Corrosion Resistance Of Micro-arc Oxidation Coatings Research Articles

Effect of annealing treatment of hot-rolled AZ31 magnesium alloy on properties and stress corrosion resistance of MAO coatings

Purpose This study aims to study the formation mechanism of micro-arc oxidation (MAO) coating on AZ31 magnesium alloy and how the annealing process affects its corrosion resistance. Design/methodology/approach This study involved immersion experiments, electrochemical experiments and slow strain rate tensile experiments, along with scanning electron microscopy, optical microscopy observation and X-ray diffraction analysis. Findings The findings suggest that annealing treatment can refine the grain size of AZ31 magnesium alloy to an average of 6.9 µm at 300°C. The change in grain size leads to a change in conductivity, which affects the performance of MAO coatings. The MAO coating obtained by annealing the substrate at 300°C has smaller pores and porosity, resulting in better adhesion and wear resistance. Originality/value The coating acts as a barrier to prevent corrosive substances from entering the substrate. However, the smaller pores and porosity reduce the channels for the corrosive solution to pass through the coating. When the coating cracks or falls off, the corrosive medium and substrate come into direct contact. Smaller and uniform grains have better corrosion resistance.

Corrosion resistance and antibacterial properties of hydrophobic modified Ce-doped micro-arc oxidation coating

Aluminium (Al) alloys with high corrosion resistance and bactericidal capability will have significant potential for broader utilization in marine applications. To achieve this, a layer of cerium (Ce)-doped anodic oxidation (AO) film was prepared on the surface of the Al alloy to modify the pore structure of the micro-arc oxidation (MAO) coating. This not only improved its corrosion resistance but also imparted certain antibacterial properties to the coating. Then, 1H, 1H, 2H, 2H- perfluoroalkyltriethyloxy-silane (PFOTES) was used to hydrophobically modify the coating, which more effectively obstructed corrosive ions and bacteria, further elevating its corrosion resistance and antibacterial properties. Electrochemical tests demonstrated that |Z|0.01Hz of the modified coating was 2.86 × 109 Ω·cm2 which was two orders of magnitude higher than before. Furthermore, it remained above 109 Ω·cm2 even after 288 h of immersion. Moreover, the antibacterial effectiveness of the modified coating against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) increased by 5.89% and 19.53%, respectively, to 98.58% and 97.34%. These findings indicate a promising direction for improving the corrosion resistance of MAO coatings on metallic materials, paving the way for large-scale industrialization.

Mechanistic analysis of improving the corrosion performance of MAO coatings on Ti-6Al-4V alloys by annealing

Titanium and its alloys find extensive application within the realms of dentistry and orthopedics due to their remarkable chemical stability. Nevertheless, they are still susceptible to corrosion in harsh environments. Micro-arc oxidation (MAO) emerges as a potent methodology for enhancing the corrosion resistance of titanium and its alloys, achieved through the generation of a ceramic stratum atop the material's surface. The corrosion resistance of MAO coatings in clinical applications is closely related to their microstructure. This manuscript systematically explores the effect of annealing temperature on the phase composition and microstructure of MAO coatings. Furthermore, the evolution of the surface properties and the crystal structure of coatings is meticulously discussed and illustrated. The results show that the corrosion resistance of coatings exhibits an ascending trajectory followed by a subsequent decline, coinciding with the augmentation of the annealing temperature. This behavior can be attributed to the structural evolution of coatings caused by the gradual increase of the annealing temperature. At the optimum temperature (400 °C), the annealing treatment leads to the crystallization of the coating as well as to a decrease in the surface porosity, resulting in the formation of a compact and continuous multilayer microstructure. The decrease in porosity can be attributed to the diffusion-reaction of oxygen from the air with titanium from the discharge channel (titanium oxide), resulting in the formation of TiO2 particles that effectively fill the pores. The decrease in corrosion resistance of the coatings after annealing at 800 °C is due to the phase transition of TiO2 which leads to the expansion of cracks inherent in the oxide layer to form A-H cracks. Therefore, this study shows that the microstructure of MAO coatings can be adjusted by post annealing treatment (400 °C) to obtain a desirable oxide layer with appropriate porosity, crystallinity and chemical composition, thus improving its corrosion resistance.

