Porous Micro-arc Oxidation Research Articles

A targeted vacuum-expansion sealing strategy for long-term protection of porous coatings

While surface treatments like anodic oxidation or micro-arc oxidation (MAO) significantly enhance the surface hardness and abrasion resistance of aluminum alloys, they often leave numerous dense pores or defects that compromise their corrosion resistance. Traditional sealing methods, such as boiling water or silane treatments, frequently fail to completely seal these micro-cavities and can introduce new defects due to volumetric shrinkage of the sealants, thereby reducing the service lifespan of the coatings. In this study, we introduce a novel vacuum-expansion sealing (VE sealing) strategy aimed at providing a more effective and durable sealing treatment. This method involves filling sealants in a vacuum environment, followed by volume expansion through ring-opening polymerization. Using this approach, the porosity of the porous MAO layer is significantly reduced, with the MAO-Ves coating porosity decreasing from 8.69 % to a remarkable 0.03 %. Electrochemical tests further demonstrate that after VE sealing, the sealed MAO sample maintains a higher low-frequency impedance |Z|0.01Hz of 1.16 × 107 Ω·cm2 after 672 h. This study not only presents a valuable strategy for enhancing the corrosion resistance of MAO coating, but also provides an efficient solution for improving the hardness, abrasion-resistance, and corrosion-resistance of surface-treated aluminum alloys and magnesium alloys.

Degradation behavior and osteogenic activity of octacalcium phosphate modified MgO coatings on magnesium

To further enhance the corrosion resistance and osteogenic activity of the porous micro-arc oxidation (MAO) derived MgO ceramic coating on biomedical magnesium (Mg), octacalcium phosphate (OCP) layers with different microstucture were prepared on MAO coating by chemical deposition (CD) method to fabricate MAO/CD composite coatings. The in vitro degradation behaviors of MAO/CD-coated samples were evaluated by immersion in simulated body fluid (SBF) and electrochemical impedance spectroscopy (EIS) measurement. Their effects on in vitro MC3T3-E1 cell differentiation were investigated by examining alkaline phosphatase (ALP) activity and expression of main osteogenic-related genes. The in vitro immersion tests show that MAO/CD coatings rarely experience destruction and only micro-cracks occur after 96 h. EIS tests indicate that MAO/CD coatings slow down the degradation rate of Mg observably. In addition, MAO/CD coatings improve osteogenic differentiation of MC3T3-E1 cells compared to MAO coating, which is ascribed the lightened alkalization of the surrounding medium as well as up-regulated intracellular Ca2+. MAO/CD coating deposited in 2 h endows Mg the slowest degradation rate and excellent osteogenic differentiation induction activity.

Improvement of laser welded TC4/CFRTP joint strength by combination of surface modification of MAO and laser texturing

Laser texturing and MAO (micro-arc oxidation) hybrid process was adopted to enhance the tensile-shear strength of laser welded CFRTP/TC4 joint. Introduction of porous MAO coating enhanced wetting ability of molten peek from CFRTP and increased surface roughness. Porous structure could also provide a mechanical interlocking effect in CFRTP/TC4 interface and improve the resistance to tensile-shear load. Highest tensile-shear force of 1587 N was obtained when 2.3 μm MAO coating was prepared. Based on this, influence of various laser texturing patterns with different MAO coating thickness on the tensile-shear forces was clarified by combination of experiment and reasonable SVR (Support Vector Regression) model. Maximum predicated surface roughness of 3.075 mm was produced at texturing width of 0.327 mm, texturing depth of 100.54 μm and MAO duration of 1.667 min. Furthermore, highest predicated tensile-shear force of 2995.1 N was achieved at texturing width of 0.331 mm, texturing of depth of 100.6 μm and MAO duration time of 1.718 min.

Preparation and properties of MAO self-healing anticorrosion film on 5B70 Al alloy

Micro-arc oxidation (MAO) was a new surface treatment technology for Al alloys. However, the MAO ceramic film could not provide long-term protective performance owing to its inherent porous structure. In this work, a new type of CeO2-sealed GO/Al2O3 composite film on 5B70 Al alloy with excellent corrosion resistance was prepared by integrating MAO and the hole sealing technique. The experimental results indicated that the loose and porous MAO ceramic film could serve as a ‘shield’ and a ‘reservoir’, respectively, to obtain improved impedance and sufficient corrosion inhibitor loading. Compared to the original MAO ceramic film or GO/Al2O3 composite film, The GO/Al2O3 composite film after sealing treatment for 60 min had lower porosity and better corrosion resistance. In addition, CeO2-sealed GO/Al2O3 composite film exhibited a positive self-healing effect in 3.5 wt% NaCl solution.

