Effects of Voltage on Microstructure and Corrosion Resistance of Micro-arc Oxidation Ceramic Coatings Formed on KBM10 Magnesium Alloy

Micro-arc oxidation (MAO) coatings on KBM10 magnesium alloy were prepared in an electrolyte system with sodium silicate, potassium hydroxide, sodium tungstate, and citric acid. The effects of voltage on the microstructure and corrosion resistance of MAO coatings were studied using stereoscopic microscopy, scanning electron microscopy, x-ray diffraction, scratch tests, potentiodynamic polarization, and electrochemical impedance spectroscopy. The results showed that the roughness of the MAO coatings, diameter, and number of pores increase with the increase in voltage. The coating formed at the voltage of 350 V exhibited the best adhesive strength when evaluated by the automatic scratch tester. The coatings were mainly composed of MgO, MgWO₄, and Mg₂SiO₄, and the content of Mg₂SiO₄ increased with the increase in voltage. The corrosion resistance of MAO coatings could be improved by changing the applied voltage, and the best corrosion resistance of MAO coating was observed at the voltage of 350 V.     

Development of anodic coatings on aluminium under sparking conditions in silicate electrolyte

Spark anodizing of aluminium at 5 A dm⁻² in sodium metasilicate/potassium hydroxide electrolytes is studied, with particular emphasis on the mechanism of coating growth, using transmission electron microscopy and surface analytical techniques, with coatings typically 10 μm, or more, thick. Two-layered coatings develop by deposition of an outer layer based on amorphous silica, associated with low levels of alkali-metal species, at the coating surface and growth of an inner, mainly alumina-based, layer, with an amorphous region next to the metal/coating interface. Formation of crystalline phases in the inner layer, mainly γ-Al₂O₃, with some α-Al₂O₃ and occasional δ-Al₂O, is assisted by local heating, and possibly also by ionic migration processes, arising from the rapid coating growth at sites of breakdown. Due to local access of electrolyte species in channels created by breakdown events, the silicon content in the inner coating regions varies widely, ranging from negligible levels to about 10 at.%. Silica deposition at the coating surface and formation of Al₂SiO₅ and Al₆Si₂O₁₃ phases is promoted by increased time of anodizing and concentration of metasilicate in the electrolyte. However, at sufficiently high concentration of metasilicate and pH, when more extreme voltage fluctuations accompany breakdown, the two-layered nature of coatings is replaced by a mixture of aluminium-rich and silicon-rich regions throughout the coating thickness.     

Fabrication and characterization of micro‐arc oxidation (MAO) coatings on Mg‐Li alloy in alkaline polyphosphate electrolytes without and with the addition of K2TiF6

Micro‐arc oxidation (MAO) has been accomplished on Mg‐Li alloy in alkaline polyphosphate electrolytes without and with the addition of 10 g/L K₂TiF₆. The surface/cross‐section microstructures of the fabricated coatings were analyzed by scanning electron microscope (SEM); the compositions of the fabricated MAO coatings were analyzed by X‐ray diffraction (XRD) and energy dispersive X‐ray analysis (EDAX); the corrosion behaviors of bare and MAO coated Mg‐Li alloys were evaluated using potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) in 3.5 wt% NaCl solution. Novel hybrid MAO coatings which showed hybrid composition of MgF₂/Ti₂O₅/Ti₆O₁₁/MgO, as well as hybrid structure of a dense inner layer, a dense outer layer and an intermediate layer with some sealed/semi‐sealed pores, had been fabricated on Mg‐Li alloy in alkaline polyphosphate electrolytes with the addition of K₂TiF₆. Meanwhile, the corrosion resistance of the fabricated MAO coatings was improved obviously as the result of the addition of K₂TiF₆ in the electrolytes. Moreover, the multiple roles of ${\rm TiF}_{{\rm 6}}^{{\rm 2}{-} } $ on the MAO process accounted for the fabrication of the corrosion‐resistant hybrid MAO coatings.     