Final surface modification for better wear resistance of ceramic coating on cast AlSi10Mg alloy

Using the design of experiment (DOE) method and the micro-arc oxidation (MAO) technique, ceramic layers on AlSi10Mg alloy were systematically prepared to design optimal process conditions for achieving the best tribological properties of the ceramic layers. The lowest concentrations of the applied 6 g/l NaOH and 12 g/l Na 2 SiO 3 resulted in the preparation of uniform MAO layers with the lowest rated parameters R a , R z and thickness achieved under micro-arc discharge conditions at 500 V and 60 min. With the increasing thickness of the coatings, there was an increase of Si in the MAO coating. Full factorial DOE was used to optimize the tribological properties in a polyalphaolefin (PAO) environment at 80 °C. The most significant influence for the preparation of abrasion-resistant layers for the investigated factors was identified on the AlSi10Mg alloy by the NaOH content in the electrolyte. The friction coefficients of MAO coatings reached an average value of 0.15. Aero-lap polishing technology was applied for increased wear resistance requirements to eliminate the deficiencies of MAO coatings, leading to decrease wear track by almost double compared to polished silumin. Removal of the outer MAO layer by polishing led to a reduction in the high corrosion resistance of the MAO coating, demonstrating the influence of the outer layer not only on the tribological properties but also on the corrosion resistance of MAO coatings.

Effect of Na2SiO3 concentration on microstructure and corrosion resistance of MAO coatings prepared on Al-Mg-Sc alloys

PurposeThe purpose of this paper is to investigate the effect of Na2SiO3 concentration on the microstructure and corrosive properties of microarc oxidation (MAO) coating on Al-Mg-Sc alloy and explore microstructure evolution rule of Al substrate in the contact area.Design/methodology/approachThe Na2SiO3 concentration in electrolytes influenced the microstructure and corrosion behavior of MAO coatings. Instantaneous high temperature and high pressure due to microarc discharge caused annealing treatment. The corrosive behavior of the MAO coating was featured with polarization curves and electrochemical impedance spectrum in 3.5 Wt.% NaCl solution.FindingsThe substrate in the contact area existed the instantaneous annealing treatment, which caused obvious recrystallization. The coating prepared in electrolyte containing 7 g/L Na2SiO3 exhibited the highest protective properties in 3.5 Wt.% NaCl solution.Originality/valueMAO treatment could increase the corrosion resistance by producing a protective layer on the Al-Mg-Sc alloy surface at a suitable Na2SiO3 concentration and microstructure evolution rule of Al substrate in the contact area was obtained.

Effect of In2(SO4)3 on characteristics of Al–Li alloy MAO coating

ABSTRACT Micro arc oxidation (MAO) coatings were prepared on the surface of 2195 Al–Li alloy by adding different amounts of indium sulphate (In2(SO4)3) into an electrolyte consisting of silicate and phosphate. The surface, cross section, phase composition, and electrochemical experiments of MAO coatings were analysed. X-ray diffraction results show that the coating was composed of γ-Al2O3, α-Al2O3, and In2O3. The results show that after adding In2(SO4)3, the discharge holes on the surface of the coating were more uniform and the thickness was also increased. The corrosion resistance of MAO coatings was improved by the addition of In2(SO4)3.

Investigations on the thermal control properties and corrosion resistance of MAO coatings prepared on Mg-5Y-7Gd-1Nd-0.5Zr alloy

In this paper, the micro arc oxidation (MAO) method was utilized to prepare the ceramic coatings on Mg-5Y-7Gd-1Nd-0.5Zr substrate. Effects of different reaction time and NH4VO3 concentration on the morphology, composition, absorptance, emissivity, corrosion resistance and adhesion strength of coatings were investigated. These results show that the as-prepared coatings were mainly composed of Mg, Si, P, V, O and Na elements, and all of them existed as amorphous state. With the increases of reaction time and NH4VO3 concentration could improve the thickness, roughness and reduce the lightness (L value), and so the absorptance and emissivity of prepared coatings increased. Meanwhile, the corrosion resistance of coatings was much better than that of Mg alloy, exhibiting an increasing tendency as the reaction time and NH4VO3 concentration. However, too more NH4VO3 might cause many pores and micro-cracks generated on the surface of coatings, which slightly reduced the corrosion resistance. Although scratch tests denote that the adhesion strength of coatings decreased with the increase of thickness, T4/N4 with the maximum thickness still had good adhesion to the substrate and remained stable after thermal shock experiments. In general, T4/N4 possessed the optimal comprehensive performances, which would expand the application range of Mg-5Y-7Gd-1Nd-0.5Zr in aerospace and optics.

