Improvement in corrosion resistance of micro-arc oxidation coating on PVD Ti-coated aluminum alloy 7075

Surface treatments are always needed to enhance corrosion-resistant performance of aluminum (Al) alloys when they are used in seawater environments. The paper aimed to prepare the composite oxide ceramic coating on Al alloy 7075 by combining micro-arc oxidation (MAO) and magnetron sputtering technology. The Al substrate was precoated with titanium (Ti) layer by using the magnetron sputtering technology and then treated by MAO in the alkaline aluminate electrolyte, resulting in a composite MAO coating, which is composed of Al₂O₃ and TiO₂ along with the complex oxide (Al₂TiO₅). The potentiodynamic polarization and electrochemical impedance spectroscopy were carried out to evaluate the corrosion performance of the MAO coatings in 3.5 wt% NaCl solution. Better corrosion resistance was observed for composite oxide coating than the reference MAO coating on the bare Al, as evidenced by the higher corrosion potential of −0.664 V versus Ag/AgCl and the lower corrosion current density of 4.41 × 10^-6 A/cm².     

Investigation of tribological properties of micro-arc oxidation ceramic coating on Mg alloy under dry sliding condition

To enhance wear resistance of Mg alloy, micro-arc oxidation (MAO) ceramic coatings on Mg substrate were prepared in silicate electrolyte under various currents. It was found that the surface roughness and thickness of MAO coating were increased with the increase of current. The dry tribological tests showed that the friction coefficient and wear resistance of thicker coatings (obtained under currents of 3?A and 4?A) were much higher than that of Mg alloy and the thin coating (obtained under current of 2?A), meanwhile the lifetime of the coating obtained under 4?A was longer than the other coatings under higher load. The wear type of thin MAO coating was slight abrasive wear under low load, whereas translated to severe adhesive wear under high load. While the main wear mechanism of thick MAO coating was slight abrasive wear or scratch under the given test condition, which was attributed to the thick intermediate layer improved load support for the soft substrate. The tribological study indicated that the MAO coating obtained under 4?A current had better wear resistance and life time due to its compact microstructure and thickness.     

Enhanced corrosion protection for MAO coating on magnesium alloy by the synergism of LDH doping with deposition of 8HQ inhibitor film

The inherent micropores of micro-arc oxidation (MAO) coatings on magnesium alloy often cause poor long-term corrosion resistance. Herein, a low-porosity and high corrosion-resistant 8HQ/LDH/MAO composite coating, comprising a layered double hydroxide (LDH)-doped MAO primer and a top layer of 8-hydroxyquinoline (8HQ) inhibitor film, was deposited onto the magnesium alloy surface. Characterizations such as high-resolution field-emission transmission electron microscopy were performed to observe the synthesized nanoparticles’ morphology, size, composition, and structure. Results confirmed the successful synthesis of nitrate ion intercalated MgAl-LDH nanosheets and demonstrated the increase in thickness and denseness of the MAO coating after LDH doping. Based on electrochemical impedance spectroscopy and Tafel curves, the corrosion current density (j_corr) of the newly fabricated 8HQ/LDH/MAO composite coating was reduced by about three orders of magnitude. The low-frequency impedance modulus (|Z|_ƒ=0.1 Hz) increased by nearly four orders of magnitude compared with that of bare Mg alloy. After 14 days of exposure to the corrosive electrolyte, the composite coating maintained a low j_corr, showing significantly enhanced corrosion resistance compared with single MAO coating. These findings demonstrated the synergism of LDH doping with 8HQ sealing to enhance the corrosion protection of MAO coating on magnesium alloy.     

铝-锰合金镀层的微弧氧化研究

在钢铁基体上采用熔盐电镀的方法获得铝-锰合金镀层,对镀层进行微弧氧化后制得铝-锰陶瓷膜。研究了不同质量浓度的硅酸钠电解液对铝-锰陶瓷膜厚度和硬度的影响;测定了相应质量浓度下的塔菲尔极化曲线,以此评价铝-锰陶瓷膜的耐蚀性;通过扫描电镜观察铝-锰陶瓷膜的微观形貌。     

Influences of electrolytes on tribocorrosion performance of MAO coating on AZ31B magnesium alloy in simulated body fluid

In this paper, the effects of electrolytes on the corrosion resistance and tribocorrosion performance of micro-arc oxidation (MAO) coatings on AZ31B magnesium (Mg) alloys in simulated body fluid (SBF) were studied. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were utilized to explore the microstructure, surface morphology, and phase components of the MAO coatings. Corrosion and tribocorrosion performance of MAO coated Mg alloys were evaluated by using potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and a ball-on-disk tribotester. It was found that MAO coating produced in electrolyte containing both Na₂SiO₃ and Na₂B₄O₇ exhibited superior corrosion resistance and tribocorrosion performance in the SBF.     

