Surface modification of biomedical magnesium alloy wires by micro-arc oxidation

Magnesium alloy wires were processed by micro-arc oxidation （MAO） in a modified silicate-phosphate composite electrolyte containing hydroxyapatite （HA） nanopowders and NaOH. Effects of NaOH content in the composite electrolyte on the microstructure and properties of the MAO ceramic coatings on magnesium alloy wires were studied. It is found that the arc voltage of magnesium alloy wires in the micro-arc oxidation process is significantly reduced while the oxidation rate is accelerated. Addition of 2 g/L NaOH in the composite electrolyte is a better choice for improving corrosion resistance of magnesium alloy wires. During early simulated body fluids （SBF） immersion, the micro-arc oxidized magnesium alloy wires undergo a slow and stable degradation. After soaking for 28 d, the protective ceramic coating still shows no damage but significant degradation is observed for magnesium alloy wires after immersion for more than 60 d.

Corrosion behavior of micro-arc oxidation coating on AZ91D magnesium alloy in NaCl solutions with different concentrations

Ceramic oxide coatings were prepared on AZ91D magnesium alloys in alkaline silicate solution using micro-arc oxidation（MAO） technique.The corrosion behavior of MAO coating on AZ91D magnesium alloys in NaCl solutions with different concentrations（0.1%,0.5%,1.0%,3.5% and 5.0% in mass fraction） was evaluated by electrochemical measurements and immersion tests.The results showed that the corrosion rate of the MAO coated AZ91D increased with increasing chloride ion concentration.The main form of corrosion failure was localized corrosion for the MAO coated AZ91D immersed in higher concentration NaCl solutions（1.0%,3.5% and 5.0%）,while it was general corrosion in dilute NaCl solutions（0.1% and 0.5%）.Two different stages of the failure process of the MAO coated AZ91D could be identified：1） occurrence of the metastable pits and 2） growth of the pits.Different equivalent circuits were also proposed based on the results of electrochemical impedance spectroscopy（EIS） for the MAO coated AZ91D immersed in different concentrations of NaCl solutions for 120 h.

Preparation and performance of coating on rare-earth compounds-immersed magnesium alloy by micro-arc oxidation

A composite ceramic coating containing Y2O3-ZrO2-MgO（YSZ-MgO） was prepared on AZ91D magnesium alloy,which was immersed in Y（NO3）3 aqueous solution as pretreatment,by micro-arc oxidation（MAO） process.The morphology,elemental and phase compositions,corrosion behavior and thermal stability of the coatings were studied by SEM,EDX,XRD,electrochemical corrosion test,high temperature oxidation and thermal shock test.The results show that the coating mainly consists of ZrO2,Y2O3,MgO,Mg2SiO4,and MgF2.Among these compounds,Y2O3 accounts for 26.7% of（Y2O3 ＋ ZrO2）.The thickness of YSZ-MgO coating is smaller than that of ZrO2-MgO coating,but its compactness and surface roughness are better than those of ZrO2-MgO coating.YSZ-MgO coating has a good corrosion resistance,and its corrosion rate in 5% NaCl aqueous solution is lower than that of ZrO2-MgO and only about 8.5% of that of AZ91D magnesium alloy.After oxidation at 410 °C,the mass gain of AZ91D magnesium alloy presents a linear increase with the oxidation time.The YSZ-MgO coating and ZrO2-MgO coating can remarkably decrease the oxidation mass gain.The oxidation mass gain of YSZ-MgO coating is lower than that of ZrO2-MgO coating,especially during a long oxidation period.The thermal shock resistance of YSZ-MgO coating is superior to ZrO2-MgO coating.