Properties of Micro-Arc Oxidation Coating Fabricated on Magnesium Under Two Steps Current-Decreasing Mode

A concept of two steps current-decreasing mode derived from constant current mode was developed to fabricate micro-arc oxidation (MAO) coatings on ZK60 Magnesium (Mg) alloy in a dual electrolyte system. The growth characteristics of coatings were analyzed by voltage-time curves, and the coating microstructures were characterized by scanning electron microscopy (SEM). Meanwhile, the roughness, corrosion behavior and microhardness of MAO coatings were investigated. The results showed that the MAO coatings exhibit a smooth and compact surface and have improved hardness and thickness, smaller roughness, uneven distribution of holes and even low electrical energy consumption. Such positive characteristics results in an improved corrosion resistance of MAO coatings. The coating produced under the two steps current mode of “1.2-0.6A” showed a smaller corrosion rate of 0.1559 g/m2h as compared to the counterpart produced under the other modes. The nanoscratch tests results showed that the coating fabricated by “1.2-0.6A” mode has strong bond strength with the substrate. Under this optimized mode, the MAO process also had the lowest energy consumption of 49.8 W·(dm2·μm)-1.

Corrosion Resistance of MAO Coatings on Al-Si Alloys

The paper presents the results of an experimental study of the influence of micro-arc oxidation (MAO) on the parameters and the corrosion resistance of MAO-treated surfaces on a high-silicon aluminum alloy, M244 (Si ~ 25 %). The article describes the study methodology, comprising forming a coating on the surface of laboratory samples by means of MAO at different process modes and the study of their parameters, properties, and corrosion resistance. The experiments were conducted in accordance with the 23 full factorial experiment design theory. The MAO process was carried out using a silicate-alkaline electrolyte. MAO factors were as follows: the concentration of the electrolyte components (potassium hydroxide – KOH and liquid glass – Na2SiO3) and electrical parameters of the process, determined by the capacitor capacitance of the processing unit. The samples were tested to define thickness, porosity and micro-hardness of the MAO layer. Being corroded in a corrosive solution for 144 hours, their corrosion resistance was estimated by a mass corrosion rate. The data obtained made it possible to form mathematical models, based on the dependence of corrosion resistance, thickness, porosity and micro-hardness on the process factors. A verification of the obtained models was carried out to determine adequacy and their analysis. Conclusions on the extent of the MAO effect on the corrosion resistance of the samples were drawn. MAO modes were stated to have a significant impact on the corrosion resistance of the samples. MAO should be carried out using low electrolyte concentrations and a low-capacity processing unit for improved corrosion-protective properties of the coating on the M244 (Mahle) aluminum alloy. It is necessary to increase the concentration of the electrolyte components and the processing unit capacity, to obtain a thicker coating with low micro-hardness and porosity.

Corrosion resistance and biocompatibility of calcium-containing coatings developed in near-neutral solutions containing phytic acid and phosphoric acid on AZ31B alloy

Calcium phosphate (Ca-P) coatings were fabricated by micro-arc oxidation (MAO) on AZ31B magnesium alloy in near-neutral pH solutions (pH 7.6–7.9) and the influences of EDTA-CaNa2, phytic acid (IP6), phosphoric acid (PA), and treatment time on coating properties were investigated by an orthogonal experiment. The results show that coating corrosion resistance is synergistically determined by coating characteristics with coating thickness playing a particularly important role. EDTA-CaNa2 acts as a corrosive agent of magnesium alloys and the increased concentration increases calcium content but decreases corrosion resistance of MAO coatings. As a strong chelating agent, IP6 can promote EDTA-CaNa2 solubility and therefore increases the calcium content more effectively than PA does. Compared with PA, IP6 more effectively improves corrosion resistance mainly by increasing coating thickness and bonding strength between coating and substrate, although the uniformity of anodic coatings becomes worse due to the greater interfacial tension of the IP6 solution than that of the PA solution. During MAO, P and F compete with each other to enter into anodic coatings. F amount in MAO coatings is closely related to the coating biocompatibility. MAO coatings developed in the solution composed of both IP6 and PA achieve low F content (3.49 at%) and good biocompatibility, while those fabricated in solutions containing only IP6 or PA achieve high F content (higher than 19.00 at%) and exhibit high toxicity.

