Additionally, heteroatoms in amino acids could share their lone pair electrons with the vacant molecular orbitals of the magnesium alloy. The adsorption of the amino group was mainly achieved through the coupling of the lone pair electrons of nitrogen atoms with the surface, whereas the carboxyl group combined with Mg 2+ via their oxygen atoms.
The results of electrochemical polarization curves, EIS, and hydrogen evolution tests demonstrated that the addition of amino acids enhanced the corrosion resistance of the Ca-P coatings, which was ascribed to the inhibition and adsorption of amino acid molecules on AZ31 magnesium alloy. The main constituents of the amino acid-induced Ca-P coatings were CaHPO 4 and Ca 10(PO 4) 6(OH) 2 (HA). The formation mechanisms of amino acid additive-induced Ca-P (Ca-P Phe, Ca-P Met, and Ca-P Asn) coatings on AZ31 magnesium alloy were probed. The corrosion resistance of the coating in simulated body fluid (Hank's solution) was investigated through electrochemical polarization, AC impedance, and hydrogen evolution tests. Additionally, the morphology of the coatings, composition distributions, and phase structures were observed and analyzed via SEM, EDS, XRD, FTIR, and XPS. Three amino acid-induced Ca-P (Ca-P Phe, Ca-P Met, and Ca-P Asn) coatings were prepared on AZ31 magnesium alloy via a constant temperature water bath method at a temperature of 60 oC. To clarify the influence of amino acid groups on film formation, Phenylalanine (Phe), Methionine (Met), and Asparagine (Asn) were used to regulate the degradation rate of magnesium alloy. Amino acids can induce the formation of calcium-bearing phosphates on biodegradable magnesium alloy. However, the Ca-P coatings may have defects like porosity, low adhesion, and coarse grains, which lead to prefailure in the early stage and then the coatings cannot meet the requirements of long-term clinical service. Ca-P coating not only enhances the corrosion resistance of biodegradable magnesium alloys but also contributes to the formation of new bones and promotes bone integration around implants.
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