Electrochemical corrosion behavior of composite coatings of sealed MAO film on magnesium alloy AZ91D

Protective composite coatings were prepared on magnesium alloy AZ91D by micro-arc oxidation (MAO) treatment plus a top coating with sealing agent using multi-immersion technique under low-pressure conditions. The corrosion resistance of AZ91D alloy with composite coatings was superior evidently to that with merely MAO film. SEM observations revealed that the sealing agent was integrated with MAO film by physically interlocking; therewith covered uniformly the surface as well as penetrated into pores and micro-cracks of MAO film. The anti-corrosion properties in 3.5% NaCl solution of the composite coatings were evaluated by using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization measurements. Based on the results of chronopotentiometric (E ∼ t) and EIS measurements for long time immersion in 3.5% NaCl solution, appropriate equivalent circuits for the composite coatings system were proposed. It follows that due to the blocking effect of the sealing agent in pores and cracks in MAO film, the composite coatings can suppress the corrosion process by holding back the transfer or diffusion of electrolyte and corrosion products between the composite coatings and solution during immersion.     

Effect of ultra accurate control of electrolyte temperature on the performance of micro arc oxidation ceramic coatings

The technique of micro arc oxidation (MAO) uses arc discharge and high-voltage breakdown to produce a ceramic layer on valve metal surfaces. However, the common method of MAO requires immersing the workpiece in an electrolyte solution, which can result in elevated temperatures due to the arc discharge, thus negatively affecting the coating's quality and performance. This article investigates the influence of electrolyte temperature on the performance of MAO ceramic coatings, with the assistance of a robotic arm enabling valve metal reaction without immersion in the electrolyte, and precise control of electrolyte temperature through a MAO temperature monitoring system. Various techniques, such as scanning electron microscopy (SEM), hardness testing, electrochemical corrosion experiments, and friction-wear experiments, were utilized to characterize the performance of the prepared coating. The results indicate a nonlinear correlation between the temperature of the electrolyte and the thickness and hardness of the ceramic coating. The corrosion and wear resistance of the MAO ceramic coatings initially improve with increasing electrolyte temperature but eventually deteriorate. At an electrolyte temperature of 40 °C, the MAO ceramic coating exhibits the optimal corrosion and wear resistance. The variation in electrolyte temperature affects the reactivity of the electrolyte ions, leading to changes in the morphology and properties of the resulting MAO ceramic coating. These findings offer valuable insights into the interaction mechanism between electrolyte temperature and the properties of the resulting MAO ceramic coating. This is of great significance in optimizing the MAO process for specific applications and improving the overall performance of ceramic coatings.     

Tribological mechanism of micro-arc oxidation coatings prepared by different electrolyte systems in artificial seawater

The micro-arc oxidation (MAO) coatings were prepared in four different electrolyte systems, including mixed acid, phosphate, phosphate-aluminate and phosphate-silicate electrolytes. The friction and wear properties of MAO coatings in ambient air, seawater and four groups of saline solutions related to seawater were investigated. The results showed that the addition of silicate to phosphate could increase the density of the coating. The phosphate-aluminate ceramic layer exhibited the lowest wear rate in various environments. Additionally, the friction coefficient and wear rate of MAO coating in seawater were lower than those in ambient air, which was due to the boundary lubrication effect of seawater. Meanwhile, the presence of divalent metal salts in seawater made its lubricity better than other salt solutions.     

铈对镁合金微弧氧化膜微观结构和耐蚀性的影响

利用扫描电镜、电化学工作站等,研究了不同铈盐添加方法对ZE10镁合金在由硅酸钠14 g/L、氟化钠14g/L、氢氧化钠2 g/L、甘油5mL/L组成的溶液中,于电压300 V、脉冲频率1 kHz、占空比15％的条件下微弧氧化20 min所得微弧氧化膜厚度、形貌和耐蚀性的影响.结果表明,先进行铈盐转化处理再微弧氧化和在电...     

