Micro-arc Oxidation Film Research Articles

Effects of H<sub>2</sub>O<sub>2</sub> on Characteristics of Films Formed on Surface of Ti6Al4V in Electrolyte by Micro-Arc Oxidation

Using micro-arc oxidation technology in preparation Ti6Al4V alloy surface oxidation film, adding to the electrolyte H2O2, influence of H2O2 addition amount of oxidation film characteristics has been studied. The results show that, H2O2 addition amount of 4.0 g·L-1, positive critical arcs voltage minimum, for 338 V. SEM analysis showed that, electrolyte of H2O2 addition amount by 1.0 g·L-1 increased to 4.0 g·L-1, film hole diameter after decreasing, micro-arc oxidation film layer of density increases. Oxidation film surface roughness reduced to 7.619 when H2O2 addition amount for 4 g·L-1. With H2O2 addition amount from 1.0 g·L-1 increased to 4.0 g·L-1, micro-arc oxidation film in the wear process mass loss is reduced, wear resistance for promotion.

The Development and Mechanism of Micro-Arc Oxidation Process in Magnesium Surface

In this study, a constant current density was used in magnesium microarc oxidation treatment. The microstructure, chemical composition, phase composition and corrosion resistance of the micro-arc oxidation film were characterized by XRD and EDS. The experimental results showed that the test pieces of magnesium would formed an silver-gray uniform densification oxide film on the surface, the film-formed materials was amorphous by XRD analysis, and on this basis we preliminary discused the film-forming mechanism.

Study of Micro-Arc Oxidation Process in Magnesium Surface

This study selected sodium silicate, sodium fluoride, glycerol hydroxyl acid, potassium hydroxide and phosphine oxide (LAP) as the stabilizer electrolyte, and then on micro-arc oxidation of magnesium with constant current density. The microstructure, corrosion resistance, chemical composition, structure and characterization of micro-arc oxidation film were determined by SEM and anodic oxidation methods. Experimental results indicated that the test pieces of magnesium would formed an silver-gray uniform densification oxide film on the surface by micro-arc oxidation treatment, and the corrosion resistance of magnesium film-formed had increased by potentiodynamic polarization curve testing.

Electrochemical corrosion behavior of modified MAO film on magnesium alloy AZ31 irradiated by high-intensity pulsed ion beam

Micro-arc oxidation (MAO) films on AZ31 magnesium alloy were modified by high-intensity pulsed ion beam(HIPIB) irradiation with ion energy of 300keV at 200A/cm2 with 1–10 shots. A compact and solidified layer was prepared on the irradiated MAO films. The electrochemical impedance spectrum (EIS) of the MAO films in 3.5% NaCl solution was measured with varying the immersion time. The Nyquist plots for EIS showed typical capacitive arc and inductive loop. The diameters for capacitive arc and inductive loop are both increased and then decreased with increasing irradiation shot number, the maximal value for diameters obtained at 200A/cm2 with 5 shots at immersion time of 5h achieved to 6×107Ω and 1.5×105Ω, respectively. With immersion time augment, the diameters for capacitive arc and inductive loop of Nyquist plots both decreased. Based on the results of EIS measurements, appropriate equivalent circuits for the modified layers were proposed to fit the EIS for MAO films. It is found that the enhancement in corrosion property of ablated MAO films is mainly caused by improvement of continuity and compaction of films irradiated by HIPIB, which suppressed the corrosion process by holding back the transfer of electrolyte through the modified films to magnesium substrate during immersion.

Preparation and Characterisation of CaP-Containing Coating on Magnesium Alloy by Micro-Arc Oxidation and Hydrothermal Treatment

Micro-arc oxidation (MAO) together with hydrothermal treatment had been performed to improve the corrosion resistance of magnesium alloy. The oxide films were hydrothermally heated at 150°C for 2h. The morphology of samples were characterized by SEM/EDS. The phase compositions were examined by TF-XRD. The corrosion resistance of the coatings was evaluated by electrochemical methods in Hank’s solution. The MAO films displayed porous and rough structure and were mainly composed of MgO and MgAl2O4. After hydrothermal treatment, a dicalcium phosphate dihydrate (DCPD) coating formed on the oxide film. The corrosion resistance of the MAO film and DCPD coating was increased by about 12 and 40 times higher than that of bare magnesium alloy.

