Advances in coatings on biodegradable magnesium alloys

The clinic applications of bioabsorbable magnesium(Mg)and its alloys have been significantly restricted owing to their poor corrosion resistance.Besides elemental alloying,surface modification and functionality is a major approach to increasing corrosion resistance for magnesium alloys.This article reviews the cutting-edge advances and progress of biodegradable surface coatings upon Mg alloys over the last decades,aims to build up a knowledge framework of surface modification on biodegradable Mg alloys.A considerable number of conversion,deposition,mechanical and functional coatings and their preparation methods are discussed.The emphasis has been placed on the composition of chemical conversion and deposited coatings to overcome the disadvantages of adhesion,corrosion resistance and biocompatibility of a single coating for biomedical materials.The issues have been addressed on the integration of the structural and functional factors of the composite coatings.

Corrosion Resistance and Durability of Superhydrophobic Coating on AZ31 Mg Alloy via One-Step Electrodeposition

To enhance durability and adhesion of superhydrophobic surface,an integrated superhydrophobic calcium myristate(Ca[CH3(CH2)12COO]2)coating with excellent corrosion resistance was fabricated on AZ31 magnesium(Mg)alloy via one-step electrodeposition process.Field-emission scanning electron microscopy,Fourier transform infrared spectrometry and X-ray photoelectron spectroscopy as well as X-ray diff raction were employed to investigate the surface characteristics(morphology,composition and structure)of the coatings.Hydrophobicity of the coating was evaluated by means of contact and sliding angles.Additionally,potentiodynamic polarization,electrochemical impedance spectroscopy and hydrogen evolution tests were conducted to characterize the corrosion resistance.Results indicated that the coating exhibited super-hydrophobicity with large static water contact angle(CA)and small sliding angle of 155.2°±1.5°and 6.0°±0.5°,respectively,owing to spherical rough structure and low surface energy(7.01 mJ m^(-2)).The average hydrogen evolution rate(HERa)and corrosion current density(icorr)of the coated sample were 5.3μL cm^(-2)h^(-1) and 5.60×10^(-9)A cm^(-2),about one and four orders of magnitude lower than that of AZ31 substrate,respectively,implying the excellent corrosion resistance.The CA of the coating remained 155.6°±0.9°after soaking for 13 days,showing the super-hydrophobicity and stability of the coating.Simultaneously,the large critical load(5004 mN)for the coating designated the outstanding adhesion to the substrate by nano-scratch test.

In vitro degradation of pure magnesium-the synergetic influences of glucose and albumin

The biocorrosion of magnesium in the external physiological environment is still difficult to accurately evaluate the degradation behavior in vivo,particularly,in the microenvironment of the patients with hyperglycemia or diabetes.Thus,we explored the synergistic effects of glucose and protein on the biodegradation of pure magnesium,so as to have a deeper understanding the mechanism of the degradation in vivo.The surface morphology and corrosion product composition of pure magnesium were investigated using SEM,EDS,FTIR,XRD and XPS.The effect of glucose and albumin on the degradation rate of pure magnesium was investigated via electrochemical and immersion tests.The adsorption of glucose and albumin on the sample surface was observed using fluorescence microscopy.The results showed that the presence of 2 g/L glucose changed the micromorphology of corrosion products on the magnesium surface by reacting with metal cations,thus inhibiting the corrosion of pure magnesium.Protein formed a barrier layer to protect the magnesium at early stage of immersion.The chelation reaction between protein and magnesium surface might accelerate the degradation at later stage.There may be a critical glucose(albumin)content.Biodegradation of pure magnesium was inhibited at low concentrations and promoted at high concentrations.The synergistic effect of glucose and protein restrained the adsorption of aggressive chloride ions to a certain extent,and thus inhibited the degradation of pure magnesium considerably.Moreover,XPS results indicated that glucose promoted the adsorption of protein on the sample surface.

Corrosion Resistance of Superhydrophobic Mg(OH)_(2)/Calcium Myristate Composite Coating on Magnesium Alloy AZ31

Hydrophobic Mg(OH)_(2)/calcium myristate(Ca[CH3(CH2)12COO]_(2),CaMS)and Mg(OH)_(2)/magnesium myristate(Mg[CH3(CH2)12COO]2,MgMS)composite coatings were prepared on the alkali-treated AZ31 substrates via both electrodeposition and dipping methods.The morphologies,compositions,and constitutes of the coatings were investigated by using field-emission scanning electron microscopy(SEM),X-ray photoelectron spectroscopy(XPS),and Fourier transform infrared spectrometry(FTIR)together with X-ray diffractometer(XRD).Furthermore,electrochemical(polarization and impedance spectroscopy)and hydrogen evolution tests were applied to evaluate the corrosion resistance.The wettability and adhesive strength of the composite coatings were characterized through water static contact angle(CA),sliding angle(SA),and nano-scratch tests.Results indicated that the better super-hydrophobicity,corrosion resistance,and adhesion were achieved via an electrodeposited process.The corrosion current density(icorr)and hydrogen evolution rate(HER)of the electrodeposited coating were three and one orders of magnitude smaller than the substrate,implying a significantly improved corrosion resistance.This scenario was ascribed to the super-hydrophobicity of electrodeposited composite coating with a contact angle(CA)and slide angle(SA)of 159.2°±0.8°and 5.2°±0.8°,respectively.However,the dipped composite coating was adverse to the improvement of corrosion resistance and adhesion due to the dissolution of the underlying Mg(OH)_(2)layer and smooth surface with less organic fatty acid salt(MgMS)

Self-catalytic degradation of iron-bearing chemical conversion coating on magnesium alloys — Influence of Fe content

A number of industrial and biomedical fields,such as hydraulic fracturing balls for gas and petroleum exploitation and implant materials,require Mg alloys with rapid dissolution.An iron-bearing phosphate chemical conversion(PCC)coating with self-catalytic degradation function was fabricated on the Mg alloy AZ31.Surface morphologies,chemical compositions and degradation behaviors of the PCC coating were investigated through FE-SEM,XPS,XRD,FTIR,electrochemical and hydrogen evolution tests.Results indicated that the PCC coating was characterized by iron,its phosphates and hydroxides,amorphous Mg(OH)2 and Mg3-n(HnPO4)2.The self-catalytic degradation effects were predominately concerned with the Fe concentration,chemical composition and microstructure of the PCC coating,which were ascribed to the galvanic corrosion between Fe in the PCC coating and the Mg substrate.The coating with higher Fe content and porous microstructure exhibited a higher degradation rate than that of the AZ31 substrate,while the coating with a trace of Fe and compact surface disclosed a slightly enhanced corrosion resistance for the AZ31 substrate.