Effect of Na2SiO3 solution concentration of micro-arc oxidation process on lap-shear strength of adhesive-bonded magnesium alloys

•We report effect of MAO electrolyte concentration on lap shear strength of Mg alloy.•MgO in film composition decreases as electrolyte concentration increases.•Adhesive bonded strength of Mg alloy decreases as electrolyte concentration increases.•MgO in film composition was related with bonding strength of MAO films.•Hydroxyl groups initiated by MgO afford hydrogen bonding and better wettability. Micro-arc oxidation films are fabricated on the surface of AZ31B magnesium alloy in the Na2SiO3 electrolyte. The mechanical performance of magnesium alloy bonding with the adhesive is examined by single lap-shear test, and the morphology and composition of the oxidation film are characterized by scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). The data indicate that the increase of Na2SiO3 solution concentration results in the decrease of bonding strength of magnesium alloy joints, while the porosity has no directly relation with the variation of the lap-shear strength. The results also show that Na2SiO3 participates into the film formation and the fabricated film is mainly composed of MgO and Mg2SiO4. In particular, with the increase of Na2SiO3 solution concentration, the content of MgO decreased while that of Mg2SiO4 increased. Moreover, MgO could initiate the formation of the hydroxyl group, which potentially enhances surface wettability and hydrogen bonding with the adhesive. This rationale is strongly supported by the results of Fourier Transform infrared spectra (FTIR), X-ray photoelectron spectroscopy (XPS) and contact angle measuring.