Corrosion characterization of microarc oxidation coatings formed on Mg–7Li alloy

Ceramic coatings with thickness of 27 µm were fabricated on Mg–7Li alloy in Na₂SiO₃–C₆H₁₈O₂₄P₆ solution by microarc oxidation (MAO). The morphology and phase composition of MAO coatings were characterized by scanning electron microscopy (SEM) and X‐ray diffraction (XRD). The corrosion behavior of the bare and MAO coated Mg–7Li alloy was investigated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). Results showed that the MAO coatings were composed of MgO, Li₂O, and Mg₂SiO₄, and there existed some micropores on the coating surface with a diameter of 3–20 µm. The corrosion potential (E_corr) and corrosion current density (I_corr) of the MAO coated alloy were about ?1.4761 V and 7.204 × 10^?7 A/cm², respectively. The E_corr of the MAO coated alloy increased by 109.6 mV and its I_corr decreased by three orders compared with that of the bare Mg–7Li alloy. The EIS plots indicated that the impedance of the MAO coated alloy was 15 times higher than that of the bare alloy. The fitting parameters showed that the resistance of the MAO coatings was far greater than that of the bare alloy. The dense intermediate layer and the transition layer of the MAO coatings acted as a barrier to hinder the proceeding of solution permeation, remarkably improving the corrosion resistance of the Mg–7Li alloy.     

Effect of voltage on microstructure and corrosion resistance of microarc oxidation coatings on CP-Ti

The surface modification of commercially pure titanium (CP-Ti) by microarc oxidation (MAO) under different voltages was investigated using 1%H₃PO₄ solution as an electrolyte. The microstructure, phase composition and elemental distribution of ceramic coatings were investigated using scanning electron microscopy (SEM) and X-ray diffraction. The corrosion behaviour of the coating was also examined by potentiodynamic polarisation testing in a 3·5 wt-%NaCl solution. Micropore oxide films were formed on all the sample groups by MAO. The thickness and micropore size of the MAO coating increased with the increasing voltage. Energy dispersive X-ray spectroscopy results indicate that Ti, O and P became incorporated into the MAO coatings. At a low voltage of 250 V, the MAO coatings were composed of amorphous, P₂O₅, TiP₂O₇ and titania phases (rutile and anatase). Variation of treatment voltages increased the ceramic coatings from an amorphous structure to a phase structure, and the P₂O₅ phase disappeared. The corrosion potential Φ_corr of the MAO sample shifted towards nobler directions, and the corrosion density I_corr fell significantly compared with that of the bare CP-Ti. Corrosion testing showed that the sizes of the micropore of the MAO samples obviously decrease, and the MAO surface becomes smooth.     

An Investigation into the Corrosion Behavior of MgO/ZrO<Subscript>2</Subscript> Nanocomposite Coatings Prepared by Plasma Electrolytic Oxidation on the AZ91 Magnesium Alloy

Plasma electrolytic oxidation (PEO) of AZ91 Mg alloys was performed in ZrO₂ nanoparticles containing Na₂SiO₃-based electrolytes. The phase composition and the microstructure of PEO coatings were analyzed by x-ray diffraction and scanning electron microscopy followed by energy dispersive spectroscopy. Pitting corrosion properties of the coatings were investigated using cyclic polarization and electrochemical impedance spectroscopy tests in a Ringer solution. The results showed the better pitting corrosion resistance of the composite coating, as compared to the oxide one, due to the thickened inner layer and the decrease in the surface defects of the composite coating. Also, the PEO process decreased the corrosion current density from 25.06 µA/cm² in the Mg alloy to 2.7 µA/cm² in the oxide coating and 0.47 µA/cm² in the composite coating.     