Structure and corrosion resistance of modified micro-arc oxidation coating on AZ31B magnesium alloy

A hydrophobic surface was fabricated on a micro-arc oxidation (MAO) treated AZ31 Mg alloys via surface modification with myristic acid. The effects of modification time on the wettability of the coatings were investigated using the contact angle measuring device. The surface morphologies and structure of the coatings were evaluated using SEM, XRD and FT-IR. The corrosion resistance was investigated by potentiodynamic polarization curves and long-term immersion test. The results showed that the water contact angle (CA) increases gradually with modification time from 0 to 5 h, the highest CA reaches 138° after being modified for 5 h, and the number and size of the micro pores are decreased. The modification method hardly alters crystalline structure of the MAO coating, but improves the corrosion resistance based on the much positive potential and low current density. Moreover, the corrosion resistance and hydrophobicity can be enhanced with increasing the alkyl chain. The wetting and spreading for the alkylcarboxylate with low surface energy become easier on the micro-porous surface, and alkylcarboxylate monolayer will be formed through bidentate bonding, which changes the surface micropores to a sealing or semi-sealing structure and makes the MAO coating dense and hydrophobic. All the results demonstrate that the modification process improves the corrosion protection ability of the MAO coating on AZ31B Mg alloy.

Corrosion resistance and mechanisms of smart micro-arc oxidation/epoxy resin coatings on AZ31 Mg alloy: Strategic positioning of nanocontainers

Smart micro-arc oxidation(MAO)/epoxy resin(EP) composite coatings were formed on AZ31 magnesium(Mg) alloy. Mesoporous silica nanocontainers(MSN) encapsulated with sodium benzoate(SB) corrosion inhibitors were strategically incorporated in the MAO micropores and in the top EP layer. The influence of the strategic positioning of the nanocontainers on the corrosion protective performance of coating was investigated. The experimental results and analysis indicated that the superior corrosion resistance of the hybrid coating is ascribed to the protection mechanisms of the nanocontainers. This involves two phenomena:(1) the presence of the nanocontainers in the MAO micropores decreased the distance between MSN@SB and the substrate, demonstrating a low admittance value(^5.18 × 10^(-8)Ω^(-1)), and thus exhibiting significant corrosion inhibition and self-healing function;and(2) the addition of nanocontainers in the top EP layer densified the coating via sealing of the inherent defects, and hence the coating maintained higher resistance even after 90 days of immersion(1.13 × 10^(10)Ω cm^(2)).However, the possibility of corrosion inhibitors located away from the substrate transport to the substrate is reduced, reducing its effective utilization rate. This work demonstrates the importance of the positioning of nanocontainers in the coating for enhanced corrosion resistance,and thereby providing a novel perspective for the design of smart protective coatings through regulating the distribution of nanocontainers in the coatings.

Enhanced corrosion resistance of micro-arc oxidation coated magnesium alloy by superhydrophobic Mg-Al layered double hydroxide coating

To further enhance the corrosion resistance of the porous micro-arc oxidation(MAO) ceramic layers on AZ31 magnesium alloy, superhydrophobic Mg-Al layered double hydroxide(LDH) coating was fabricated on MAO-coated AZ31 alloy by using in-situ growth method followed by surface modification with stearic acid. The characteristics of different coatings were investigated by XRD, SEM and EDS. The effect of the hydrothermal treatment time on the formation of the LDH coatings was studied. The results demonstrated that the micro-pores and cracks of MAO coating were gradually sealed via in-situ growing LDH with prolonging hydrothermal treating time. Electrochemical measurement displayed that the lowest corrosion current density, the most positive corrosion potential and the highest impedance modulus were observed for superhydrophobic LDH/MAO coating compared with those of MAO coating and LDH/MAO coating. Immersion experiment proved that the superhydrophobic LDH/MAO coating with the active anti-corrosion capability significantly enhanced the long-term corrosion protection for MAO coated alloy.

Investigation of a novel self-sealing pore micro-arc oxidation film on AM60 magnesium alloy

Micro-arc oxidation(MAO)is one of the promising methods to improve the corrosion resistance of magnesium alloys.However,there are plenty of micro-pores in the traditional MAO films,deteriorating their protection property.A novel self-sealing pore MAO film was developed in this paper.The morphologies and chemical composition of the film were detected by scanning electron microscopy(SEM)and energy dispersive X-ray spectroscopy(EDX).The corrosion behavior was investigated by electrochemical and salt spray tests.The possible film formation and corrosion mechanisms were proposed.The self-sealing pore film presents a blue appearance.Most of the micro-pores in the surface of the film are sealed during the film formation process.The chemical composition of the film mainly contains Mg,O,Ti,F and P.The self-sealing pore film exhibits better corrosion resistance compared with the traditional silicate film.Especially,the self-sealing pore film keeps intact after salt spray test for 2000 h,which can be attributed to its high compactness.