Preparation and Characterization of MAO Film with High Dense Layer

The influence of different negative current density on oxidation film thickness and dense layer thickness was studied. According to the results and experimental phenomenon，the negative current was adjusted steply, and the dense layer with high rates for 70% of micro-arc oxidation film was achieved. EDS and X-ray diffraction were used to analyse the composition and phase structure of the surface layer and the dense layer of oxide film .The friction coefficient and wear of the surface layer and the dense layer of oxide film were measured by wear machine measurement ,and were compared with hard anode oxidation film. The results show that the oxidation film thickness and the dense layer thickness increase with the rising of negative current density.The micro sparks can be avoided out, through adjusting negative current and a high dense layer of MAO film is got .The dense layer of MAO film contains only oxygen and aluminum elements, so it can not be effected by the electrolyte composition .The alpha Al2O3 level of the dense layer is higher than that of the surface layer, and the friction coefficient and abrasion loss of the dense layer is the lowest among the surface layer and hard anode oxidation film.

Research on Tribological Property of Micro-Arc Oxidation Film on AZ91D Magnesium Alloy

Magnesium alloys were treated by micro-arc oxidation at a constant applied current density. The friction and wear properties were investigated by optical microscope and universal friction and wear tester in 3.5%NaCl solution. The result reveals that the film formed by micro-arc oxidation enhances the corrosion resistance of the magnesium alloys. The Ecorr of micro-arc oxidation film improved 94mV than the Ecorr of magnesium alloys matrix. Wear resistance of the film was weakened in 3.5%NaCl solution. The wear mechanism of film depends on the load. Abrasive wear was predominant at low load. Peeling wear was predominant at high load, which result from damage of the film adhesion.

The Influence of Micro-Arc Oxidation on Corrosive-Wear Behavior of Magnesium Alloy

Using an alternative pulse current source and in silicate solution electrolyte，a uniform coating on the surface of AZ9lD alloy was fabricated by micro-arc oxidation(MAO) with current density of 40mA/cm2. Friction wear testing machine was used to study the wear behavior under corrosive environment of the film and scanning electron microscopy(SEM)were used to study microstructures of film and worn surface．The result shows that the corrosion resistance of micro-arc oxidation film is large better than that of magnesium alloy. The corrosive solution great weaken the abrasion resistance of the coating. As the influence of the corrosive solution, the wearing mechanism is varying with the loading. Abrasive wear is the main mechanism when the loading is small, if the loading is large enough, the binding force will be destroyed and the coating will peel off from magnesium alloy.

Corrosion and drug release properties of EN-plating/PLGA composite coating on MAO film

The electroless nickel plating/poly( dl-lactide-co-glycolide) composite coating (EN-plating/PLGA composite coating) was fabricated on the surface of the micro-arc oxidation (MAO) film of the magnesium alloy AZ81 to double control the corrosion and drug release in the hanks' solution. The EN-plating was fabricated on the MAO coating to improve the corrosion resistance by overlaying most pores and micro-cracks on the surface of the MAO film. Meanwhile, a double layered organic poly( dl-lactide-co-glycolide)/paclitaxel (PLGA/PTX) drug releasing coating with a top layered PLGA drug controlled releasing coating on EN plating was prepared to control the drug release rate by adjusting the different lactide: glycolide (LA:GA) ratio of PLGA. Scanning electron microscopy (SEM) and the X-ray powder diffraction (XRD) were used to analyze the morphology and the composition of the EN-plating. The corrosion behavior of the magnesium alloy substrate and the status of the drug in the PLGA matrix were respectively evaluated by Potentiodynamic polarization and Differential scanning calorimetry (DSC). The drug release was determined by ultraviolet–visible (UV–visible) spectrophotometer. EN-plating coating which was composed of compact cauliflower nodules was uniform in size and defect free with no pores or cracks. EN-plating could seal the microcracks and microholes on the outer layer of the MAO coating effectively. The corrosion resistance was improved by preventing the corrosive ions from diffusing to the magnesium alloy substrate. The drug release rate of PTX exhibited a nearly linear sustained-release profile with no significant burst releases.