Enhancement of corrosion resistance and mechanical properties of Mg–2.0Zn–0.5Zr–1.5Dy (mass%) alloy in a corrosive medium by a microarc oxidation/polylactic acid composite coating

AbstractTo enhance the corrosion resistance of a micro‐arc oxidation (MAO) coating and enable the alloy to maintain high mechanical properties in a corrosive medium, polylactic acid (PLA) was used to seal the porous MAO coating on the surface of extruded Mg–2.0Zn–0.5Zr–1.5Dy alloy by using a dipping coating method. Assessments were conducted by electrochemical experiments, immersion experiments, and tensile tests. The results showed that after the MAO coating was dipped in PLA solution four times, the pores of the MAO coating were largely sealed by PLA. Compared with the MAO coating, the MAO/PLA composite coating had a lower surface roughness, higher hydrophobicity, and better bonding strength with the matrix. In addition, the MAO/PLA composite coating mitigated the damage of the coating during the tensile and corrosion processes and provided a method to maintain higher corrosion resistance and mechanical properties of magnesium alloys in a corrosive medium.

Influence of a Micro-Arc Oxidation/Poly-Lactic Acid Composite Coating on Corrosion Resistance of Extruded Mg-2Zn-0.5Zr-1.5Dy (Mass%) Alloy

Micro-arc oxidation (MAO) coating can significantly slow down the repaid degradation rate of biodegradable magnesium alloy, but the porous characteristics of the coating cannot provide long-term protection to magnesium alloy. In this paper, poly-lactic acid (PLA) was used to seal the porous MAO coating on the surface of extruded Mg-2Zn-0.5Zr-1.5Dy (mass%) magnesium alloy by a dipping coating method. Assessments were conducted by electrochemical experiment, immersion test, and hydrogen evolution experiment. The result shows that after the MAO-coated sample was dipped in PLA solution four times, the PLA could largely seal the porous and cracks of the MAO coating, and a dense MAO/PLA composite coating with a thickness of ~ 50 μm was prepared. The MAO/PLA composite coating provides good and stable protection to the alloy under 0~56 d immersion in the simulated body fluid than the single MAO coating, which shows an excellent application potential in the field of biodegradable magnesium alloy.

Properties of Micro-Arc Oxidation Coatings on 5052 Al Alloy Sealed by SiO2 Nanoparticles

Micro-arc oxidation (MAO) treatment can effectively improve the wear resistance, corrosion resistance, and mechanical strength of aluminum alloy substrates. Improving the porous structure of MAO film and effectively sealing the pores is a significant research issue. In this study, the MAO treatment of 5052 aluminum alloy was carried out in silicate electrolytes. The MAO films were sealed with different concentrations of SiO2 nanoparticles. The effects of SiO2 nanoparticle content on the MAO films’ microstructure, mechanical properties, and corrosion performance were systematically investigated. When adding SiO2 nanoparticles to electrolytes, the particles were deposited at the micropores of the film, which could effectively seal the porous MAO film and significantly improve its corrosion and wear resistance. The corrosion resistance and wear resistance properties were optimal with 5.0 g/L SiO2 addition. Compared to the unsealed film, the corrosion current density and corrosion rate decreased from 1.24 × 10−9 A/cm2 and 1.47 × 10−5 mm/a to 7.78 × 10−10 A/cm2 and 9.15 × 10−6 mm/a, respectively. Moreover, the average friction coefficient of the sealed film was 0.606, which was ~19.3% lower than that of the substrate and 3.3% lower than for the unsealed film.

Enhanced corrosion resistance of porous microarc oxidation coating sealed with electroless nickel plating under pressure impregnation

The corrosion protection ability of the microarc oxidation (MAO) coating cannot be maximized due to its inherent defective structure, and further hole sealing treatment has been a prevailing strategy for enhancing the corrosion resistance. Herein, the electroless Ni plating under pressure impregnation was demonstrated to be significantly effective to remedy the defects of the porous MAO coating, the voids and cracks in the MAO coating were filled with Ni well, resulting in the blockage of the invasive route of the corrosive Cl− ions, and the ionization of the metal matrix was unable to occur, the MAO coating with ultrahigh electrical resistivity hardly transferred the electron from the Ni layer to substrate, hence, the corrosion property of the MAO coating was obviously enhanced.

