Anodic Alumina Films Research Articles

Anodization of Aluminum in a Sodium Hydroxide Solution: Effect of pH on Chemical Dissolution

Abstract The chemical dissolution of anodic alumina film was investigated using the re-anodization technique. The formation of thick oxide films in an alkaline electrolyte is thought to be difficult to achieve due to the high pH value and high solubility of anodic alumina. The dissolution rate of anodic film was found to be strongly affected by the pH of the solution used for chemical dissolution; the film dissolved significantly faster in a sodium hydroxide solution having a high pH of 13.11 than in acidic solutions commonly used for anodization, such as sulfuric acid (pH 0.98) and phosphoric acid (pH 1.54). Nevertheless, the addition of glycerol to the sodium hydroxide solution effectively suppressed the chemical dissolution of the anodic film. The change in solubility of the anodic alumina film in solution was greatly affected by the change in the dissociation of the solute in solution. The results demonstrated that oxide films more than 10 μm thick were produced in an alkaline electrolyte by adding glycerol to the solution. The suppression of the chemical dissolution of alumina by the addition of alcohol has thus been shown to occur not only in acid solutions but also in alkaline solutions.

Anodic Oxide Films Grown in Dehydrated Phosphate/Glycerol Electrolytes at High Temperatures.

Traditional anodization was generally conducted in aqueous or water-containing solutions at room temperature or even lower temperatures. The two best known examples of the application of traditional anodization in the fabrication of nanostructured oxides are porous anodic alumina and titania nanotube films. In 1998, Melody et al. first observed that anodic oxide films on some valve metals such as tantalum can continue to grow at low anodization voltages in phosphate/glycerol solutions containing less than 0.1% water at elevated temperatures (>150 °C) in a non-thickness-limited (NTL) manner. Clearly, this type of anodization at high temperatures is conceptually different from traditional anodization. This Perspective provides a relatively thorough review of this topic. Particular attention is given to the main features for the NTL growth of anodic oxide films and the mechanism behind this behavior. The anodization behaviors for the NTL growth and typical microscopic morphologies of the resultant films on various metals are discussed. The main applications of the attained anodic films with different nanostructures and anodization in other dehydrated electrolyte systems such as the molten orthophosphoric acid are also involved. Finally, we highlight some unresolved issues for this type of anodization at high temperatures and the potential future directions of this field.

Synthesis, characterization, and optical sensing of hydrophilic anodic alumina films

Herein, the anodization of 2.6 μm Al-0.2 at. % Cu (Al-0.2Cu) films on TiN/p-type Si substrate were performed using common acidic mediums at different ranges of anodization voltage (Va) to produce porous anodic alumina (PAA) nanostructures with high porosity. The anodization of Al-0.2Cu in 1 M H2SO4 at Va range of 10–30 V produced a higher oxide thickness, and hence, a higher volume expansion factor, compared to the anodization in 0.75 M H3PO4 at Va range of 100–140 V. The increase in Va, as expected, increases the inter-pore distance, pore size, and porosity of the PAA and improves the anodization rate. The optical sensing performance of the synthesized PAA under different conditions was investigated. The maximum surface wettability was attained for PAA anodized in 1 M H2SO4 at 20 and 30 V. Moreover, the Vickers hardness (HV) of PAA was improved by forming a thin alumina layer on the Al-0.2Cu surface, and the anodized Al-0.2Cu in 0.3 M C2H2O4 revealed the highest HV compared to other acids. The synthesized PAA was tested as an optical sensor for some liquids by monitoring the refractive index. The synthesized PAA structure demonstrated a sensitivity of 180 nm/RIU. The anodization of Al-0.2Cu films using a one-step anodization process in different acidic mediums, and the resulting multifunctional properties (improved hardness, tunable wettability, and competitive sensitivity) existing a novel and potentially impactful approach.