镁基材料表面微弧氧化生物医用陶瓷涂层研究进展

利用微弧氧化技术在镁基材料表面原位生成陶瓷膜层,能显著增强镁基材料的耐磨性、耐腐性、生物相容性等表面性能,在医用镁基材料表面改性中具有广泛的应用前景.本文主要对镁基材料表面微弧氧化所用的电解液组成、陶瓷膜的结构组成、复合生物涂层及其生物相容性的研究现状进行了评述,并对今后的研究前景进行了展望.     

Effect of Na2SiO3 concentration on corrosion resistance and wear resistance of MAO ceramic film on the Al-Mg-Sc alloy

Although Al-Mg-Sc alloy was widely applied to aviation aerospace field, they were vulnerable to local corrosion and wear in the process of long-term service in severe environmental conditions. In this paper, micro-arc oxidation (MAO) ceramic films on Al-Mg-Sc alloy substrate were prepared in electrolyte solutions with different Na₂SiO₃ concentrations, and the corrosion resistance and wear resistance of the MAO samples were studied. The experimental results of potentiodynamic polarization (PDS) and long-term immersion tests indicated that the MAO ceramic film prepared in 10 g/L Na₂SiO₃ electrolyte solution had the best corrosion resistance, as manifested by no obvious cracks, serious collapse and corrosion products on the sample surface and no deep cracks and corrosion paths in the cross-sectional area. The increase of Na₂SiO₃ concentration in electrolyte solution also improved the wear resistance of MAO ceramic film, as manifested by low wear depth (10 μm)and width (1 mm) of the MAO ceramic film prepared in 10 g/L Na₂SiO₃ electrolyte solution against GCr15 steel ball. Studies in mechanisms suggested that as the Na₂SiO₃ concentration in the electrolyte increased, the MAO ceramic film became denser, which could prevent the penetration of corrosive medium, promote the generation of the anti-wear layer with SiO₂ as the main component to enhance the wear resistance. MAO ceramic film formed in Na₂SiO₃ electrolyte solution provided good protective performance for Al-Mg-Sc alloy in the corrosion and wear conditions, which had a broader application prospect.     

Electrochemical study of corrosion behavior of graphene coatings on copper and aluminum in a chloride solution

Electrochemical characteristics and corrosion behavior of graphene coatings on Cu and Al in a 0.1 M NaCl solution were investigated. The graphene coatings were deposited on a Cu surface by chemical vapor deposition. Multiple graphene layers were then mechanically transferred from the growth substrate, Cu, onto Al surface by a transfer technique. The corrosion stability of graphene coatings was determined by electrochemical impedance spectroscopy and open circuit potential, while the corrosion rate was evaluated using potentiodynamic sweep measurements. Surface morphologies of the graphene coatings were analyzed by scanning electron microscopy and energy dispersive spectroscopy. Obtained results indicate that Cu coated with graphene grown using chemical vapor deposition shows corrosion-inhibiting properties in 0.1 M NaCl. On the other hand, Al coated with a multilayer graphene film mechanically transferred from the Cu surface exhibits electrochemical characteristics similar to an Al oxide on bare Al. Better protective properties of graphene coating on Cu compared to the graphene coating on Al were observed, probably due to the breakage of Al oxide film, causing the corrosion of Al to proceed rapidly in the presence of chloride electrolyte.     

Electrochemical corrosion behaviour of plasma electrolytic oxidation coatings on AM50 magnesium alloy formed in silicate and phosphate based electrolytes

PEO coatings were produced on AM50 magnesium alloy by plasma electrolytic oxidation process in silicate and phosphate based electrolytes using a pulsed DC power source. The microstructure and composition of the PEO coatings were analyzed by scanning electron microscopy (SEM) and X-ray Diffraction (XRD). The corrosion resistance of the PEO coatings was evaluated using open circuit potential (OCP) measurements, potentiodynamic polarisation tests and electrochemical impedance spectroscopy (EIS) in 0.1 M NaCl solution. It was found that the electrolyte composition has a significant effect on the coating evolution and on the resulting coating characteristics, such as microstructure, composition, coating thickness, roughness and thus on the corrosion behaviour. The corrosion resistance of the PEO coating formed in silicate electrolyte was found to be superior to that formed in phosphate electrolyte in both the short-term and long-term electrochemical corrosion tests.     