Electroless Plating of Ni-P and Ni-P-PTFE on Micro-Arc Oxidation Coatings for Improved Tribological Performance

Although micro-arc oxidation (MAO) coatings are widely used for anti-wear protection of metals, they usually contain lots of pores and exhibit high coefficient of friction. In this work, porous MAO coatings consisting of Al2TiO5 were fabricated on the titanium substrate, and then Ni-P and Ni-P-polytetrafluoroethylene (PTFE) were deposited via electroless plating to fill MAO pores and improve the tribological performance. Electroless deposited Ni-P has high wear resistance and the incorporated PTFE particles have self-lubricating property. Results show that the micro-pores of the MAO coating were completely filled by deposited Ni-P and Ni-P-PTFE, and the self-lubricating PTFE particles distributed uniformly in the coating. For the MAO-Ni-P-PTFE coating, the MAO and Ni-P components mechanically interlocked and played a load-carrying role during the tribological test, and the dispersed PTFE particles produced low friction. Compared with the MAO and MAO-Ni-P coatings, the MAO-Ni-P-PTFE coating exhibited much lower coefficient of friction and wear rate.

Dual self-healing composite coating on magnesium alloys for corrosion protection

In this work, a new self-healing composite coating for corrosion protection of magnesium (Mg) alloy, which is based on the dual actions of the corrosion inhibitor M-16 embedded in the micro-arc oxidation (MAO) coating and self-healing polyurethane (PU) modified by disulfide bonds, is developed. Electrochemical impedance spectroscopy (EIS) and scanning electrochemical microscopy (SECM) are used to study the anti-corrosion performance of the scratched and healed composite coatings immersed in 3.5 wt% NaCl solution. Results show that the porous MAO coating is suitable to carry inhibitor M-16 and the anti-corrosion performance of the scratched coating is significantly improved by the inhibitor embedded in the MAO coating. The scratched coating exhibits an excellent recovery of the anti-corrosion performance after the heat treatment, which is attributed to the cooperation of dynamic disulfide bonds and shape-memory effect of the self-healing PU coating. Furthermore, it is found that the inhibitor M-16 embedded MAO coating can enhance the repair ability and anti-corrosion performance of the PU coating when the corrosion product was pre-formed inside the scratch of the composite coating.

Biodegradation behaviour of hydroxyapatite-containing self-sealing micro-arc-oxidation coating on pure Mg

ABSTRACTThis work successfully prepared a hydroxyapatite (HA)-containing self-sealing micro-arc oxidation (MAO) coating on pure Mg by adding HA particles into a Ca–P based electrolyte solution. Biodegradation behaviour of the obtained coatings was studied. The HA particles transformed the porous MAO coating into a self-sealing coating. The presence of different melting point MAO coating constituents was a key factor. HA particles increased the corrosion resistance of the coating by increasing compactness of the coating which counteracted the decrease in thickness. The MAO coating containing 1 g/L HA particles had the best corrosion resistance.

Improved corrosion resistance of friction stir welded magnesium alloy with micro-arc oxidation/electroless plating duplex coating

In this study, the corrosion behavior of duplex coating on friction stir welded AZ31 alloy was investigated by a series of characterization methods and electrochemical tests. The duplex coating consisted of a micro-arc oxidation (MAO) intermediate layer and an electroless nickel (EN) top layer was applied to the friction stir welding (FSW) joint to enhance its corrosion resistance. The experimental results exhibited that the heterogeneous microstructure in the different zones of the FSW joint caused inhomogeneous susceptibilities to local corrosion. The coarse second phase in the heat-affected zone (HAZ) decreased the corrosion resistance. Although the microstructure differed in the different zones, the morphology of the duplex coating is no significant difference in different zones of the FSW joint. The dense EN top layer and porous MAO intermediate layer were bonded by mechanical interlocking because parts of the EN layer were embedded in the micro-pores of the MAO layer. After depositing the duplex coating, the corrosion potential was raised, and the corrosion current density was reduced. Therefore, the duplex coating provided the FSW joint with improved corrosion resistance. The protection mechanism of the duplex coating against corrosion is depicted in schematic diagrams that explain the corrosion behavior and corrosion mechanism.