Influence of micro-arc oxidation on the microstructure and dielectric properties of anodic aluminum oxide

To improve the dielectric performance of the anodic alumina film used in aluminum electrolytic capacitors, this study comparatively investigated the microstructure and dielectric properties of anodic aluminum oxide obtained through micro-arc oxidation (MAO) and conventional anodic oxidation (CAO). It is found that from the perspective of microstructure, the internal structure of the MAO treated oxide film has more and larger pores than that of CAO. This was attributed to the generation and overflow of numerous oxygen bubbles from within the oxide film at the locations where plasma sparks occurred during the process, thus forming larger pores. Regarding dielectric properties, the leakage current of the oxide film after MAO treatment was significantly reduced compared to CAO, with reductions of 58%, 56%, 64%, and 74% for the tested electrolytes Y1–Y4, respectively.

Effect of voltage and time on microstructure and corrosion resistance of anodic oxidation film on Al-5.5Zn-0.03In-1Mg-0.03Ti-0.08Sn alloy

Anodic aluminum oxide (AAO) films were prepared on the surface of Al-5.5Zn-0.03In-1Mg-0.03Ti-0.08Sn alloy in a 160 g/L H2SO4 electrolyte. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), immersion weight loss, and electrochemical testing were used to analyze the element and phase composition, surface and cross-section morphology and corrosion resistance of AAO films in NaCl solution. The effects of anodic oxidation voltage and time on the microstructure and corrosion resistance of AAO films were discussed. The results show that the AAO film on the surface of Al-5.5Zn-0.03In-1Mg-0.03Ti-0.08Sn alloy was mainly composed of Al2O3, with a dense barrier layer near the aluminum matrix and a porous layer on the outer layer. With the increase of anodic oxidation voltage and oxidation time, the thickness and pore diameter of AAO film increases gradually, and the corrosion resistance increases gradually. After anodizing for 15V-60min, the thickness of AAO film on the surface of Al-5.5Zn-0.03In-1Mg-0.03Ti-0.08Sn alloy reached 45.51μm, the corrosion current density decreased to 0.79 µA/cm2, and the weight loss decreased to 0.19 µg·cm−2·h−1, showing the best corrosion resistance. This study provides a new method to improve the corrosion resistance of Al-5.5Zn-0.03In-1Mg-0.03Ti-0.08Sn alloy used in the Marine environment and discusses the corrosion behavior of Al-5.5Zn-0.03In-1Mg-0.03Ti-0.08Sn alloy with AAO films in the Marine environment.

Formation mechanism of nanopores in dense films of anodic alumina

Constant-current anodization of pure aluminum was carried out in non-corrosive capacitor working electrolytes to study the formation mechanism of nanopores in the anodic oxide films. Through comparative experiments, nanopores are found in the anodic films formed in the electrolytes after high-temperature storage (HTS) at 130 °C for 240 h. A comparison of the voltage−time curves suggests that the formation of nanopores results from the decrease in formation efficiency of anodic oxide films rather than the corrosion of the electrolytes. FT-IR and UV spectra analysis shows that carboxylate and ethylene glycol in electrolytes can easily react by esterification at high temperatures. Combining the electronic current theory and oxygen bubble mold effect, the change in electrolyte composition could increase the electronic current in the anodizing process. The electronic current decreases the formation efficiency of anodic oxide films, and oxygen bubbles accompanying electronic current lead to the formation of nanopores in the dense films. The continuous electronic current and oxygen bubbles are the prerequisites for the formation of porous anodic oxides rather than the traditional field-assisted dissolution model.

Formation of Through-Hole Porous Anodic Aluminum Oxide Layer Locally with 3D Printed Solution Flow Type Microdroplet Cell

In this study, a 3D-prinited solution-flow type microdroplet cell (SF-MDC) is employed as a new technique for the fabrication of porous anodic aluminum oxide (AAO) layer using oxalic acid electrolyte on aluminum. The surface morphology of the porous AAO film was characterized by a scanning electron microscope. The aim of this study was to fabricate a through-hole porous alumina layer in a single step anodizing process and to investigate the influence of anodized voltages and scanning speeds on the thickness and pore structure of alumina layer. The results showed that the pore diameter and interpore distance were directly proportional to the anodizing voltage. The thicknesses of formed AAO films were found to be 35.5, 50.7, and 81.6 μm at scanning speeds of 10, 5, and 2.5 μms−1, respectively. Through-hole porous AAO was successfully fabricated at room temperature without chemical etching. The SF-MDC fabrication technique is proposed as an environmentally attractive and suitable process for the fabrication of porous AAO layers.

