Corrosion Resistance Of Micro-arc Oxidation/Layered Double Hydroxides Composite Coating On Biomedical Magnesium Alloys

Micro-arc oxidation(MAO),with a high bonding strength to the substrate,is popularly used to modify the surface of Mg alloys to regulate its degradation rate.Unfortunately,a porous structure of the MAO coating provides a convenient channel for aggressive ions to attack the substrate,which may lead to a galvanic corrosion between the oxide film and the substrate,thereby accelerating the late corrosion.Layered double hydroxide(LDH)is a nanostructured material with an ion-exchange capability that can act as a physical barrier to inhibit corrosion.In this paper,the top layers of Mg(OH)2,Mg-Al-LDH,and Mg-Al-Ga-LDH,respectively,are constructed on the bottom layer of the MAO coating in the strong alkaline treatment solution containing EDTA-2Na with a low-temperature water bath method.Firstly,a Mg(OH)2 top layer was prepared on the surface of the MAO coating to explore the conditions suitable for the growth of the top coating.Results show that low preparation temperature,high pH value and the addition of EDTA are conducive to the growth of Mg(OH)2;the corrosion current density(icorr)of the MAO/Mg(OH)2 coating is reduced by three orders of magnitude compares to the Mg alloy substrate,suggesting the composite coating has an excellent corrosion resistance;after immersion in Hank’s solution for 156 h,more Ca-P product was deposited on the surface of the composite coating,indicating that the nanostructure is favorable for Ca-P product deposition.Secondly,under the above preparation conditions,a certain amount of AI source was added to prepare the top layer of Mg-Al-LDH with nano-sheet composition on the surface of the MAO coating.The results show that the thickness of the LDH coating is about 3.8 μm;EDTA complexes free Al3+ ions throughout the coating growth period to promote LDH growth;MAO/Mg-Al-LDH coating,with superior a self-healing and an ion-exchangcablc ability,shows an excellent corrosion resistance,and its icorr decreases four orders of magnitude than that of its substrate.MC3T3-E1 osteoblasts have a higher cell survival rate on the MAO/Mg-Al-LDH coating,indicating that the nanostructure is beneficial to improve cell compatibility.Finally,the Al source was replaced with Ga3+ ions of the same family,and a Mg-Ga-LDH top layer was prepared on the surface of the MAO coating.The results show that Ga3+is successfully doped into the LDH coating,which is conducive to improving antibacterial properties;the icorr of the MAO/Mg-Al-Ga-LDH coating decreases three orders of magnitude than that of the substrate;during the long-term immersion process,the surface of the MAO/Mg-Al-Ga-LDH coating contains more Cl-ions,confirming the ion-exchange ability of LDH.