Anodic Oxide Layer Research Articles

Tuning the geometry of porous alumina layers via anodization in mixtures of different acids

AbstractPorous anodic aluminum oxide (AAO) layers have been obtained by two-step anodization of high-purity Al in two types of acid mixtures, i.e., in H2C2O4–H3PO4 and, for the first time, in H2SO4–H3PO4 systems. The kinetics of oxide formation was examined by monitoring the current vs. time curves while the morphology of the resulting layers was carefully verified by scanning electron microscopy (SEM). A special emphasis was put on establishing correlations between electrolyte composition, the kinetics and effectiveness of oxide growth, and the morphological features of AAO layers (pore and cell diameter, porosity), as well as pore arrangement. It was confirmed that the addition of H3PO4 to both H2C2O4 and H2SO4 electrolytes results in a significant decrease in oxide growth rate, and worsening of pore arrangement, while the values of pore diameter and interpore distance are much less affected. Moreover, the presence of a small amount of phosphoric acid in the reaction mixture allowed for a noticeable increase in pore ordering if anodization was carried out beyond the self-ordering regime, or performing controlled anodization even at voltages at which the burning phenomenon is typically observed. It is strongly believed that manipulating the electrolyte composition by adding another acid may provide another degree of freedom to control the morphology of the resulting nanostructured alumina layers.

Designing bipolar anodizing towards white anodic aluminum oxide (AAO)

Obtaining white anodic aluminum oxide (AAO) is a major challenge in the surface finishing industry. The present work presents a cost-effective route to design light-scattering morphologies through submicrometric hollows homogeneously dispersed in the depth of the oxide layer, created by localized hydrogen evolution. To this end, cathodic steps are added in alternance with anodic steps in a bipolar pulse anodizing process performed in 2 M sulfuric acid. The mechanisms of nucleation and growth of the targeted hollows within the AAO are highlighted by combining in situ electrochemical measurements and high magnification SEM observations. The aesthetic performances in terms of white color, evaluated by reflectance spectroscopy in comparison with a white reference, are directly related to the size, shape and distribution of the hollows, controlled by both the parameters of the cathodic step and the thickness of the unit anodic oxide layers.

Microstructure and topography of laterally confined porous anodic oxides produced with high growth rate in a maskless two-phase jet setup

AbstractMaskless anodic oxidation with a continuous free jet of electrolyte can be used for the local surface functionalization and structuring of aluminum materials. In this study, a two-phase jet was applied with the aim of enhanced lateral confinement of the anodic oxide on the aluminum alloy EN AW-7075. The two-phase jet was realized by a coaxial arrangement. While the inner electrolyte nozzle with a diameter of 100 µm acted as the cathode and was used to provide the electrolyte with a flow rate of 10 ml min−1 resulting in an average jet velocity of approximately 21 m s−1, the outer nozzle with a diameter of 3000 µm was used to provide deionized water with a flow rate of 383 ml min−1 resulting in an average water jet velocity of 1 m s−1, which is sufficient to realize a continuous free two-phase jet. Process voltages from 10 to 60 V were investigated. The realized oxide layers were characterized by non-destructive as well as destructive methods to determine their microstructure and their thicknesses. Optically transparent anodic oxide layers were achieved in the voltage range between 10 and 25 V. Maximum total layer thicknesses between 1.1 µm and 16.9 µm were measured in these cases. Thicknesses of more than 50 µm were determined for higher voltages up to 60 V; however, burning effects and stronger discolorations were determined for voltages ranging from 30 to 60 V. Consequently, 25 V was derived as best suitable voltage for two-phase anodizing with high growth rate leading to oxide layers with low defects. Graphical abstract

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.

Modeling and validation of hydrogen porosity formation in aluminum laser welding

Laser welding is used to weld aluminum alloy 1100 for battery cell manufacturing in the automotive industry. However, these welds can have porosity that can decrease their performance in the battery packs. One type of porosity is gas porosity, commonly understood to be due to hydrogen entrapment in the weld during solidification. Hydrogen is dissolved in the molten aluminum at elevated temperatures during laser heating and can be entrapped in the weld pool upon solidification. When welding anodized aluminum, the amount of hydrogen porosity is increased because the anodized oxide layer is broken up and the broken-up oxide can act as heterogeneous nucleation sites for hydrogen pores. A cellular automaton model has been developed to predict the nucleation and growth of hydrogen porosity during laser welding. This model has been validated with microstructure analysis, LECO® hydrogen analysis, and X-ray micro-computed tomography. This model can be used by welding engineers to reduce porosity and control weld quality during laser welding of aluminum alloys.