Exploring a nanostructured X-ray optical device for improved spatial resolution in laboratory X-ray diffraction imaging

Analytical methods with wide field range and high spatial resolution are required to observe the distribution of the crystal structure in micro-regions undergoing macroscopic chemical reactions. A recent X-ray diffraction (XRD) imaging method combines XRD with an X-ray optical device such as a glass polycapillary consisting of a bundle of numerous monocapillaries. The former provides the crystal structure, while the latter controls the shape of the incident or diffracted X-rays and retains the positional information of the sample. Although reducing the monocapillary pore size should improve the spatial resolution, manufacturing technology challenges must be overcome. Here, an anodic aluminium oxide (AAO) film, which forms self-ordered porous nanostructures by anodic oxidation in an electrolyte, is applied as an X-ray optical device. The AAO film (pore diameter: 110 nm; size of the disc: 11 mm; and thickness: 620 µm) was fabricated by anodization in a mixture of oxalic acid and ethylene glycol. The film was incorporated into a laboratory XRD instrument. Compared with using a glass polycapillary alone, using a combination of a glass polycapillary and the AAO film improved the spatial resolution of the XRD imaging method by 40%. This XRD imaging method should not only provide practical analysis in a laboratory environment but also support various observations of the crystal structure distribution.

Ultra‐high bonding strength of carbon fiber–doped polyamide–aluminum alloy composite structure achieved by changing crystallinity

AbstractLightweight and high‐strength polymer–metal hybrid structures are favored for reducing material and energy consumption. In this study, carbon fiber‐reinforced polyamide 6 (CFPA) composite materials were prepared through melt blending, and 30 wt% CFPA exhibited the highest shear strength and lowest shrinkage rate among other carbon fiber contents. With pretreatments of anodizing and etching treatment, injection molded direct joining was employed to create polyamide 6 (PA)/CFPA–Aluminum alloy (Al) composites. The impacts of the injection temperature and speed on the bonding strength of the PA/CFPA–Al composite materials were investigated. The characterization results confirmed that carbon fiber doping reduces the coefficient of linear thermal expansion (CLTE) and enhances crystallization ability of PA. The higher injection temperatures and lower injection speed significantly increase the crystallinity of PA and the infiltration of the molten polymer into the nanoporous anodic aluminum oxide films. When the injection temperature was 270°C and the speed was 6 mm/s, the bonding strength of the PA–Al hybrid structure reached a maximum of 36.6 MPa. When the injection temperature was 300°C, the bonding strength of the CFPA–Al hybrid structure reached a maximum of 40.8 MPa. Adhesive parts with high shear bonding strength are critical to satisfying engineering requirements and have significant potential, particularly in polymer–metal composite structures. The polymer‐metal hybrid structure can meet the high interfacial bonding strength required in most engineering applications.Highlights Injection temperature and speed in IMDJ affect PA crystallinity. Carbon fiber doping reduces the CLTE of PA and increases the shear strength. High temperatures and low injection speeds are beneficial to adhesives. The bonding strength of PA–Al reached a maximum of 36.6 MPa. The bonding strength of CFPA–Al reached a maximum of 40.8 MPa.

Research on variation in nanopore parameters and surface characteristics of anodic aluminum oxide (AAO) films with time-controlled anodization processes

Research on variation in nanopore parameters and surface characteristics of anodic aluminum oxide (AAO) films with time-controlled anodization processes

Effects of Pores in Porous Anodic Alumina Films on Adhesive Strength to Resin

Effects of Pores in Porous Anodic Alumina Films on Adhesive Strength to Resin

Influence of Normal-to-High Anodizing Voltage on AAO Surface Hardness from 1050 Aluminum Alloy in Oxalic Acid.