Effect of Fe3+ and F− on black micro-arc oxidation ceramic coating of magnesium alloy

This study focuses on a black micro-arc oxidation ceramic coating prepared on the surface of magnesium alloy by the technology of micro-arc oxidation in the electrolyte containing F^– and Fe³⁺ as well as its mechanism of F^– and Fe³⁺. It needs coatings to experience detail analyses on their thickness, roughness, corrosion resistance, thermal control property, valence states of elements, phase composition, and morphology of coatings, respectively, through coating thickness gauge, roughness tester, electrochemical workstation, AE radiometer, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electron microscope (SEM), and energy-dispersive X-ray spectroscopy (EDS). Results showed that with the help of F^– in electrolyte, Fe³⁺ can be complexed and MgF₂ can be obtained in the coating, which reduces the pores on the surface of micro-arc oxidation coating. In addition, Fe³⁺ in the electrolyte contributes to the preparation of Fe₂O₃ and Fe₃O₄ in the coating, which can blacken the surface of the coating. Both F^– and Fe³⁺ benefit to improve the corrosion resistance and thermal control performance of micro-arc oxidation coating. There is higher iron oxide in the outer layer but higher fluoride in the inner layer of the coating.     

Effect of cobalt oxide on characteristics of micro-arc oxidation coatings on TC11 alloy

In order to improve the comprehensive performance of TC11 titanium alloy and make it more widely used in the field of oil and gas wells, cobalt oxide micro-powder from 0 to 0.8 g·L⁻¹ was added in the electrolyte. The effects of cobalt oxide micro-powder concentration on properties of micro-arc oxidation (MAO) coatings were analyzed by scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and electrochemical workstation. With cobalt oxide micro-powder increased, the oxide voltage increased, and the discharge coating micropores became more uniform. The increase in thickness and hardness also improved corrosion resistance. The comprehensive performance after added 0.6 g·L⁻¹ of cobalt oxide was the best, with a doubling increase in thickness and hardness, reaching 32.1 µm and 721.2 HV, respectively. The corrosion rate reduced to 4.19 × 10⁻¹⁰ mm·a⁻¹, which reduced by three orders of magnitude. In addition, the substrates and samples prepared with 0.6 g·L⁻¹ cobalt oxidation were used for the erosion corrosion tests in different NaCl solutions, and it was found that the weight loss and rate of the MAO sample (slight corrosion) were lower than substrates, indicating that the cobalt oxide doped coatings had better corrosion resistance.     

Fabrication of Ti/TiO2(Ca)/hydroxyapatite bioceramic material by micro-arc oxidation and electrochemical deposition

Titanium with a bioceramic hydroxyapatite (HA) coating has been widely used in biomaterials owing to its excellent mechanical characteristics and high osteoconductivity. However, the interfacial strength of Ti/HA prepared by electrochemical deposition (ED) is relatively low because the physical combination is typically inadequate. In this study, to improve the interfacial strength, a micro-arc oxidation (MAO) process with calcium was introduced for preparing a connecting interlayer known as the MAO coating. Pulsed ED was employed to synthesise the HA coating on the MAO surface using an electrolyte with 6 wt% H₂O₂. Sample characterisations revealed that the MAO coating comprised porous TiO₂ (rutile and anatase) with Ca or CaTiO₃. The formation of CaTiO₃ depends on the current density, reaction time, and concentration of Ca²⁺, in addition to voltage. The MAO coatings exhibited a higher corrosion resistance than that exhibited by Ti substrates. Furthermore, the HA coating on the MAO coating was confirmed to be plate-like Ca-deficient HA. The final sample had a Ti/TiO₂(Ca)/HA structure, and its adhesive strength was approximately double that of the Ti/HA sample. In particular, the MAO coating synthesised at a high Ca²⁺ concentration exhibited an improved adhesive strength (2.326 MPa). The application of the MAO coating containing Ca as a connecting interlayer is a promising strategy for improving the HA adhesion strength.     