Electro-Coloring Mechanism of Aluminum Anodic Oxides in Tin-Based Electrolytes

A method for accurately determining the chemical composition of deposits at the bottom of pores during the electrocoloring (e-coloring) of aluminum anodic oxide (AAO) layers in tin-based solutions is developed. The aluminum samples were AC e-colored after DC sulfuric anodization. Free-standing, tin e-colored aluminum oxide film was obtained by selective dissolution of the metallic aluminum from the AAO in copper chloride solution to access the deposit directly at the bottom of the pore. This allowed us to conduct XPS analysis directly on the deposits at pore bottoms without any interference from the base material or insulating barrier layer. The results revealed the presence of a mixture of tin oxide and metal in the deposits, which were richer in oxide content. Furthermore, a cyclic voltammetry experiment mimicking real polarization conditions during AC conditions was optimized and used to gain a deeper understanding of the electrochemical reactions that occur during AC electrocoloring. The comparison of CV results in tin-free and tin-containing electrolytes indicated that the tin deposited during a cathodic cycle is oxidized in the anodic cycle. The formation of tin-based deposits radically changed the CV behavior. The XPS and cyclic voltammetry results consistently show that the deposits formed during e-coloring comprised a mixture of metallic and oxidic tin species richer in oxide content.

Self-Ordered Anodic Porous Oxide Layers as a High Performance Electrocatalyst for Water Oxidation

Self-Ordered Anodic Porous Oxide Layers as a High Performance Electrocatalyst for Water Oxidation

Effect of anodic oxide layer on friction stir spot welding between anodized A5052 and CFRTP

Dissimilar joining between carbon fiber reinforced thermoplastic (CFRTP) and metal has attracted attention of transportation equipment industry for reduction of weight and cost by multi-material structure. Friction stir spot welding (FSSW) between anodized A5052 (AO-A5052) and carbon fiber reinforced thermoplastic (CFRTP) was investigated for the improvement of joining strength. Average tensile shear strength of AO-A5052/CFRTP joint was 2505 N and more than 500 N higher than that of untreated-A5052/CFRTP joint. In the AO-A5052/CFRTP joint, probe plunge induced cracks and microwrinkles in the anodic oxide layer on the weld surface. Matrix resin was immersed into the cracks and the microwrinkles underneath of the shoulder of the welding tool. Reinforced carbon fiber and matrix resin was immersed into large crack at the outer peripheral corner of probe. Thus, those increased the joining strength by mechanical anchoring effect. Thermal stress test and machining stress test were performed for revealing the mechanism of cracking of anodic oxide layer. Although thermal stress did not generate cracks in the anodic oxide layer, machining stress by probe plunge generated cracks in the anodic oxide layer. The generated cracks in anodic oxide layer underneath of the shoulder were distorted to the rotating direction of welding tool by the friction stirring.

Friction Behavior of Anodic Oxide Layer Coating on 2017A T4 Aluminum Alloy under Severe Friction Solicitation: The Effect of Anodizing Parameters

This article aims to highlight the wear mechanisms and friction behavior of the 2017A T4 anodized aluminum alloy used for automotive and aerospace applications. The effect of the processing parameters on the durability of the anodized layer under high friction is studied. Scratch tests were carried out to study the level of the friction coefficient with the increase in the thickness of the oxide layer formed on the Al 2017 A (AU4G) substrate. The results of the scratch tests show that the variation in the anodization duration, which influences the thickness of the oxide layer, induces an increase in the coefficient of friction. Besides, the variations in friction coefficient with sliding distance are influenced by the changes in wear morphology and degree of oxidation. Treated surfaces with a thickness of 50 μm have the lowest friction coefficients and wear rates. Their improved wear resistance may be related to the increased bond strength compared to other anodized surfaces. The tribological damage was characterized by the detachment of debris, which increases with the increase of the duration of anodization. Upon sliding, its detachment leads to delamination of the underlying anodic aluminum oxides and subsequent abrasion of the aluminum substrate.