Anodic aluminum oxide (AAO) has been widely applied for the surface protection of electronic component packaging through a pore-sealing process, with the enhanced hardness value reaching around 400 Vickers hardness (HV). However, the traditional AAO fabrication at 0~10 °C for surface protection takes at least 3-6 h for the reaction or other complicated methods used for the pore-sealing process, including boiling-water sealing, oil sealing, or salt-compound sealing. With the increasing development of nanostructured AAO, there is a growing interest in improving hardness without pore sealing, in order to leverage the characteristics of porous AAO and surface protection properties simultaneously. Here, we investigate the effect of voltage on hardness under the same AAO thickness conditions in oxalic acid at room temperature from a normal level of 40 V to a high level of 100 V and found a positive correlation between surface hardness and voltage. The surface hardness values of AAO formed at 100 V reach about 423 HV without pore sealing in 30 min. By employing a hybrid pulse anodization (HPA) method, we are able to prevent the high-voltage burning effect and complete the anodization process at room temperature. The mechanism behind this can be explained by the porosity and photoluminescence (PL) intensity of AAO. For the same thickness of AAO from 40~100 V, increasing the anodizing voltage decreases both the porosity and PL intensity, indicating a reduction in pores, as well as anion and oxygen vacancy defects, due to rapid AAO growth. This reduction in defects in the AAO film leads to an increase in hardness, allowing us to significantly enhance AAO hardness without a pore-sealing process. This offers an effective hardness enhancement in AAO under economically feasible conditions for the application of hard coatings and protective films.

Facile one-step nanoporous alumina for modifying the surface optical properties of ITO glass

Structure color is a useful technique with good brightness, variuos color, and friendly environmental properties, compared with the dyes and pigments colors. However, the structure colors are based on the micro- or nano structures, which is still a challenge for fabricating the desired colors. Anodic aluminum oxide (AAO) is a common nanomaterial for fabricating the structure colors because of the properties of hardness, stability, and the self-assembly. However, current techniques mostly manufactured the structure color on the aluminum foils or the alloy, which limited these techniques from the glass-based applications. Additionally, current technologies usually require a time-consuming fabrication process lasting several tens of hours. In this study, a facile method is proposed for producing structure colors on transparent ITO glass through AAO coating with a short fabrication time. The vibrant structure colors result from constructive interference of visible light due to the nanoporous AAO film on the ITO glass substrate. Furthermore, a correlation between the anodization time of AAO and color is established, enabling the prediction of anodization time to achieve the desired color without resorting to a trial-and-error approach. A spectrum of colors within the visible light range, spanning wavelengths from 420 to 740 nm, is successfully demonstrated on the ITO glass using this uncomplicated one-step anodization process.

Enhanced bonding of polyphenylene sulfide-aluminum alloy composites using combined mild and hard anodizing techniques

High-strength direct joining between polymers and Al has attracted considerable attention, particularly for reducing the weight device. In this study, porous anodic alumina (PAA) films were fabricated on a commercial 6063 Al alloy using with mild and hard anodizing with varying durations, followed by etching. Subsequently, polyphenylene sulfide (PPS) was directly joined with Al through injection molding. The correlations between the interface bond strength of PPS-Al joints and the Al oxidation duration were systematically investigated. The porosity, diameter of nanopore, and wettability of anodic aluminum oxide (AAO) film surface significantly influenced the bonding strength of PPS-Al composite structure. The porosity increased with an etching duration, increasing the bonding strength. However, excessive porosity (>54 %) weaken the films and reduced their bonding strengths. When the durations of mild anodizing (MA), hard anodizing (HA), and etching treatment were set to 30, 30, and 60 min, respectively, the maximum bonding strength reached 25.2 MPa, approximately 77 % of the base-material shear strength. Scanning electron microscopy revealed nanopillars with two different top sizes (approximately 80 and 110–130 nm) on the surface of injection-molded PPS, which were attributed to the different anodizing preparation voltage. These findings are generally significant for the realization for high-strength direct-bonded polymer-Al joints.