A comparative study on the microstructure and corrosion resistance of MAO coatings prepared in alkaline and acidic electrolytes

Ceramic coatings were prepared on 6061 aluminum alloy using micro-arc oxidation (MAO) method in alkaline (pH = 11.3) and acidic (pH = 2.78) electrolytes, respectively. The MAO process was carried out in constant current mode using a unipolar pulse power supply. The voltage variation curves and discharge phenomena have been recorded during the process of MAO. For further comparison of the surface and cross-section microstructures, phase composition and corrosion resistance between the ceramic coatings produced in two different electrolytes, the scanning electron microscopy (SEM), x-ray diffraction (XRD), potentiodynamic polarization tests and electrochemical impedance spectroscopy (EIS) were employed. It was found that there were great differences in the microstructures, and the discharge intensity in alkaline electrolyte was much stronger than that in acidic electrolyte. The coatings obtained in alkaline electrolyte were composed of Al2O3, while coatings fabricated in acidic electrolyte was composed of ZrO2 and Al2O3. Besides, the coatings prepared in acidic electrolyte showed better corrosion resistance than that in alkaline electrolyte due to the presence of ZrO2.

Investigation on Corrosion Resistance and Formation Mechanism of a P–F–Zr Contained Micro-Arc Oxidation Coating on AZ31B Magnesium Alloy Using an Orthogonal Method

In this study, the synergistic effects of NH4HF2, sodium phytate (Na12Phy), K2ZrF6, and treatment time on corrosion resistance of a micro-arc oxidation (MAO) treated magnesium alloy and the entrance mechanism of P, F, and Zr into anodic coatings were investigated using an orthogonal method. In addition, the roles of NH4HF2, Na12Phy, and K2ZrF6 on coating development were separately studied. The results show that NH4HF2 and Na12Phy, the corrosion inhibitors of magnesium alloys, are beneficial but K2ZrF6 is harmful to developing anodic coatings. The corrosion resistance of MAO coatings is synergistically determined by coating characteristics, though the coating thickness plays a main role. Na12Phy significantly improves but NH4HF2 decreases the corrosion resistance of MAO coatings, while excess high K2ZrF6 is harmful to the coating corrosion resistance. Treatment time can increase the coating thickness but is the least important factor in corrosion resistance. During MAO, NH4HF2, Na12Phy, and K2ZrF6 take part in coating formation, causing P, F, and Zr to compete with each other to enter into anodic coatings.

Microstructure and corrosion resistance of MAO coatings on AZ31 magnesium

In this study, ceramic coatings were deposited on AZ31 magnesium alloy by micro arc oxidation (MAO) technique in sodium phosphate electrolytes (Na3PO4.12H2O) containing different concentrations of sodium hydroxide (NaOH) and borax (Na2B4O7) under voltage of 420 V for 5 min. Hank’s solution was used to investigate the corrosion behavior of the samples. Potentiodynamic polarization and electrochemical impedance spectroscopy were also employed. X-ray diffraction and scanning electron microscopy were applied for phase and microstructure characterization. The prepared MAO layers consisted mainly of ZnO and Mg3PO4. Results indicated that use of micro arc oxidation technique for coating may result in significant increase of corrosion resistance of magnesium alloy in Hank’s solution. One of the important parameters which affects this coatings properties is the electrolyte composition which used in the process. The coating deposited by application of 10 g/l Na3PO4 + 1.5 g/l NaOH, exhibited the highest corrosion resistance in Hank’s solution among the samples.

Effect of Current Density on the Microstructure and Corrosion Properties of MAO Coatings on Aluminum Alloy Shock Absorber

In order to improve the corrosion resistance of shock absorber for ships, the alumina ceramic coatings are carried out on the surface of aluminum alloy shock absorber by micro arc oxidation (MAO) technology. The microstructure and anti-corrosion performance of the MAO coatings were investigated experimentally. This paper mainly focuses on the experimental work to determine the effect of current density on the structural characteristics and corrosion resistance of MAO coatings. The results show that the current density has a significant influence on the preparation of MAO coating during the process. The surface of the coating becomes more compact and smooth with the cathode voltage of 7 A.dm-2. Furthermore, the anti-corrosion performance of the MAO coatings can effectively be improved at the current density of 7 A.dm-2.

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.

Preparation, antibacterial effects and corrosion resistant of porous Cu–TiO2 coatings

Abstract Antibacterial TiO 2 coatings with different concentrations of Cu (Cu–TiO 2 ) were prepared by micro-arc oxidation (MAO) on pre-sputtered CuTi films. The effect of Cu concentrations in CuTi films on the MAO process was investigated. The Cu–TiO 2 coatings were analyzed by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). The corrosion resistance of Cu–TiO 2 coatings was evaluated via potentiodynamic polarization method. The antibacterial properties were assessed by two methods: spread plate method and fluorescence staining. The experimental results demonstrate that the coatings are porous and consist of anatase phase, rutile phase and unoxidized titanium. The CuTi films are almost completely oxidized and the thickness of all MAO coatings is about 5–10 μm. Cu mainly exists as CuO in the TiO 2 coatings. The Cu–TiO 2 coatings exhibit excellent antibacterial activities, and the antibacterial rate gradually rise with the increase in Cu concentration in the MAO coatings. The corrosion resistance of MAO coatings is also improved slightly.