The influence of sodium tungstate concentration and anodizing conditions on microarc oxidation (MAO) coatings for aluminum alloy

Microarc oxidation (MAO) coatings on 5052 aluminum alloy are prepared in silicate–hypophosphite electrolytes with sodium tungstate. The effects of sodium tungstate concentrations and current density on the surface morphology, phase composition and properties of the coatings are investigated. With the addition of sodium tungstate in the electrolyte and increase of current density, the final voltage at the microarc discharge process increases. The results also show that the MAO coatings are composed mainly of α-Al₂O₃ and γ-Al₂O₃ and the proportion of α-Al₂O₃ and γ-Al₂O₃, pore size, surface roughness as well as thickness of the coatings strongly depend on the sodium tungstate concentration and current density. Thus, the hardness, friction coefficient and corrosion resistance of the coatings are significantly influenced by the magnitude of the current density and sodium tungstate concentration. These oxide films on aluminum were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), thickness gage, and polarization curves, respectively.     

Effect of treatment time on a PEO-coated AZ31 magnesium alloy

Plasma electrolytic oxidation (PEO) coatings were formed in a phosphate–silicate-based electrolyte containing K₂ZrF₆ on an AZ31 Mg alloy. The physical and chemical properties of the coatings were investigated using scanning electron microscopy, atomic force microscopy, X-ray diffraction (XRD), potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS). The results showed that the thickness of the PEO coatings increased linearly with increased treatment times. Additionally, the micropores on the coating surfaces increased in size, but decreased in porosity with increased PEO treatment time. The XRD results showed that the coatings were mainly composed of MgO, MgF₂, MgSiO₃, and ZrO₂, and the electrochemical tests revealed that the corrosion resistance of the coatings increased with increased treatment time. Besides, the EIS results correlated well with the potentiodynamic polarization test results.     

终止电压对MB8镁合金微弧氧化膜耐蚀性的影响

采用SEM、XRD、动电位极化曲线及电化学阻抗等测试方法,研究了MB8镁合金微弧氧化过程中不同终止电压下获得的陶瓷膜层的耐蚀性能.结果表明:终止电压越高,膜层越厚;微火花阶段,膜层表面均匀、结晶细致,腐蚀电流密度较小,阻抗较大;弧放电阶段,膜层孔径变大,陶瓷层内显微缺陷增多,腐蚀电流密度增大,阻抗减小.由此得出结论:膜层耐蚀性能由膜层厚度与终止电压共同决定,微火花放电末期膜层的耐蚀性能优于弧放电阶段的耐蚀性能.     

Study of the structure and corrosion behavior of PEO coatings on AM50 magnesium alloy by electrochemical impedance spectroscopy

In this work coatings were developed on the surface of AM50 magnesium alloy using four different electrolytes containing 10 wt.% each of K₃PO₄ and Na₃PO₄ in combination with either potassium or sodium hydroxides. Electrolyte conductivity and breakdown voltage were measured in order to correlate the property of the coating to the nature of electrolyte. Further, the coatings were examined using scanning electron microscopy for surface morphology and cross sectional investigation, X-ray diffraction for phase determination, and electrochemical impedance spectroscopy for corrosion resistance evaluation. The effect of employing different ions in the electrolytes results in different surface morphologies, chemical phases and, consequently, the corrosion resistance of the coatings. The EIS results indicate the presence of porous and compact layers in the structure of the PEO coatings, whilst the overall coating resistance mainly results from the compact layer, the role of the porous layer as a barrier against corrosion is negligible. Finally, a correlation between the passive current density of the bare alloy and the corrosion resistance of the PEO coating is proposed.     

Structure and corrosion resistance of ZrO2 ceramic coatings on AZ91D Mg alloys by plasma electrolytic oxidation

The aim of this work is to study the structure and the corrosion resistance of the plasma electrolytic oxidation ZrO₂ ceramic coatings on Mg alloys. The ceramic coatings were prepared on AZ91D Mg alloy in Na₅P₃O₁₀ and K₂ZrF₆ solution by pulsed single-polar plasma electrolytic oxidation (PEO). The phase composition, morphology and element distribution in the coating were investigated by X-ray diffractometry, scanning electron microscopy and energy distribution spectroscopy, respectively. The results show that the coating thickness and surface roughness were increased with the increase of the reaction time. The ceramic coatings were of double-layer structure with the loose and porous outer layer and the compact inner layer. And the coating was composed of P, Zr, Mg and K, of which P and Zr were the main elements in the coating. P in the coating existed in the form of amorphous state, while Zr crystallized in the form of t-ZrO₂ and a little c-ZrO₂ in the coating. Electrochemical impedance spectra (EIS) and the polarizing curve tests of the coatings were measured through CHI604 electrochemical analyzer in 3.5% NaCl solution to evaluate the corrosion resistance. The polarization resistance obtained from the equivalent circuit of the EIS was consistent with the results of the polarizing curves tests.     