Effect of Anodizing Voltage and Tobacco Extract Addition on the Structure of Porous Anodic Aluminum Oxide (PAAO) Layer

Porous Anodic Aluminum oxide (PAAO) is a porous oxide layer resulting from anodization. The structure of PAAO is influenced by anodization parameters, i.e., voltage and electrolyte composition. Increasing anodization voltage can affect the process of pore formation and oxide growth during anodization. Adding additives such as ethanol, propanol, and polyethylene glycol (PEG) can increase pore regularity and affect the structure of PAAO. In this study, tobacco extract (TE) was added to the oxalic acid-based anodizing solution. TE has many active compounds that may affect pore formation and oxide growth. Morphological analysis shows decreased pore diameter when adding tobacco extracts with concentrations of 0, 0.1, and 0.5 g/L, namely 43.92, 41.42, and 37.8 nm at anodization voltage 40 V. In anodization with a voltage of 60 V, a decrease in pore diameter was obtained with 46.47, 34.24, and 26.8 nm for adding tobacco extract 0, 0.1, and 0.5 g/L. The thickness of PAAO increases from 6.45 µm to 16.87 µm with increasing anodization voltage and tobacco extract concentration. The increase of tobacco extract concentration can lead to the decrease of the XRD peak intensity, where the sequence of the most significant decrease was observed for the peaks of (111), (220), (200), and (311), respectively. A decrease in the intensity ratio of (111) and (220) AAO peaks indicates the influence of tobacco extract on the anodization process. Further thermal analysis by Thermo-gravimetric (TG) shows an increase in mass loss from 1.47 to 5.37% with increasing tobacco extract concentration from 0 g/L to 0.5 g/L. TG results indicate the incorporation of tobacco extract in the inner pore wall.

Anodic Oxide Layers on Aluminum-Lithium Alloy

Al-Cu-Li alloys regained attention with Cu as the major addition and Li around 2 %wt. However, oxide blistering of is frequently found by the sulfuric acid anodizing of these alloys, which has been sometimes attributed to Li-rich precipitate dissolution or to thermomechanical stresses in the anodic oxide layer. In this work, anodic layer properties (composition, morphology, pitting resistance) were studied in dependence of anodizing process parameters. Blisters form by higher anodizing charges, independent of the applied current density and its density increases at lower temperatures, at which the anodizing efficiency is higher. Blistering cannot be associated to second phase precipitates, as their diameters (100-200 µm) are much larger than the precipitates size. The 45º-cracks at the mid section of the porous film are typical of compressive stresses perpendicular to the growth direction, indicating that blistering is possibly caused by a higher molar volume compared to other Al-Cu alloys.

Formation of Pores on Pure Ti in Highly Concentrated Sulfuric Acids

TiO2 nanotubes formed by anodization in fluoride-containing electrolytes have been extensively examined due to their wide rage of applications. It is reported that fluoride ions move faster to the interface between anodic oxide layer and underlying Ti substrate, forming a fluoride-rich layer at the interface as the ionic radius of fluoride ion is smaller compared to that of oxygen ion. The fluoride-rich layer is also reported to decrease the adhesion of the anodic oxide layer to the substrate. Therefore, the formation of nanotubular oxide layer in fluoride-free electrolytes is highly desired. In the present work, we examined the formation of nanotubular oxide layers on pure Ti in highly concentrated sulfuric acids. Specimens with the dimension of 5 mm x 80 mm x 0.6 mm were prepared from pure Ti sheet (purity ; 99.5%). Prior to anodization, the specimens were cleaned in acetone, methanol and deionized water, successively. Anodization was carried out in highly concentrated sulfuric acids at elevated temperatures using two-electrode configuration cell with platinum counter electrode. The applied voltage was increased with the sweep rate of 1 V/s to desired voltages and then kept at the voltages for various durations. After the anodization, the surface of the specimens was cleaned with deionized water and dried. The structure of the surface was evaluated using FE-SEM. In a highly concentrated sulfuric acid, the anodic current increased with increasing the applied voltage and after switching constant voltage, decreased with time. The formation of oxide layer was confirmed at the surface, but no porous structure was visible. However, a porous structure was observed on the specimen above the electrolyte surface, indicating that the anodization occurred under the meniscus consisting of highly concentrated sulfuric acids. The optimization of water concentration in sulfuric acid and temperature led to the formation of porous structure on pure Ti. This indicates that porous oxide layers can be formed in fluoride-free electrolytes although further optimization is required to improve the ordering of pores.

The effect of current density on the anodic oxidation hydrogen barrier film on ZrH1.8 surface

To improve the density and hydrogen barrier performance of the anodic oxide layer on the surface of ZrH1.8, anodization of the ZrH1.8 substrate was performed under a constant current mode. In an alkaline electrolyte environment, the film growth process was investigated under varying current densities (233, 280, 326, and 373 A/m2). The morphology, phase structure, thickness, and adhesion of the film were analyzed. The hydrogen barrier properties of the films at different current densities were evaluated through dehydrogenation experiments. The barrier properties of the film were further analyzed via electrochemical tests. The results indicate that zirconia hydrogen barrier coatings with thicknesses of 12.77, 18.74, 23.16 and 22.26 µm can be fabricated on the surface of ZrH1.8 at different current densities. The coatings contain both monoclinic zirconia (M-ZrO2) and tetragonal zirconia (T-ZrO2) phases, among which M-ZrO2 prevails. The current density has no substantial effect on the phase composition of the coatings. At a current density of 280 A/m2, a more homogeneous and compact film with superior dense adhesion and the thickness of 18.74 µm was fabricated on the ZrH1.8 surface. The hydrogen permeation temperature reached 660 °C, demonstrating a relatively good hydrogen barrier performance.