Enhanced Corrosion Performance By Micro Arc Oxidation Coating with the Addition of 2-Aminobenzimidazole Loaded Halloysite Nanotubes on AZ31 Alloy

The application of Magnesium alloys has drawn great attention in recent years. As the lightest structure metal on earth, the usage of Mg is attractive to the automotive, aerospace and electronics industries owing to its high weight-to strength ratio. Unfortunately, the poor corrosion resistance impedes the development of Mg alloys. To date, several methods have been proposed to improve the anti-corrosion ability, such as conventional anodizing, sol-gel coating, and micro arc oxidation (MAO). MAO is an innovative technique for generating surface oxide coatings on valve metals and alloys, which exhibits strong adhesion to the substrate, high thickness, hardness, corrosion and wear resistance, and provides an environmentally-friendly alternative to acid-based anodizing treatment. Despite all these advantages, the intrinsic structural imperfection of MAO coatings, including a loose structure featured by micro-pores and micro-cracks, facilitate quicker penetration of the corrosive ions into the base magnesium, and thus, increase the rate of corrosion. To overcome the problem of porous MAO film, composite coatings with self-healing ability that are capable to repair themselves when damage occurs have been utilized. In this study, the concept of self-healing is carried out by incorporating corrosion inhibitors, 2-aminobenzimidazole (2-ABI), loaded nanocontainers, halloysite nanotubes (HNT), to silicate-based electrolyte to obtain MAO coatings on AZ31 alloy in single step. The coatings are evaluated by SEM, EDX, and XRD. Corrosion performance is studied with EIS, PDP and salt spray test. The effects of various electrical parameters, including current density, frequency and working time, on the coating are also investigated. Experimental results indicate that the corrosion resistance of MAO films with the addition of 2-ABI loaded HNT is significantly improved, and thus confirms the feasibility of 2-ABI as corrosion inhibitor to Mg alloys.

Corrosion behavior of a self-sealing coating containing CeO2 particles on pure Mg produced by micro-arc oxidation

CaP based micro-arc oxidation (MAO) coatings containing CeO2 particles were prepared on pure magnesium. The coatings were characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The addition of CeO2 particles transformed the porous MAO coating into a self-sealing coating, and significantly increased the corrosion resistance of the coated Mg substrate. The optimum concentration of CeO2 particles was 2 g/L. The MAO coating containing CeO2 particles provided a long-term stable protection in Hank's solution for the Mg substrate. The mechanism by which the CeO2 particles promoted self-sealing behavior of the MAO coating was clarified: the different melting point of the coating constituents was a key factor for self-sealing of pores.

Fabrication and Characterization of Porous Micro-arc Oxidation/Al Composite Coating: Bio-inspired Strategy from Cancellous Bone Structure

Based on the trabecular bone structure, a porous micro-arc oxidation (MAO) coating and a MAO/Al composite coating were fabricated on AZ31D Mg alloy. Surface morphology, cross-section morphology, coating composition, and coating thickness were observed by scanning electron microscopy, energy dispersive spectrometry, and x-ray diffraction. Then, force-dampening and energy-absorbing effects of the coatings were explored by a constant kinetic energy impact experiment, to explore their dampening stress and reducing vibration values applying in engineering structure subjected to impact or vibration stress. Results show that large plastic deformation and cracks formed in the AZ31D Mg alloy after impact, and the MAO coating hindered the formation of the defects due to its higher stiffness. In addition, MAO enhanced the energy-absorbing ability of the Mg alloy, which is beneficial for vibration reduction. However, the MAO coating had a rough and brittle surface, and when Al was used to protect it, its force-dampening effect, energy-absorbing effect, and plastic deformation resistance had the evolution trend approaching the Mg alloy substrate but with improved crack resistance. MAO can be an optimization method for enhancing the plastic deformation resistance and vibration reduction effect of Mg alloy as structure material, and Al can be used to enhance its crack resistance.

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

Abstract 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.

Synergistic effect of hydrophobic film and porous MAO membrane containing alkynol inhibitor for enhanced corrosion resistance of magnesium alloy