TiO2纳米添加剂对6063铝合金微弧氧化陶瓷涂层性能的影响（英文）

通过在硅酸盐电解液中加入TiO2纳米添加剂,研究纳米添加剂浓度的变化对6063铝合金微弧氧化陶瓷涂层性能的影响。结果表明,纳米添加剂进入到陶瓷涂层中,而添加剂浓度的选取有一个较合理的范围。随着纳米添加剂的浓度增加到3.2g/L,涂层的结合力逐渐增大,平均摩擦因数和质量损耗逐渐减小。当浓度增加到4.0g/L时,涂层的结合力减弱,而平均摩擦因数增加,这与涂层显微硬度的测试结果一致。     

Structure, morphology and chemical composition of sputter deposited nanostructured Cr-WS2 solid lubricant coatings

WS₂ and Cr-WS₂ nanocomposite coatings were deposited at different Cr contents (approximately 15-50 at.%) on silicon and mild steel substrates using an unbalanced magnetron sputtering system. X-ray diffraction (XRD) was used to study the structure of Cr-WS₂ coatings and the bonding structure of the coatings was studied using X-ray photoelectron spectroscopy (XPS). The characterization of different phases present in Cr-WS₂ coatings was carried out using micro-Raman spectroscopy. The XPS and Raman data indicated the formation of a thin layer of WO₃ on the surface of Cr-WS₂ coatings and the intensity of the oxide phase decreased with an increase in the Cr content, which was also confirmed using energy-dispersive X-ray analysis results. The surface morphologies of WS₂ and Cr-WS₂ coatings were examined using field emission scanning electron microscopy (FESEM) and atomic force microscopy. It has been demonstrated that incorporation of Cr in WS₂ strongly influences the structure and morphology of Cr-WS₂ coatings. The XRD and FESEM results suggested that increase in the Cr content of Cr-WS₂ coatings resulted in a structural transition from a mixture of nanocrystalline and amorphous phases to a complete amorphous phase. The cross-sectional FESEM data of WS₂ coating showed a porous and columnar microstructure. For the Cr-WS₂ coatings, a mixture of columnar and featureless microstructure was observed at low Cr contents (≤ 23 at.%), whereas, a dense and featureless microstructure was observed at high Cr contents. Detailed cross-sectional transmission electron microscopy (TEM) studies of Cr-WS₂ coatings prepared at Cr content ≤ 23 at.% indicated the presence of both nanocrystalline (near the interface) and amorphous phases (near the surface). Furthermore, high-resolution TEM data obtained from the nanocrystalline region showed inclusion of traces of amorphous phase in the nanocrystalline WS₂ phase. Potentiodynamic polarization measurements indicated that the corrosion resistance of Cr-WS₂ coatings was superior to that of the uncoated mild steel substrate and the corrosion rate decreased with an increase in the Cr content.     

Effects of electric parameters on microstructure and properties of MAO coating fabricated on ZK60 Mg alloy in dual electrolyte

Micro-arc oxidation (MAO) process was carried out in a dual electrolyte system of NaAlO2 and Na3PO4 on ZK60 Mg alloys to explore the effect of electric parameters including current density, frequency, duty cycle and oxidation time on the evolution of coatings and other characteristics. The microstructural characteristics of coatings were observed by scanning electron microscopy (SEM) combined with the analysis of voltage-time responses during MAO process. Test of weight loss was conducted at a 3. 5％ NaCl solution to assess the resistance to corrosion. The results indicate that the current density and duty cycle play key roles on the coating quality. The peak voltage during MAO process increased with the increase in current density but the coating would be more easily detached when the current density was beyond a critical value. Voltage during MAO and microstructure of the coating were affected remarkably by duty cycle, and corrosion resistance was improved greatly when duty cycle was 40％. By means of single variable experiments, MAO process with optimized electric parameters was developed, which corresponds to the current density of 20 A·dm-2, frequency of 500 Hz, duty cycle of 40％ and oxidation time of 15 min.     