镁合金微弧氧化膜的制备及其性能研究

通过对微弧氧化电解液进行优化,得出了最佳配方,并对微弧氧化膜的性能进行了测试。镁合金微弧氧化的最佳工艺条件为:Na_2SiO_320g/L,Na_2B_4O_730g/L,NaOH 30g/L,氧化时间20min。向电解液中加入醋酸镍后,生成含有Ni_2SiO_4的深灰色微弧氧化膜,该膜层较厚且耐蚀性进一步提高。微弧氧化膜表面光滑、致密,但存在少量的微孔和裂纹。微弧氧化膜的主要成分为MgO、Mg_2SiO_4、Ni_2SiO_4和SiO_2,微弧氧化处理显著提高了镁合金基体的耐蚀性。     

Preparation and properties of wear-resistant carbonized-ceramic composite coating on pure aluminum surface

In this paper, a new composite coating was prepared on the surface of pure aluminum (Al) by combining the micro-arc oxidation (MAO) technology with the polyethylene glycol (PEG 400) carbonization technology. The composite coating and the single MAO coating were observed by a scanning electron microscope, and an energy-dispersive spectrometer, finding that the single MAO coating surface with volcano-like pores and microcracks, was covered by the carbonized layer of the composite coating where the overall coating thickness was around 19.5 µm, including 17.5 µm of inner MAO coating. The material properties of the composite coating were characterized by X-ray diffraction and X-ray photoelectron spectroscopy. The wear resistance of the composite coating was tested under dry friction conditions, finding that the wear width on the composite coating surface was 909.6 µm only, which was around 55.7%, 50.4%, and 58.2% of those for pure Al substrate, single carbonized coating, and single MAO coating, respectively. Then the comprehensive wear resistance of the composite coating was explored under different sliding speeds and lubrication mediums. Finally, the wear-resisting mechanism of the composite coating was discussed, concluding that the composite coating could effectively reduce adhesive wear and abrasive wear of the Al substrate.     

Effect of various additives on morphology and corrosion behavior of ceramic coatings developed on AZ31 magnesium alloy by plasma electrolytic oxidation

Plasma electrolytic oxidation (PEO) coatings were developed on AZ31 magnesium alloy using alkaline silicate with KOH as a base electrolyte system, and with the addition of sodium aluminate, sodium tetra borate, potassium titanium fluoride, tri sodium ortho phosphate and urea as additives. The phase composition and surface morphology of these multi-phase coatings were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The corrosion behavior of the coated samples was evaluated by potentiodynamic polarization tests and electrochemical impedance spectroscopy (EIS) in 3.5 wt.% NaCl solution. The results showed that the anions namely, SiO₃^2?, AlO₂^?, B₄O₇^2?, F^? and PO₄^3?, effectively participated in the coating formation influencing its chemical composition and surface morphology and thereby corrosion resistance. The mechanism of corrosion process of each coating was explained in detail with the help of Electrochemical Impedance Spectroscopy (EIS) analysis and equivalent circuit modeling. It was observed that the sample treated by PEO in the electrolyte solution containing sodium tetra borate as an additive showed higher corrosion resistance which could be attributed to its morphological characteristics.     

Formation of a compact oxide layer on AZ91D magnesium alloy by microarc oxidation via addition of cerium chloride into the MAO electrolyte

Magnesium and its alloys have been used in many industries, but they are reactive and require protection against aggressive environments. In this study, a compact and relatively pore-free oxide coating was formed on AZ91D magnesium alloy to improve its corrosion resistance by means of the microarc oxidation (MAO) process via the addition of CeCl₃ as an additive into the MAO electrolyte. Morphologies and compositions of the coatings were studied with scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), respectively. It was found that with the addition of CeCl₃ into the MAO electrolyte, the obtained coating showed more uniform morphology compared with the coating produced in the CeCl₃ free electrolyte. EDS analysis confirmed the presence of cerium in the coating formed in the electrolyte with CeCl₃. Polarization tests results showed that the corrosion current density of the coating formed in the CeCl₃ containing electrolyte was less than that of the coating formed in the electrolyte without CeCl₃. Furthermore, electrochemical impedance spectroscopy (EIS) tests indicated that the coating formed in the electrolyte with the addition of CeCl₃ improved the corrosion resistance of the substrate significantly.