Research of growth mechanism of ceramic coatings fabricated by micro-arc oxidation on magnesium alloys at high current mode

Micro-arc oxidation(MAO)coatings of ZK60 magnesium alloys were formed in a self-developed dual electrolyte composed of sodium silicate and phosphate at the high constant current of 1.8 A(15 A/dm^(2)).The MAO process and growth mechanism were investigated by scanning electron microscopy(SEM)coupled with an energy dispersive spectrometer(EDS),confocal laser scanning microscopy and X-ray diffraction(XRD).The results indicate that the growth process of MAO coating mainly goes through“forming→puncturing→rapid growth of micro-arc oxidation→large arc discharge→self-repairing”.The coating grows inward and outward at the same time in the initial stage,but outward growth of the coating is dominant later.Mg,Mg_(2)SiO_(4) and MgO are the main phases of ceramic coating.

Preparation of micro-arc oxidation coatings on magnesium alloy and its thermal shock resistance property

In the NaAlO2-Na2SiO3 compound system, the ceramic coatings were prepared on magnesium alloy by micro-arc oxidation. The morphology, phase composition, and thermal shock resistance of the ceramic coatings were studied by scanning electron microscope, X-ray diffraction and thermal shock tests, respectively. The results showed that the ceramic coating contains MgO, MgAl2O4, as well as a little amount of MgESiO4. The thickness of the ceramic coatings increases with the current density increasing, when the current density is 12 A·dm^-2, the thermal shock resistance of the produced ceramic coating is the best. The hardness of the ceramic coating is up to 10 GPa or so.

Characterization of ceramic coating on ZK60 magnesium alloy prepared in a dual electrolyte system by micro-arc oxidation

Micro-arc oxidation （MAO） process was cartied out in an optimized dual electrolyte system to fabricate a compact, smooth, and corrosion resistant coating on ZK60 Mg alloy. The microstructural characteristics of coating were investigated by scanning electron microscopy （SEM） coupled with an energy dispersive spectrometer （EDS） and X-ray diffraction （XRD）. Test of mass loss was conducted at a 3.5 % NaCl solution to assess the resistance to corrosion. The bonding strength between the coating and ZK60 substrate was evaluated using scratch experiment. The results reveal that MgA1204 and MgO are the main phases of ceramic coating obtained in the dual electrolyte system. The corrosion rate of coating prepared in the optimized dual electrolyte is only 0.0061 g.m-2.h-1, which demonstrates excellent corrosion resistance. This is mainly due to the compact, uniform coating with high bonding strength.

Growth process and corrosion resistance of micro-arc oxidation coating on Mg-Zn-Gd magnesium alloys

A Mg-6Zn-3Gd(mass fraction,%) alloy,noted as ZG63,was coated by different micro-arc oxidation(MAO) processes,and the coating structure and corrosion resistance of the alloy were studied using scanning electron microscopy(SEM),glancing angle X-ray diffractometry(GAXRD) and various electrochemical methods.The micro-arc oxidation process consists of three stages and corresponds with different coating structures.In the initial stage,the coating thickness is linearly increased and is controlled by electrochemical polarization.In the second stage,the coating grows mainly inward and accords with parabolic regularity.In the third stage,the loose coating forms and is controlled by local arc light.The looser coating is mainly composed of MgSiO3 and the compact coating is mainly composed of MgO.From micro-arc oxidation stage to local arc light stage,the corrosion resistance of the coated alloy firstly increases and then decreases.The satisfied corrosion resistance corresponds to the coating time ranging from 6 to10 min.

Discharge mechanism of ultrasound micro-arc oxidation on magnesium alloy

Magnesium alloy was treated by micro-arc oxidation with ultrasound and the discharge phenomenon was studied. The effect of ultrasound on the discharge life and the number of discharge charges was studied asing high speed photography technique. The results indicate that 60 W power ultrasound increases discharge life of single charge and the total number of discharge charges in one discharge period. The size of micro-pore increases and the number of cracks in coating significantly reduce. The dendritic area with a width of 15 μm in magnesium alloy forms during micro-arc oxidation process. Ultrasound eliminates the dendritic structures of micro-arc oxidation magnesium alloy.

