Study On Corrosion Of Mg Alloys And Their Ti/Al Coating In Hank's Solutions

Magnesium alloys are potential implant materials due to their excellent biocompatibility and good mechanical property. Clinical applications of them, however, are restricted oweing to their poor corrosion resistance. The study on corrosion and protection of magnesium alloys as biomaterials is focoused.Ti/Al coatings were prepared by magnetron sputtering. And diffusion treatment of them was conducted. The microstructure and coating morphologies of these alloys were observed by means of an optical microscope and a scanning electron microscope (SEM). Immersion tests and potentiodynamic electrochemical tests were applied to investigate the corrosion behavior of magnesium alloys AZ31, Mg-3.3Li, Mg-8Li-1Al, Mg-14Li-1Al and the Ti/Al coatings on AZ31 in simulated body fluids (SBFs). The influence of anions: HCO₃^-, SO₄^2-, HPO₄^2- and H₂PO₄^-, as well as glucose (C₆H₁₂O₆), and their content on the corrosion behavior of magnesium alloy AZ31 were studied. The influence of alloying element lithium content on the surface films and corrosion behavior of Mg-Li alloys, as well as the Ti/Al coatings on the corrosion resistence of AZ31 alloy were also disscused. Energy dispersive spectrum (EDS), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction analysis (XRD) were employed to characterize the microstructure and composition of the alloys, coatings and corrosion products.The results demonstrated that HCO₃^-, HPO₄^2-, and H₂PO₄^- etc anions decreased the the free corrosion potential (Ecorr) of AZ31 significantly, while C₆H₁₂O₆ and SO₄^2- ions slightly decreased Ecorr. Nevertheless, Ecorr of AZ31 increased surprisingly with the increasing immersion time in 0.9% NaCl+2.0 g/L C₆H₁₂O₆ solution. HCO₃^-, SO₄^2- ions and C₆H₁₂O₆ changed the anodic polarization behavior of AZ31, there was an obvious passivation process of AZ31 in these solutions. C₆H₁₂O₆ could inhibit the corrosion of magnesium alloy drastically, and HPO₄^2-, H₂PO₄^- could slightely inhibit the corrosion of magnesium alloy, while there was no obvious influence of SO₄^2- ions on corrosion rate.The oxide film of Mg-3.3Li alloy was composed ofαphase, Li₃Mg₁₇ and Li₂CO₃; and the film of Mg-8Li-1Al was consisted ofα,β, Li₃Mg₁₇, Li₃Mg₇, Li₂O₂, Li₂CO₃ and MgCO₃; while the film on the Mg-14Li-1Al includedβ, Li₃Mg₇, Li₂CO₃, Li₂O₂ and Mg(OH)₂ as well as MgCO₃·2H₂O. The corrosion rates of the alloys increased with an increas in Li content in Hank's solutions. The film thickness increased with increasing lithium content, which leads to an improvement in the corrosion resistance of Mg-Li alloys. The Ti/Al composite coatings significantly increased the corrosion potential and corrosion resistance of the substrates. Ti film improved the anti-corrosion property of the AZ31B alloy more effectively than Al film. Diffusion treatment improved the bonding between the coatings and substrate by increasing the inter-diffusion between Mg/Al and Al/Ti, but decreased the corrosion resistance of the coatings. This probably resulted from more secondary phases or intermetallic compounds formed in the coatings after diffusion treatment, and they leaded to micro-galvanic corrosion of the coatings.