Effect of Ti interlayer on corrosion behavior of nanostructured Ti/TiN multilayer coating deposited on TiAl6V4

To improve the corrosion properties of TiAl₆V₄ alloy, TiN monolayer and Ti/TiN multilayer coatings are deposited by reactive magnetron sputtering. The phase, structure, and morphology properties are investigated by grazing‐incidence X‐ray diffraction, field‐emission scanning electron microscopy, and atomic force microscopy, respectively, and the corrosion behavior is evaluated by electrochemical impedance spectroscopy and potentiodynamic polarization. The TiN monolayer and Ti/TiN multilayer with thickness of 1,350 and 1,410 nm have the (111) and (002) preferred orientation and crystallite size of 42.5 and 24.3 nm, respectively. Columnar growth in TiN is hindered by the Ti interlayers and no cracking is observed between the layers indicating strong adhesion. The nanostructured Ti/TiN coating forms stable surface titanium oxide which improves the corrosion resistance by approximately 80 and four times compared with TiAl₆V₄ alloy and TiN coating, respectively. Hindrance of the columnar structure in TiN by the Ti interlayer decreases the local corrosion rate and enhances the galvanic corrosion resistance by forming a layer on the β‐phase enriched with vanadium as well as a TiO₂ stable layer. The nanostructured Ti/TiN coating demonstrates capacitive behavior with phase angles approximately ?50° and high impedance values at low frequency to be the corrosion resistance mechanism.     

Microstructural characterization of radio frequency and direct current plasma-sprayed Al2O3 coatings

Microstructures of radio frequency (RF) and direct current (DC) plasma-sprayed Al₂O₃ coatings deposited onto steel substrates were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), electron microprobe analysis (EMPA), polarizing optical microscopy (OM), and transmission electron microscopy (TEM). Because RF and DC plasmas produce different particle heating and acceleration, the morphology, phase structure, and fracture modes of the coatings vary substantially. In the case of RF coatings, a clear lamellar microstructure with relatively thick lamellae was observed, which is due to the large particles and the low particle velocities, with α-Al₂O₃ as the predominant phase and with delamination type of fracture detected on the fracture surface. In contrast, the DC coatings consisted of predominantly metastable γ-Al₂O₃ as well as amorphous phases, with a mixed fracture mode of the coating observed. In spite of limited interfacial interdiffusion detected by EMPA, TEM showed an interfacial layer existing at the interface between the coating and the substrate for both cases. For RF coatings, the interfacial layer on the order of 1 μm was composed of three sublayers, each of which was different in composition and morphology. However, the interfacial layer for the DC coating consisted primarily of an amorphous phase, containing both coating and substrate materials with or without platelike microcrystals; although in some regions a thick amorphous Al₂O₃ layer was in direct contact with the substrate.     

钛合金表面GO/HA/MAO复合膜层的制备及其性能

为了改善钛合金种植体在体液中的腐蚀及摩擦腐蚀行为,延长其在人体环境中的服役时间,在微弧氧化 (MAO)膜层上采用溶胶凝胶(Sol-gel)法于羟基磷灰石(HA)和氧化石墨烯(GO)的混合溶胶中浸渍提拉成膜,从而在 Ti6Al4V 合金表面成功地制备了 GO/ HA/ MAO 复合膜层。 结果表明,MAO 膜层表面的微孔及微球被 GO/ HA 薄膜有效的覆盖且较为致密;膜层的物相组成主要为金红石相及锐钛矿相的 TiO₂、HA、SiO₂ 和GO;根据电化学腐蚀和摩擦腐蚀结果分析知,GO/ HA/ MAO 复合膜层在模拟体液(SBF)中的耐蚀性及耐摩擦腐蚀性相比于 MAO 膜层和 Ti6Al4V 基体均得到了显著提高。     

