| Magnesium alloys have been used in the fields of aerospace,electronics and biomedical materials due to their properties such as low density and high specific strength.However,magnesium alloys are highly electrochemically active and prone to corrosion failure under practical working conditions.Therefore,one of the main ways to expand the wide application of magnesium alloys is through the reasonable construction of efficient surface film layers.This paper significantly improves the corrosion resistance of magnesium alloy substrate by growing Li-Al LDH/Mg(OH)2 composite film layer in situ on the surface,and introducing Ca2+ to further modify the LDH composite film layer according to the characteristic of LDH having biocompatibility.The main studies are as follows:(1)Formation and corrosion resistance mechanism of in situ Li-Al LDH/Mg(OH)2 composite film on AZ91D surface.A Li-Al LDH and Mg(OH)2 composite film intercalated with F-and OH-anions was constructed by hydrothermal method.The film structure shows that Li-Al LDH nanosheets and regular hexagonal Mg(OH)2 sheets are vertically interlaced and covered on the surface of the substrate.In an alkaline environment,α-Mg is preferentially dissolved to form Mg(OH)2,and a small amount of Mg17Al12 is dissolved and combined with Li+ and F-ions in the solvent to grow Li-Al LDH.Among them,the impedance value of the Li-Al LDH/Mg(OH)2 composite film layer increased by 5 orders of magnitude compared with the substrate,from 103 Ω/cm2 to 108 Ω/cm2.The corrosion current density decreased from 1.163 × 10-6 A/cm2 to 2.036 × 10-10 A/cm2,which was reduced by 4 orders of magnitude.After long-term immersion for 20 days,the impedance value of Li-Al LDH/Mg(OH)2 sample at low frequency can still reach 107 Ω/cm2.Li-Al LDH/Mg(OH)2 film can significantly reduce the corrosion rate.The fitting calculation shows that the average interplanar spacing of Li-Al LDH is 0.401 nm,which is higher than the interlayer distance of traditional Mg-Al LDH.The expansion of Li-Al LDH interlayer spacing improves the ion exchange capacity and delays the erosion rate of corrosive ions on the substrate.After the ion exchange capacity reaches the maximum,the corrosion products of the failed LDH structure continue to protect the magnesium alloy substrate as the second corrosion physical barrier.By introducing Li+ions and F-ions,a high-impedance surface LDH film is prepared,which extends the application of the LDH film in the field of corrosion protection.(2)The structure and corrosion resistance mechanism of Ca2+ modified Mg-Al LDH/Mg(OH)2 film.Ca2+ was introduced into the LDH film layer prepared by hydrothermal method to realize the Ca2+modification treatment of LDH nanosheets on the surface of magnesium alloy.Under the high temperature and high pressure conditions of the hydrothermal process,the α-Mg phase is preferentially dissolved to form Mg(OH)2,and LDH nucleates and grows around Mg17Al12.The film layer presents the characteristics of the alternating distribution of LDH and Mg(OH)2,and Ca(OH)2 is physically adsorbed at the pits formed by LDH nanosheets and near some Mg(OH)2.The interlayer spacing of modified LDH increased from 0.359 nm to 0.430 nm,which was attributed to the chemical potential driving force caused by the anion concentration difference during the modification process,and more OH-entered the LDH interlayer structure.The Ca2+modified film in 3.5 wt%NaCl solution increased the breakdown potential of the film,and the re-passivation effect of the film appeared.Ca(OH)2 covers the surface of LDH.On the one hand,it prolongs the start time of Cl-ion exchange;on the other hand,it acts as a physical barrier for corrosion products together with LDH,reducing the corrosion rate of magnesium alloys.Human simulated body fluid tests showed that the corrosion current density of the Ca2+ modified film and that of the unmodified film were 8.436×10-9 A/cm2 and 5.277×10-6 A/cm2,respectively.Ca2+ ion modification also improves the application possibility of LDH in biomedical implant materials. |
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