Optimization of dual electrolyte and characteristic of micro-arc oxidation coating fabricated on ZK60 Mg alloy

Micro-arc oxidation （MAO） process was carried out in a dual electrolyte system of NaAlO 2 and Na 3 PO 4 to develop compact, smooth and corrosion-resistant coatings on ZK60 Mg alloy by single factor experiments. The microstructural characteristics of coatings were investigated by X-ray diffractometry （XRD） and scanning electron microscopy （SEM） coupled with energy dispersive X-ray spectroscopy （EDS）. Test of mass loss was conducted at a 3.5% NaCl solution to assess the resistance to corrosion. The effect of every element in the dual electrolyte system on voltage—time responses during MAO process and the coating characteristic were also analyzed and discussed systematically via single factor experiments. The results reveal that the main components of NaAlO 2 and Na 3 PO 4 as well as additives of NaOH, NaB4O7 and C6H5Na3O7 demonstrate different effects on MAO process and coating characteristics. By means of single factor experiments, an optimized dual electrolyte system was developed, containing 17.5 g/L NaAlO 2, 5.0 g/LNa3 PO4, 5.0 g/L NaOH, 3.0 g/L NaB4O7 and 4.2g/LC6H5Na3O7 .

Effect of current density on corrosion resistance of micro-arc oxide coatings on magnesium alloy

Oxide coatings were prepared on magnesium alloys in electrolyte solution of Na2SiO3 at different current densities(3,4 and 5 A/cm 2 )with micro-arc oxidation process.X-ray diffractometry(XRD)results show that the oxide coatings formed on magnesium alloys are mainly composed of MgO and MgAl2O4 phases;in addition,the content of MgO increases with increasing the current density.The morphology and surface roughness of the coatings were characterized by confocal laser scanning microscopy (CLSM).The results show that the surface roughness(Ra)decreases with increasing the current density.Moreover,the electrochemical corrosion results prove that the MgO coating produced in the electrolyte Na2SiO3 at current density of 5 A/cm 2 shows the best corrosion resistance.

Structure and Corrosion Resistance of Microarc Oxidation Coatings on AZ91D Magnesium Alloy

Magnesium alloys are widely used as shells of 3C (computer, mobile phone and consumer electronics) equipments for its impressive mechanical and physical properties, such as low density, good resistance to electromagnetic radiation, suitable for high pressure diecasting and easily recycling, etc. But poor corrosion resistance confines its extensively application. In this paper, protective coatings was successfully prepared on AZ91D magnesium alloys by micro-arc oxidation (MAO) and painting process. Microstructures and phases of MAO coatings were invesgated with scanning electron microscope (SEM) and X-Ray diffractometer. Mechanical properties of MAO coating, such as adhesive force and corrosion resistance, were also tested. Results showed that MAO coatings were a good base for painting process. MAO coatings with paint have good adhesive properties to base metal and excellent corrosion resistance. Micro-arc oxidation with painting process is a good kind of surface treatment to improve the corrosion resistance of mobile phone shell made of AZ91D magnesium alloys.

Quick Surface Treatment of AZ31B by AC Micro-arc Oxidation

In order to explore an effective way to shorten treatment time and enhance the quality of treatment coating, AC micro-arc oxidation was conducted to treat the surface of AZ31 B deformation magnesium alloy in KF＋KOH treatment solution. The infl uences of micro-arc oxidation parameters such as concentration of KF, concentration of KOH, output voltage of booster, temperature of treatment solution, and treatment time on treatment coating thickness were raveled out under different conditions. The structure and composition of treatment coating were determined, the growth mechanism of treatment coating was discussed, and the quick surface treatment technology for compact treatment coating with maximum thickness was developed. The experimental results show that： A maximum 33 μm-thick compact treatment coating, consisting of MgF2 and MgO mainly, can be formed on AZ31 B in 112 s under the conditions of 1 132 g/L KF, 382 g/L KOH, 66 V for output voltage of booster and 34 ℃ of treatment solution which were optimized by a genetic algorithm from the model established by artifi cial neural networks. There are no ＂crater-shaped＂ pores in this treatment coating as the heat shock resulting from the smooth variation of AC sinusoidal voltage is far smaller than that of the rigidly varied DC or pulse current. The treatment time is only one sixth of that adopted in the other surface treatment technology at best, principally for the reason that the coating can always grow irrespective of the electric potential of AZ31 B. This investigation lays a fi rm foundation for the extensive application of magnesium alloy.