Incorporation mechanism of colloidal TiO2 nanoparticles and their effect on properties of coatings grown on 7075 Al alloy from silicate-based solution using plasma electrolytic oxidation

Plasma electrolytic oxidation (PEO) was applied using a pulsed unipolar waveformto produce Al₂O₃−TiO₂ composite coatings from sol electrolytic solutions containing colloidal TiO₂ nanoparticles. The sol solutions were produced by dissolving 1, 3, and 5 g/L of potassium titanyl oxalate (PTO) in a silicate solution. Scanning electron microscopy, energy dispersive spectrometry, X-ray diffraction, and Raman spectroscopy were applied to characterizing the coatings. Corrosion behavior of the coatings was investigated using polarization and impedance techniques. The results indicated that TiO₂ enters the coating through all types of micro-discharging and is doped into the alumina phase. The higher level of TiO₂ incorporation results in the decrease of surface micro-pores, while the lower incorporation shows a reverse effect. It was revealed that the higher TiO₂ content makes a more compact outer layer and increases the inner layer thickness of the coating. Electrochemical measurements revealed that the coating obtained from the solution containing 3 g/L PTO exhibits higher corrosion performance than that obtained in the absence of PTO. The coating produced in the absence of PTO consists of γ-Al₂O₃, δ-Al₂O₃ and amorphous phases, while α-Al₂O₃ is promoted by the presence of PTO.     

Effects of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Nano-Particles on Corrosion Performance of Plasma Electrolytic Oxidation Coatings Formed on 6061 Aluminum Alloy

Corrosion resistance improvement of plasma electrolyte oxidation coatings on 6061 aluminum alloy in silicate electrolyte containing Al₂O₃ nano-particles was studied, with particular emphasis on the microstructure, coating growth, and corrosion behavior in 3.5 wt.% NaCl solution. The microstructure of coatings, their thickness, and phase composition were characterized using scanning electron microscopy and x-ray diffraction. All characterization data showed that the maximum coating thickness and lowest amount of porosity were obtained in a low concentration of KOH, a high concentration of Na₂SiO₃, and moderate concentration of Al₂O₃ nano-particles in the electrolyte. This combination describes the optimum plasma electrolytic oxidation electrolyte, which has the best conductivity and oxidizing state, as well as the highest incorporation of electrolyte components in the coating growth process. On the other hand, incorporation and co-deposition of Al₂O₃ nano-particles were more pronounced than SiO₃ ^2? ions in some level of molar concentration, which is due to the higher impact of electron discharge force on the adsorption of Al₂O₃ nano-particles. The electrochemical results showed that the best protective behavior was obtained in the sample having a coat with the lowest porosity and highest thickness.     

纳米Si3N4 对TC4钛合金微弧氧化层微观结构和性能的影响

在基电解液中加入氮化硅纳米颗粒,对TC4钛合金进行微弧氧化（MAO）处理,研究了Si₃N₄浓度对微弧氧化层表面形貌、耐蚀性和耐磨性的影响。添加Si₃N₄的MAO层呈现多孔结构,当Si₃N₄浓度为1 g/L时,涂层厚度最大,且经过7 d的酸腐蚀试验,该涂层的耐蚀性良好,腐蚀速率最低,约为0.057 mg·cm^-2·d^-1。随着Si₃N₄的加入,MAO涂层的抗菌性能先升高后降低。当Si₃N₄的添加量为1 g/L时,该MAO层的抗菌性能最好。Si₃N₄的加入能明显提高涂层在模拟海水中的耐磨性。当Si₃N₄的添加量为3和4 g/L时,所得涂层的摩擦系数低且稳定,且添加3 g/L Si₃N₄制备来的MAO涂层表现出优异的耐磨性。