Electrochemical properties of microarc oxidation films on a magnesium alloy modified by high-intensity pulsed ion beam

Microarc oxidation (MAO) films on AZ31 magnesium alloy were treated by high-intensity pulsed ion beam (HIPIB) irradiation with ion energy of 300 keV at ion current density of 200 A/cm 2. A remelted layer of a few micrometers was produced on irradiated MAO films. The corrosion resistance of MAO films was characterized by potentiodynamic electrochemical test in 3.5% NaCl solution. The anodic polarization behavior of MAO samples exhibited a transition from the active anodic dissolution for the original one to the passivation–pitting breakdown for the modified films. The passivation–pitting breakdown voltage of modified films increased with multi-shot irradiation, i.e. from a value of − 1420 mV(SCE) at 1 shot to − 800 mV(SCE) at 5 shots, and the corresponding passivation current density decreased by two orders of magnitude. The irradiated MAO films have a higher corrosion potential than the original one, reaching a maximal value of − 1350 mV(SCE) at 5 shots from the original − 1580 mV(SCE). The electrochemical impedance spectrum (EIS) of modified MAO films was measured with varying the immersion time in 3.5% NaCl solution. The Nyquist impedance plots from modified films may show only a capacitive loop at the immersion time of 5 h, and the inductive loop occur at longer immersion time, whereas original films presenting both capacitive and inductive loops at all the immersion time. Bode plots of MAO films before and after irradiation were obviously distinguished in shape at low frequency range, especially at short immersion time. The evolution of Nyquist and Bode plots with immersion time was discussed in association with the processes of electrolyte penetration into the MAO structure and resultant reaction at the film-substrate interface. It is concluded that the improvement in the continuity and compaction of MAO films accounts for the enhanced corrosion resistance of the films irradiated by HIPIB.

The Research on Anticorrosion and Abrasion Behavior in Corrosive Medium of Micro-Arc Oxidation Film on Magnesium Alloy

<b></b>In the sodium silicate electrolyte system, the uniform ceramic films formed on AZ91D magnesium alloys were obtained by the pulse power when the current density is 40mA/cm2. The morphology, phase composition, anticorrosion and abrasion resistance behaviors in corrosive medium were studied scanning electron microscopy (SEM). The results show that Micro-arc oxidation film is of double-layer structure (loose layer and compact layer), uniform surface and local cracks. Micro-arc oxidation treatment enhanced the anticorrosion of magnesium alloy. The corrosion potential of micro-arc oxidation film is more increased by 94mV than that of the magnesium alloy substrate. The abrasion of the film will be reduced in corroding medium.

Preparation and osteoinduction of active micro-arc oxidation films on Ti-3Zr-2Sn-3Mo-25Nb alloy

A layer of porous film containing Ca and P was prepared by the micro-arc oxidation method on the surface of a novel near β biomedical Ti-3Zr-2Sn-3Mo-25Nb alloy, and then NH 2 − active group was introduced to the films by activation treatment. The phase composition, surface micro-topography and elemental characteristics of the micro-arc oxidation films were investigated with XRD, SEM, EDS and XPS, and the osteoinduction of the micro-arc oxidation films was tested using the simulated body fluid immersion, the in-vitro osteoblast cultivation test and animal experiment. The results show that the oxide layer is a kind of porous ceramic intermixture and contains Ca and P. The films in the simulated body fluid can induce apatite formation, resulting in excellent bioactivity. The cell test discovers that osteoblasts can grow well on the surface of micro-arc oxidation films. And the Ti-3Zr-2Sn-3Mo-25Nb biomedical alloy coated with active porous calcium-phosphate films shows better osteoinduction in vivo.

