which contains four active phosphonic groups, improves considerably the coating's resistance, as demonstrated by EIS, due to the formation of multiple covalent interactions with silica network and the passivated metal substrate.

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