Effects of sodium tungstate on properties of micro-arc coatings on magnesium alloys

Anodic coatings were prepared by micro-arc oxidation on AZ91HP magnesium alloys in a base solution containing 10 g/L NaOH and 12 g/L phytic acid with addition of 0-8 g/L sodium tungstate.The effects of sodium tungstate on the coating thickness, mass gain,surface morphology and corrosion resistance were studied by eddy current instrument,electronic scales,scanning electron microscope and immersion tester.With the addition of sodium tungstate,the electrolytic conductivity increases and the final voltage decreases.The sodium tungstate has a minor effect on the coating thickness,but lightens the coating color.With increasing sodium tungstate concentration,the size of micropores on the coatings is enlarged and the corrosion resistance of the anodized samples decreases.

Influence of nano-SiC on microstructure and property of MAO coating formed on AZ91D magnesium alloy

Ceramic coating incorporated with nano-SiC was obtained on AZ91D magnesium alloy during MAO by adding nano-SiC into the silicate-aluminate-based composite electrolyte. The microstructure, thickness, phase analysis, element composition and hardness of the coatings were respectively investigated by scanning electron microscopy（SEM）, film thickness meter, X-ray diffraction （XRD）, energy disperse spectroscopy（EDS） and Vickers hardness tester. The wear resistance of Mg alloy and coatings were evaluated by friction and wear apparatus, while the corrosion resistance of Mg alloy and coatings were evaluated by potentiodynamic polarization test and electrochemical impedance spectroscopy （EIS）. The results show that after adding nano-SiC into the electrolyte, both the striking voltage and final voltage decrease, the size and number of the micropore on the surface of the coating decrease, the thickness and hardness of the coating increase, both the wear resistance and corrosion resistance of the coating raise.

Influence of electric parameters on MAO of AZ91D magnesium alloy using alternative square-wave power source

Micro-arc oxidation （MAO） process was carried out on AZ91D alloy in alkaline borate solution using an alternative square-wave power source with different parameters. The effects of voltage, frequency and duty cycle on the coatings were investigated by orthogonal experiment. It is found that the thickness of coatings increases with the increase of voltage and duty cycle, but decreases with the increase of frequency. The structure and morphology of the coatings also depend on voltage, frequency and duty cycle. The coatings become more porous and crack with increasing voltage and duty cycle. The coating is thin and transparent when the voltage is lower than 120 V. The corrosion resistances of different coatings were evaluated by polarization curves and electrochemical impedance spectroscopy （EIS） in 3.5% NaCl （mass fraction） solution. When the optimized values of voltage, frequency and duty cycle are 140 V, 2 000 Hz and 0.4, respectively, the anodic coating shows the best corrosion resistance.

Investigation of rare earth sealing of porous micro-arc oxidation coating formed on AZ91D magnesium alloy

Magnesium and its alloys have been used in many industries, but they are reactive and require protection against aggressive environments. In this study, oxide coatings were applied on AZ91D magnesium alloy using micro-arc oxidation （MAO） process. Then, in order to seal the pores of the MAO coatings, the samples were immersed in cerium bath for different times. The surface morphologies and compositions of the coatings were analyzed by scanning electron microscopy （SEM） and X-ray energy dispersive spectroscopy （EDS）, respectively. The corrosion behavior of the coatings was investigated with electrochemical impedance spectroscopy （EIS） and potentiodynamic polarization tests in 3.5 wt.% NaCl solution. The amount of the porosity of the coating was measured by electrochemical method. It was found that the sealing treatments by immersion in cerium bath successfully sealed the pores of the MAO coatings. The results of the corrosion tests showed that the MAO coating which was sealed in Ce bath for 10 min enhanced the corrosion resistance of the substrate significantly. Furthermore, this coating had the lowest amount of the porosity among the coatings.