Surface phenomena and electrochemical behavior of phosphate ion incorporated titania nanotubes for boosting bone cell growth

The addition of bone-native calcium and phosphate ions to implant materials can improve biocompatibility. The objective of the work was a comprehensive analysis of phosphate incorporation in nanotubes utilising orthophosphoric acid as the electrolyte and its effect on nanotube development during anodization process. Also, the material’s biological compatibility was investigated. A traditional anodization method was utilised to create nanotubes on commercially pure titanium (Cp-Ti) metal surfaces. The creation of nanotubes was detected at an optimal concentration of orthophosphoric acid, while other experimental variables such as voltage, time, temperature, etc. were maintained constant. The surface phenomena of anodized TIOP’s were examined through High-resolution scanning electron microscopy (HR-SEM), X-ray diffraction (XRD) analyser and contact angle. Fourier transform RAMAN study validated the penetration of phosphate ions into nanotubes. Potentiodynamic polarisation investigation in SBF was used to analyse the corrosion behaviour of anodized TIOP’s surface and Scanning Electrochemical Microscopy (SECM) mapped the reactive surface of the anodic oxide layer. The biomineralization studies were performed for 14 days in SBF and analysed with SEM/EDX and ATR-FTIR. The cell compatibility of the anodized sample surface was carried out in the MG63 cell line. The result indicated the addition of bone-native phosphate ions to anodized surface influences the differentiation of cells. The apatite formations on anodized surface influence the corrosion behaviour of the sample and result in high corrosion resistance in SBF.

Modification of the structure and properties of oxide layers on aluminium alloys: A review

Abstract Aluminium alloys are a material that is increasingly used in industry. This is due to very good strength properties with low specific weight and low production costs. The disadvantage of kinematic system aluminium elements is their surface’s susceptibility to adhesive wear. One method of eliminating the adverse impact of adhesive tacks on the surfaces of cooperating aluminium components of machinery is the application of the method based on the anodic oxidation of alloys surface. The layers obtained by this method are widely used in sliding connections of kinematic machine parts. The modification of anodic oxide layers with admixtures has been an uninterrupted area of interest since the 1990s. This article is a review of selected methods of modifying the structure and properties of aluminium oxide layers on aluminium alloys.

Self-Healing Coatings Consisting of an Outer Electrodeposited Epoxy Resin Layer and an Inner Porous Anodic Oxide Layer with Healing Agents for the Corrosion Protection of Al Alloys

Recently, new surface treatments for the corrosion protection of Al alloys by forming self-healing layers have attracted the attention of many researchers. The authors of this paper have previously developed self-healing polyurethane coatings with micro-capsules containing healing agents and porous anodic oxide films filled with healing agents. In this study, self-healing coatings consisting of an outer electrodeposited epoxy resin layer and an inner porous anodic oxide layer with healing agents were developed for the corrosion protection of Al alloys. The corrosion protection abilities of the self-healing coating were shown in Cu2+/Cl− solutions after damaging with indenters and were affected by freezing treatments and the tip angles of the indenter.

Morphological and semiconductive properties of the anodic oxide layers made on Fe3Al alloy by anodizing in tartaric-sulfuric acid mixture

It has recently been found that the anodizing of FeAl alloys allows the formation of iron-aluminum oxide layers with interesting semiconducting properties. However, the lack of systematic research on different anodizing regimes is hampering their full exploitation in numerous photoelectrochemical-related applications. This study address, for the first time, the systematic effect of the electrolyte composition on the formation of self-ordered oxide films by anodizing on cast Fe3Al alloy. The Fe3Al alloy was anodized in 3 electrolytes with different water-ethylene glycol (EG) ratios (pure water, 25 vol.%-EG, and 50 vol.%-EG solutions) at a constant tartaric-sulfuric acids concentration, different voltages (10–20 V) and treatment times (2–60 min). After anodizing, all anodic oxide layers were annealed at 900 °C to form semiconductive iron-aluminum crystalline phases. Conventional techniques were used to systematically ascertain the morphological (SEM/EDS, XRD, eddy-current measurements) and semiconductive (UV–VIS reflectance spectroscopy) properties of these oxide layers. The results confirmed the formation of homogeneous and self-ordered anodic oxide layers at 10 and 15 V, regardless of the electrolyte composition. Namely, anodic films formed in electrolytes containing EG showed lower pore sizes, growth rates, and film thicknesses than those anodic films formed in the aqueous-based electrolyte. The annealing post-treatment results in different Fe-Al oxides (FexOy, FeAl2O4, etc.) with superior band gap values than those for non-annealed films.