Magnesium and magnesium alloys have important applications in aviation, electronic devices, medical equipment, and automotive industries, while the corrosion is a common and real problem that must be solved for these applications. In this paper, a novel and effective anti-corrosive composite coating on AZ31 Mg alloys was prepared by the combination of micro-arc oxidation (MAO) layer, corrosion inhibitor and hydrophobic wax film technologies. And among these, the MAO treated micro- and nanopores as the container of corrosion inhibitor and the solid hydrophobic wax as the multifunctional sealing isolating agent. The morphology and phase composition of the resulting composite coatings were investigated by field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD), respectively, indicating the successful synthesis of the organic-inorganic composite coating. The potentiodynamic polarization and electrochemical impedance spectroscopy measurements showed that compared with the MAO membrane covered Mg alloys, the organic-inorganic composite coating has superior corrosion resistance with the a lower corrosion current (5.764 × 10−9 A/cm2) and a higher protection efficiency (99.7%) after immersion in 3.5 wt% NaCl solution, attributing to the synergistic effect of effective inhibition of inhibitor and the physical barrier ability of hydrophobic film and MAO membrane. This simple and low cost strategy to fabricate organic-inorganic composite anticorrosion coatings would have promising applications in the corrosion protection of the light metals and their alloys.

Degradation Behavior of Micro-Arc Oxidized ZK60 Magnesium Alloy in a Simulated Body Fluid

Bio-ceramic coatings were synthesized on ZK60 magnesium alloys by micro-arc oxidation (MAO). The degradation behavior of the ZK60 alloys with and without MAO coating in the simulated body fluid (SBF) was studied. The samples were characterized by means of scanning electron microscopy (SEM), laser scanning confocal microscopy (CLSM), and X-ray diffraction (XRD). Electrochemical impedance spectroscopy (EIS) was used to study the degradation behavior. The results showed that the porous MAO coating mainly consisted of MgO, Mg2SiO4, Mg3(PO4)2, and CaCO3. The pH values of both coated and uncoated samples increased over time. However, the pH values of the SBF for coated samples always maintained a lower level compared with those for the uncoated samples. Thereby, the coated samples showed a much lower degradation rate. After immersion in SBF for 5 days, corrosion product containing Ca and P was found on both samples, while the deposition was more active on the coated samples. The degradation models for the uncoated and coated samples in the SBF are also proposed and discussed.

New Method for the Corrosion Resistance of AZ31 Mg Alloy with a Porous Micro-Arc Oxidation Membrane as an Ionic Corrosion Inhibitor Container.

This work introduces a new composite anticorrosion coating for the AZ31 magnesium alloy, based on the synergistic effect of an organic/inorganic composite coating with a micro- and nanoporous micro-arc oxidation (MAO) membrane as the container of ionic corrosion inhibitor (M-16). The surface morphologies and size of the micro/nanocontainers in the porous MAO membrane before and after filling with M-16 corrosion inhibitor are examined by scanning electron microscopy (SEM). The effectiveness of M-16 for corrosion suppression on AZ31 Mg alloy with and without epoxy coating as the top sealing layer is demonstrated by electrochemical impedance spectroscopy (EIS) and salt spray tests. The potentiodynamic polarization and electrochemical impedance spectroscopy measurements show that, compared with the bare AZ31 Mg alloys, the composite coating has superior corrosion resistance with the a lower corrosion current (9.7 × 10-9 A/cm2) and a higher protection efficiency (99.3%) after immersion in 3.5 wt % NaCl solution and, meanwhile, has stronger salt spray resistance within 30 days. The results demonstrate the synergistic effect of the isolation protection of the micro-arc oxidation layer and the inhibition of M-16 and that the epoxy coating contributed to the protection for AZ31 Mg substrate to some extent. Therefore, it is anticipated that the composite coating has a potential application in the protection of metals and their alloys.

Corrosion resistance of a superhydrophobic surface on micro-arc oxidation coated Mg-Li-Ca alloy

A superhydrophobic surface was successfully fabricated on micro-arc oxidation (MAO) coating on Mg-1Li-1Ca magnesium alloy via surface modification with stearic acid (SA). The microstructure, composition and superhydrophobic characteristics of the MAO/SA composite coatings were investigated by means of SEM, EDS, XRD, XPS, FT-IR, and contact angle (CA). The corrosion resistance of the coatings was assessed by potentiodynamic polarization, electrochemical impedance spectrum (EIS), and immersion test in 3.5 wt % NaCl solution. The results demonstrated that the morphology of the porous MAO surface was transformed to a novel petal-like clusters after modified with SA. The magnesium stearate and SA were deposited in/on the micro-pores and the surface of the MAO coating. The CA of the SA-modified surface reached 155.5°. The corrosion current density of the MAO/SA-7h coating decreased about three orders of the magnitude compared to the substrate. The SA-modified surface can provide a long-term corrosion protection for the Mg-1Li-1Ca alloy. Additionally, the coating formation mechanism as well as the corrosion mechanism were proposed and discussed.