Influence of K2TiF6 in electrolyte on characteristics of the microarc oxidation coating on aluminum alloy

Black and gray microarc oxidation (MAO) coatings were prepared in a phosphate electrolyte with and without K2TiF6 on 2A70 aluminum alloy, respectively. Voltage–time curves were recorded during the MAO process. The effects of K2TiF6 on the morphology, composition, abrasive resistance and corrosion resistance of MAO coatings were investigated. The results showed that the MAO coating produced in the electrolyte with K2TiF6 was thicker, and more uniform than that produced in the electrolyte without K2TiF6. Ti was detected in the surface of the MAO coating formed in the electrolyte with K2TiF6. The results of abrasive resistance and corrosion resistance showed that the MAO coating formed in the electrolyte with K2TiF6 exhibited better abrasive resistance and corrosion resistance.

Study on the Corrosion Resistance and Wear Resistance of Micro-Arc Oxidation Coatings on the Clad Plate of Aluminum and Magnesium Alloy

Aluminium plate was cladded to magnesium alloy plate by using the explosive welding. The bonding morphology and composition of the explosive cladding plate was inspected by SEM and EDS. There is a wave bonding at the interface between aluminum plate and magnesium alloy plate. Then ceramic coatings were directly prepared on the surface of aluminum and magnesium alloy by micro-arc oxidation (MAO) in the same solution and at the same time. The microstructure and composition of MAO coatings were studies by SEM and EDS. The corrosion and wear resistance of MAO coatings on the two sides of the clad plate were investigated by salt spray tests and friction-wear test. The results show that the MAO coating on the Al surface consists of Al, O and Si elements, while MAO coating on the Mg surface consists of Mg, O and Si elements. The corrosion resistance of MAO coating on the Al surface was better than that on Mg surface of the explosive clad plate. The MAO coatings both on the Al surface and on the Mg surface can obviously improve the wear resistance of substrate.

Influence of Current Density on Structure and Performance of Micro-Arc Oxidation Coatings on AZ31 Magnesium Alloy

In the Na2SiO3-NaOH-C3H8O3electrolyte system,the micro-arc oxidation (MAO) coatings were prepared on AZ31 magnesium alloy when J=2A/dm2,4A/dm2,8A/dm2and 12A/dm2.The relations of current density with microstructure and performance of the MAO coatings on AZ31 were studied by means of SEM, XRD, HVS-1000 micro Vicker, MFT-4000 Scratch Tester and electro-chemical method. The results show that when current density increases,the thickness,growth rate,micro-hardness and the contents of MgO and Mg2SiO4of MAO coatings increase,but the adhesion force between MAO coating and substrate decreases, the millipores and the roughness become larger.The corrosion resistance of MAO coatings increases firstly and then decreases with the increasing current density.The corrosion resistance of MAO coating,which J=6 A/dm2,is the best in 4.5% NaCl water solution,whose corrosion current density reduces by 3 orders of magnitude, and corrosion potential increases 1078mV in positive direction.

Effects of KMnO4 on microstructure and corrosion resistance of microarc oxidation coatings on 2024 aluminum alloy

Microarc oxidation (MAO) coatings were prepared on 2024 aluminum alloy in a Na2SiO3-KOH electrolyte with KMnO4 addition varying from 0 to 4 g/L. The microstructure and phases of the coatings were characterized by scanning electron microscopy (SEM) and X-ray diffractometry (XRD), respectively. The corrosion resistance of MAO coatings was evaluated by electrochemical potentiodynamic polarization in 5% (mass fraction) NaCl solution. The results show that when KMnO4 is added into base electrolyte, the growth speed of oxide coatings is increased obviously. The main phase of oxide coatings is Al2O3, and the contents of MnO2 and Mn2AlO4 phases are increased at the top of oxide coatings with increasing the concentration of KMnO4. The solute elements participate in forming the oxide coatings. When a proper concentration of KMnO4 (2.5 g/L) is added into the base solution, the micropores of the MAO coatings are small and compact, and the corrosion resistance of oxide coatings is increased largely.