Application of Anodic Titanium Oxide Modified with Silver Nanoparticles as a Substrate for Surface-Enhanced Raman Spectroscopy.

The formation of nanostructured anodic titanium oxide (ATO) layers was explored on pure titanium by conventional anodizing under two different operating conditions to form nanotube and nanopore morphologies. The ATO layers were successfully developed and showed optimal structural integrity after the annealing process conducted in the air atmosphere at 450 °C. The ATO nanopore film was thinner (1.2 +/- 0.3 μm) than the ATO nanotube layer (3.3 +/- 0.6 μm). Differences in internal pore diameter were also noticeable, i.e., 88 +/- 9 nm and 64 +/- 7 nm for ATO nanopore and nanotube morphology, respectively. The silver deposition on ATO was successfully carried out on both ATO morphologies by silver electrodeposition and Ag colloid deposition. The most homogeneous silver deposit was prepared by Ag electrodeposition on the ATO nanopores. Therefore, these samples were selected as potential surface-enhanced Raman spectroscopy (SERS) substrate, and evaluation using pyridine (aq.) as a testing analyte was conducted. The results revealed that the most intense SERS signal was registered for nanopore ATO/Ag substrate obtained by electrodeposition of silver on ATO by 2.5 min at 1 V from 0.05M AgNO3 (aq.) (analytical enhancement factor, AEF ~5.3 × 104) and 0.025 M AgNO3 (aq.) (AEF ~2.7 × 102). The current findings reveal a low-complexity and inexpensive synthesis of efficient SERS substrates, which allows modification of the substrate morphology by selecting the parameters of the synthesis process.

Fretting tribocorrosion properties of anodized TiNbSn implant alloy

Titanium and its alloys have poor wear resistance, and the biological response to generated wear debris leads to aseptic loosing and pathological bone resorption. In the fretting wear of artificial hip joints, a significant part of the wear surface is contacted by elastic deformation, and electrochemical anodization is a promising method to control the mechanical properties of the surface and improve wear resistance. To suppress wear debris generation and metal ions elution from implant materials into body fluids, the tribocorrosion properties of a newly developed anodized TiNbSn implant alloy were evaluated in the presence of simulated body fluid and compared to those of pure Ti. During anodization under high voltage, spark discharge occurred due to dielectric breakdown occurred for the TiNbSn electrode, but not for the Ti electrode. The primary phase of the anodic oxide layer on TiNbSn alloy was rutile TiO2, whereas that on Ti was anatase TiO2. The anodic oxide layer on TiNbSn had a higher thickness, roughness, and surface area than that on anodized Ti. In addition, the hardness and exfoliation strength were higher than those of Ti. The coefficient of friction of anodized TiNbSn in fretting tests was similar or slightly lower than that of anodized Ti, and the open circuit potential remained unchanged for anodized TiNbSn but shifted to a more negative voltage for anodized Ti. The results suggest that the hard and strongly-adhered rutile TiO2 on TiNbSn formed by spark discharge during the high-voltage anodization process improves fretting wear resistance and reduces the generation of wear debris and elution of metal ions.

Modification of Anodic Titanium Oxide Bandgap Energy by Incorporation of Tungsten, Molybdenum, and Manganese In Situ during Anodization.

In this research, we attempted to modify the bandgap of anodic titanium oxide by in situ incorporation of selected elements into the anodic titanium oxide during the titanium anodization process. The main aim of this research was to obtain photoactivity of anodic titanium oxide over a broader sunlight wavelength. The incorporation of the selected elements into the anodic titanium oxide was proved. It was shown that the bandgap values of anodic titanium oxides made at 60 V are in the visible region of sunlight. The smallest bandgap value was obtained for anodic titanium oxide modified by manganese, at 2.55 eV, which corresponds to a wavelength of 486.89 nm and blue color. Moreover, it was found that the pH of the electrolyte significantly affects the thickness of the anodic titanium oxide layer. The production of barrier oxides during the anodizing process with properties similar to coatings made by nitriding processes is reported for the first time.