Comparison of corrosion resistance of microarc oxidation coatings prepared with different electrolyte concentrations on AM60 magnesium alloy

The microarc oxidation (MAO) of AM60 magnesium alloy in two kinds of electrolytes is studied in this paper to compare the influence of electrolytes on the properties of the MAO coatings. One concentrated electrolyte is composed of 100 g L−1 NaOH+20 g L−1 Na2B4O7.10H2O+50 g L−1 C6H5Na3O7.2H2O+60 g L−1 Na2SiO3.9H2O and the other is a dilute solution, which is composed of 5 g L−1 NaOH+15 g L−1 Na2SiO3.9H2O. It was found that the sparking behaviour in dilute electrolyte is more intensive and the breakdown voltage UB is higher (the UB in dilute electrolyte is 282 V, while it is 82 V in the concentrated electrolyte). The thermal effect during the MAO in the two electrolytes is also measured, and the temperature increment in dilute electrolyte is more than twice of that in the concentrated electrolyte. The corrosion resistance of the MAO films is studied by electrochemical impedance spectroscopy and potentiodynamic polarisation techniques and the microstructure of the films is examined by SEM. The electrochemical tests show that the MAO film formed in the concentrated electrolyte is more corrosion resistant. Scanning electron microscope shows that the microstructure of the MAO film in dilute electrolyte is porous and thin, which accounts for its poor corrosion resistance. The experiment results show that high UB does not mean good corrosion resistance.

Effect of Negative Pulse Voltage on the Microstructure and Corrosion Resistance of Microarc Oxidation Film of A356 Aluminum Alloy

The effect of negative pulse voltage on the microstructure and corrosion resistance of microarc oxidation film of A356 aluminum alloy treated by microarc technique was investigated by SEM, coating thickness gauge and electrochemical workstation etc. The results show the negative pulse voltage greatly influences the microstructure and corrosion resistance of microarc oxidation film by its electrode reaction. The film thickness increases while the size of pore and roughness of the film surface decreases initially and then increases with negative pulse voltage increasing. The microarc oxidation treatment considerably improved corrosion resistance, and the highest corrosion potential was -1.16V, which was 0.38V higher than that of substrate, and the corrosion current was lower than that of substrate about three orders of magnitude.

Porous TiO 2 film prepared by micro-arc oxidation and its electrochemical behaviors in Hank's solution

Porous titanium oxide film was prepared by micro-arc oxidation (MAO) method on the surface of titanium alloy in electrolyte containing Ca and P. Surface characterizations of the film were carried out using X-ray diffractometer (XRD), scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) before and after immersion in Hank's solution. Electrochemical behaviors and corrosion resistance were studied by electrochemical techniques. The film was mainly composed of titania, α-tricalcium phosphate (α-TCP) and amorphous Ca–P compounds. α-TCP and amorphous compounds could transformed into hydroxyapatite (HA) when immersed in Hank's solution. MAO film showed higher corrosion potential and lower corrosion current than the titanium alloy and its chemical stability was slightly changed after formation of HA. Fitted electrochemical impedance spectroscopy (EIS) data indicated that after immersion for 2 weeks the MAO film kept good corrosion resistance. Porous TiO 2 film on titanium alloy by MAO method showed good chemical stability in Hank's solution and the transformation of Ca–P compounds into HA indicated that MAO was an effective method for preparing titanium alloys as bioactive artificial bone substitute even when Ca and P in the tissue environment were not abundant.

Effect of the electrolytic solution composition on the performance of micro-arc anodic oxidation films formed on AM60B magnesium alloy

Anodic oxidation in the micro-arc regime is one of the most investigated techniques used to coat magnesium alloys with ceramic coatings for protection from corrosion and wear. The composition of the electrolytic solution affects the anodic oxide layer morphology and composition and its behaviour in aggressive environments. In this paper the influence of the composition of the electrolytic solution on the anodic oxidation process of AM60B magnesium alloy and oxide properties is discussed. Scanning electron microscopy and x-ray diffraction were employed to assess morphology, crystallographic structure and composition of the anodic oxide. Electrochemical polarization tests were performed to evaluate the corrosion resistance behaviour of the coated magnesium alloys.