A multi-strategy coating for Al alloy based on orientationally arranged nanosheets doped SiO2 coating and corrosion inhibitor loaded porous anodic alumina film

Although aluminum alloys are widely used in various industries, their wear and corrosion resistance are still needed to be enhanced. In this study, a robust anti-corrosion composite coating was prepared on the surface of Al alloy based on multiple reinforcement strategies. The composite coating is consisted of horizontally arranged and functionally modified h-BN nanosheets doped SiO2 coating (SiO2-hBN(NH3+) coating) and corrosion inhibitor MBT loaded porous anodic alumina (PAA-MBT) film. Results show that the composite coating prepared in this work has excellent corrosion resistance due to the effect of the extended penetration path of corrosive medium by h-BN(NH3+) nanosheets, the full-thickness hydrophobicity of the SiO2 coating, the corrosion inhibition of the MBT released from the PAA film and the physical barrier of the entire coating system. In addition, the composite coating also exhibits good mechanical properties including high hardness, high bonding strength and superior wear resistance. This work provides a novel design of protective coating with excellent anti-corrosion resistance and mechanical properties, which has a promising prospect in the development of superior long-term anti-corrosion coatings for aluminum alloy.

Anodic alumina/carbon composite films: extraction and characterization of the carbon-containing component

Alumina/carbon composites are modern nanomaterials used as adsorbents, catalysts, catalyst supports, supercapacitors, and electrode materials for fuel cells. Among other methods, aluminum anodizing is fairly fast and inexpensive for producing anodic alumina/carbon composites with controllable properties. In the present study, the morphology and composition of carbon-enriched anodic alumina films were obtained during aluminum anodic oxidation in formic acid with ammonium heptamolybdate (C content is ca. 5.0 mass%) or oxalic acid (C content 3.4 mass%) additives. The anodic alumina films have a wide blue fluorescence (FL) in the 400–650 nm wavelength range with a maximum at ca. 490 nm. The FL decay is nonexponential and has an average lifetime of 1.54 and 1.59 ns for ammonium heptamolybdate and oxalic acid additives, respectively. As samples obtained in sulfuric acid (i.e. without carbon) do not possess detectable FL in the 400–650 nm wavelength range, it was concluded that carbon-containing inclusions are responsible for the FL properties of the films. The initial samples were dissolved in the hot aqueous HCl solution and then dialyzed to extract the carbon-containing component. It was shown that the solutions contain nanoparticles of amorphous carbon with a 20–25 nm diameter. Carbon nanoparticles also exhibit an excitation-dependent emission behavior at 280–450 nm excitation wavelengths with average lifetimes of 7.25–8.04 ns, depending on the composition of the initial film. Carbon nanoparticle FL is caused by the core of carbon nanoparticles (CNPs) and various emission centers on their surface, such as carbonyl, carboxyl, and hydroxyl groups. As CNPs could be exceptional candidates for detection technologies, the biocompatibility assays were performed with living COS-7 mammalian cells, showing a minimal negative impact on the living cells.

Correlation of Fabrication Methods and Enhanced Wear Performance in Nanoporous Anodic Aluminum Oxide with Incorporated Molybdenum Disulfide (MoS2) Nanomaterials.

Wear performance is integral to component longevity, minimizing industrial waste and excess energy costs in a wide variety of applications. Anodized aluminum oxide (AAO) has many beneficial properties leading to its wide use across industries as a surface treatment for many aluminum components, but the wear properties of the coating could be improved significantly. Here, we used an electrochemical method to incorporate molybdenum disulfide (MoS2), a nanomaterial used as a dry lubricant, to modify alloys of aluminum during AAO preparation. Using Raman spectroscopy and tribological scratch measurements, we thoroughly characterized the structure and wear behavior of the films. The MoS2 deposition procedure was optimal on aluminum 5052 anodized in higher acid concentrations, with friction coefficients at around 0.05 (~10× better than unmodified AAO). Changing anodization conditions to produce harder films with smaller pores led to worsened wear properties, likely because of lower MoS2 content. Studying a commercial MoS2/AAO film of a different Al alloy (7075) showed that a heat treatment step intended to fully convert all deposited MoSx species to MoS2 can adversely affect wear in some alloys. While Al 6061 and 1100 produced films with worse wear performance compared to Al 5052 or 7075, our results show evidence that acid cleaning after initial anodization likely removes residual alloying elements, affecting MoS2 incorporation. This study demonstrates a nanomaterial modified AAO film with superior wear characteristics to unmodified AAO and relates fabrication procedure, film structure, and practical performance.

Effect of voltage on structure and properties of 2024 aluminum alloy surface anodized aluminum oxide films

In this study, anodized aluminum oxide (AAO) films were fabricated in situ on the surface of a high-strength and robust 2024 aluminum alloy using step voltage and constant voltage modes, respectively. Our study included examination of the microstructure, thickness, microhardness, and electrochemical characteristics of the AAO films. Cross-sectional morphology analysis revealed that the AAO film produced via the step-voltage approach exhibited remarkable density, uniformity, and good quality. In contrast, conspicuous cracks were observed in the AAO film generated under constant-voltage conditions when applied to the 2024 aluminum alloy substrate. Notably, the microhardness of the AAO film prepared using the step voltage method surpassed that of its constant-voltage counterpart. Specifically, the microhardness of the AAO film produced at 30 V via the step voltage reached 289 HV, whereas the equivalent AAO film produced at a constant voltage of 30 V registered only 174 HV. Furthermore, the self-corrosion potential and polarization resistance of the AAO films created using the step voltage were markedly higher than those achieved using a constant voltage. Specifically, the AAO films produced under step voltage conditions, specifically at 30 V and 60 min, exhibited the highest self-corrosion potential and polarization resistance values of namely −0.52 V and 34,888.6 Ω, respectively. In contrast, AAO films prepared under constant voltage conditions displayed values of −0.61 V and 23,292.0 Ω, respectively. Conducting AC impedance measurements and subsequent fitting calculations, the maximum thickness of the AAO film generated via step voltage and constant voltage methods amounted to 132.43 nm and 106.70 nm, respectively. Overall, the microstructure and corrosion resistance of the AAO film formed on the surface of the 2024 aluminum alloy using the step voltage approach demonstrated that it outperformed that of the AAO layer prepared under constant-voltage conditions.Chinese library classification number:TG178 Document ID:A Article ID.

Formation behavior of anodizing films on various aluminum alloys in oxalic acid solution

The formation behavior of anodic aluminum oxide (AAO) films on various aluminum alloys, was studied in 6 wt% oxalic acid solution. There were observed two different voltage-time behaviors with a sharp peak in the initial stage of galvanostatic anodizing for pure Al, AA1050 and AA5052 (group 1), and with voltage plateaus instead of a peak for AA7075, AA6061 and AA2024 (group 2). AAO film growth rate was found to be higher for the group 1 specimens than the group 2 alloys. Two different anodizing films appeared, with regularly ordered cylindrical nano-pores for the group 1 specimens and irregularly distorted nano-pores along with numerous nano-cavities for the group 2 alloys. As the anodizing time increased, the AAO films on the group 1 specimens became darkened, while those on the group 2 alloys were not darkened but only changed in color. In addition, the surface hardness of the AAO films was much higher on the group 1 specimens than the group 2 alloys.

High bonding strength of polyphenylene sulfide-aluminum alloy composite structure achieved by constant current anodizing in tartaric acid

The high-strength and facile direct joining of metal to polymer has attracted increasing attention in adhesion. The aluminum alloy (Al) specimen was fabricated by constant current anodization and enlarging treatment. The influences of surface energy, porosity, and anodization time on the interface bonding strength of the injection molding polyphenylene sulfide (PPS)–Al composite structures were quantitatively discussed. Results showed that the bonding strength of the joints was significantly affected by the wettability, porosity, thickness, and pore diameter of the anodized aluminum oxide (AAO) films. When the current density and anodization time were 120 A/m2 and 30 min, the Al surface energy and the bonding strength reached the maximum of 74.8 mN/m and 11.7 MPa. With the enlargement treatment of the AAO film, when the etching duration was 45 min, the bonding strength increased to 25.0 MPa. In general, these results are of great importance to the further design and optimization of the polymer-metal hybrid with reliable interface adhesive strength.