The influences of duty cycle on the bonding strength of AZ31B magnesium alloy by microarc oxidation treatment

The process of microarc oxidation was studied as a pre-treatment for adhesive bonding of magnesium alloys. Porous microarc oxidation films on AZ31B magnesium alloy were prepared in a sodium silicate electrolyte by a high-frequency bipolar pulsing mode. The influences of duty cycle on film porosity and lap-shear strength of the bonding joints were studied. The higher duty cycle increased the film porosity and slightly decreased the thickness of the oxide film. The microarc oxidation film consisted mainly of MgO and MgSiO 3. The increase of duty cycle resulted in the increase of MgO in the oxide film. As the duty cycle increased, the lap-shear strength increased. The highest lap-shear strength 24.5 MPa was obtained under the duty cycle of 40%. The reason was attributed to the larger porosity and enhanced mechanical interlocking effect.

Local arc discharge mechanism and requirements of power supply in micro-arc oxidation of magnesium alloy

To study the requirements of the power supply in micro-arc oxidation (MAO) of magnesium alloy, many experiments were performed under the DC, unipolar, and ambipolar pulse output modes. Based on the experimental results and electric arc theory, the separate local arc discharge mechanism was put forward. It is considered that magnesium MAO process consists of three stages including anodic oxidation, micro-arc oxidation, and large-arc discharge in turn with increasing source voltage. The MAO film is composed of metal oxides that resulted from numerous discrete local arc discharges, which accumulate the non-equilibrium structure after undergoing sudden heating and cooling cycles. Separate local arc discharge is caused by the process in which the oxygen-based gas is ionized in the conduct channel bearing electric field intensity, changed from insulator to conductor that presents sharp negative resistance effect, and produced partially high temperature to ignite locally metal oxidation. The local arc discharge model is described as four courses: gas created from electrolysis, arc discharge, metals oxidization, and cooling and shrinking of oxides. The purpose of pulse supply is to inhibit the large-arc discharge by intervening proper cooling time, which cannot be actualized by a unipolar pulse mode because of the strong capacitive load characteristics but can be reached by an ambipolar pulse supply because the negative pulse period acts as cooling time. Using a discharge loop to remove the influence of load capacitive, a new type of pulse power supply for MAO is developed, so that the large-arc problem is resolved effectively, the film-forming efficiency is improved, and the pollution of the film and electrolyte caused by negative voltage is avoided.

Cross-linked gelatin/nanoparticles composite coating on micro-arc oxidation film for corrosion and drug release

A composite coating which could control drug release and biocorrosion of magnesium alloy stent materials WE42 was prepared. This composite coating was fabricated on the surface of the micro-arc oxidation (MAO) film of the magnesium alloy, WE42, by mixing different degrees of cross-linked gelatin with well-dispersed poly(dl-lactide-co-glycolide) (PLGA) nanoparticles. The PLGA nanoparticles were prepared by emulsion solvent evaporation/extraction technique. Nano ZS laser diffraction particle size analyzer detected that the size of the nanoparticles to be 150–300 nm. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) was used to analyze the morphology of the nanoparticles and the composite coating. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) were used to evaluate the corrosion behavior of the composite coating. Drug release was determined by ultraviolet–visible (UV–vis) spectrophotometer. The corrosion resistance of the composite coating was improved by preventing the corrosive ions from diffusing to the MAO films. The drug release rate of paclitaxel (PTX) exhibited a nearly linear sustained-release profile with no significant burst releases.

Effects of electrolytic concentration on properties of micro-arc film on Ti6Al4V alloy

To study the effect of electrolytic concentration, bioactive ceramic films containing Ca and P on the surface of the Ti6Al4V alloy were prepared by micro-arc oxidation (MAO) in aqueous solutions of different concentrations. Composition, micro-morphology, wettability of the films and their corrosion behavior in a Hank's SBF were studied. Our experimental results show that the film is mainly composed of anatase, rutile and amorphous phases. With an increase in electrolytic concentration, the ratio of rutile in films enlarge and small amounts of calcium phosphate (Ca 3(PO 4) 2) and hydroxyapatite (HA) appear. The number of micropores in films increases but their dimensions decrease and their porosities increase slightly. As the surface roughness of MAO film increases with concentration, the wettablility of the oxide film improves continually, while micro-hardness increases at first and then decreases. MAO treatment clearly improves the corrosion resistance of substrates in a Hank's SBF.