Formation Of Anodic Films Research Articles

Electrokinetics-Driven Anodic Oxide Pore Formation: Linear and Weakly Nonlinear Analysis

Anodization of aluminum in an acidic medium facilitates the formation of well-ordered nanoporous anodic oxide films. The mechanism of pore formation is investigated as a morphological instability using a simplified model. The model accounts for the high field conduction law and field-assisted reactions (oxide formation/dissolution) only at the oxide-solution interface. The role of Butler-Volmer electrokinetics, electrolyte pH, anodic efficiency, and interface curvature on reaction kinetics are taken into account. Linear stability analysis suggests that the oxide film is unstable to well-defined wavelengths in specific ranges of parameters such as anodizing efficiency, applied voltage and electrolyte pH. Subsequently, a weakly nonlinear analysis is carried out to determine the nature of bifurcation beyond the stability threshold. Our findings indicate that the instability exhibits a subcritical nature, well in agreement with experimental observations.

Preparation of Ordered Nanohole Arrays by Two-Step Anodization of Austenitic Stainless Steel Substrates.

An anodic oxide film with a regular pore arrangement was formed by the two-step anodization of an austenitic stainless steel (JIS SUS304) substrate. In this study, we determined the anodization conditions under which the pore arrangement in the anodic oxide film became self-organized. After removing the anodic oxide film formed by the first anodization with a mixture containing CrO3 and HF, the residual substrate was anodized under the same conditions as those in the first anodization. As a result, we found that it was possible to form an anodic oxide film with pores arranged in a regular pattern from the surface to the bottom. This is the first report on the formation of anodic oxide films with ordered nanohole array structures by the two-step anodization of austenitic stainless steel. The interpore distance of the resulting nanohole array structures can be controlled by changing the anodization conditions. By measuring the capacitance of the anodic oxide film, we confirmed that its properties depended on the heat-treatment temperature and pore depth. In addition, no significant decrease in the capacity of anodic oxide films with ordered nanohole array structures was observed, even when the scan rate of the cyclic voltammogram measurement was increased. This is considered to be because the cylindrical pores facilitate ion diffusion to the bottom of the holes. Anodic oxide films obtained by the anodization of austenitic stainless steel substrates can be applied as electrodes for capacitors.

Effect of Oxalic Acid Additives on Aluminum Anodizing in Formic Acid Containing Ammonium Heptamolybdate

This paper reports a systematic study of the role of oxalic acid additives in aluminum anodizing in formic acid containing ammonium heptamolybdate. Adding oxalic acid in a concentration range of 5–20 mM to the 0.4 M formic acid solution containing 0.03 M ammonium heptamolybdate improves anodic film growth, increasing the film thickness and smoothing strongly wavy interface between the film and aluminum, and adding 100 mM of oxalic acid results in an almost complete block of the regular anodic film formation. In the case of aluminum anodizing in formic acid, the ammonium heptamolybdate additive prevents aluminum dissolution more effectively than only oxalic acid. The role of oxalic acid in this process is only to improve film growth and morphology. However, ammonium heptamolybdate improves film growth by increasing its thickness. Linear sweep voltammetry studies combined with SEM investigations of alumina growth show that in heptamolybdate-containing electrolytes, a thin porous alumina film is formed at the beginning of the process. Then, when the electrolyte oxidation potential is reached, the thin film on the surface breaks, resulting in a significant increase in the anodizing surface, and anodic oxide begins to grow rapidly.

Effect of Al Polishing Conditions on the Growth and Morphology of Porous Anodic Alumina Films.

The conditions applied during the electrochemical polishing of aluminum were found to be important parameters for the successive formation of nanoporous alumina films. First, a high-purity Al foil was electrochemically polished in an aqueous solution containing C2H5OH and HClO4 at various sets of conditions, such as applied potential (5-35 V), temperature (0-20 °C), and process duration (10-180 s). Extensive studies of the topography of Al after polishing by scanning electron microscopy and atomic force microscopy allow verification of the correlations between conditions applied during the substrate pretreatment and dimensions of the nanopatterns generated on the metal surface. Next, Al polished samples at two different sets of conditions were used as starting materials for anodization. Unpolished Al samples were also anodized for reference. It was confirmed that electropolishing conditions do not significantly affect the oxide growth rate during anodization and the efficiency of anodic film formation. On the contrary, it was proved that the dimensions of the surface texture formed during Al polishing significantly affect the morphology and pore order within the anodic film. Therefore, it can be stated that it is possible to tune to some extent the arrangement of nanochannels within anodic aluminum oxide films by simply changing conditions during the electropolishing procedure..

Performance and Properties of a Ti-Al Composite Anodic Oxide Film on AC-Etched Al Foil

AC-etched aluminum foils for an Al electrolytic capacitor were covered with a TiO2 film by a sol–gel coating and then anodized to 25 V in an ammonium adipate solution. The structure, properties, and performance of the anodic oxide films were examined by transmission electron microscopy (TEM), scanning electron microscopy (SEM), electrochemical impedance measurements (EIS), a general digital LCR meter, a TV characteristic tester, and multicycle pulse charging–discharging. It was found that the anodizing of aluminum coated with TiO2 films led to the formation of Al-Ti composite anodic oxide films, which consist of an outer Al-Ti composite oxide layer and an inner Al2O3 layer on the metal substrate. The capacitance (C25V) of the anodic oxide films formed on specimens with a TiO2 coating was about 10% larger than without a TiO2 coating. The specific resistance (Rox) of the Al-Ti composite film measured by EIS was lower than the blank one, accounting for a greater increase in the rise time (Tr) and a slight reduction in the withstand voltage (Vt). After hydration and a multicycle pulse charging–discharging destructive test, the Al-Ti composite anodic oxide film maintained the same good, comprehensive dielectric properties and performance as the blank one, thereby proving to be promising for acting as dielectric layers.

Composition and growth mechanism of nanoporous anodic fluoride films on stainless steel

Anodizing of 304L stainless steel performed in ethylene glycol solution containing 0.1 M NH4F and 0.1 M H2O at constant voltage under static conditions at 5 °C results in the formation of porous anodic films. Several analysis techniques revealed a rather complex composition of the anodic layer for stainless steel compared to that reported in the literature for iron in the same anodizing conditions. Contrary to what might be expected, the anodic layers consist mainly of iron and chromium fluorides rather than oxides. Furthermore, the multilayer fitting of the Rutherford Backscattered spectroscopy shows a decreasing content of chromium and nickel fluorides from the outermost layer to the innermost layer at the metal/film interface, which is composed only of iron fluoride. Film-assisted dissolution mechanisms and the Gibbs-free energy appear to be responsible for the cation distribution and compounds formed throughout the anodic film. In addition, the thickness and final composition of the anodic layer appear to be dependent on the cleaning process carried out after the anodizing.Graphical abstract

Electrochemical and surface characterization of anodized and fs-laser treated Ti6Al4V for osseo-repellent bone screws and dental implants

The surfaces of titanium grade 5 (Ti6Al4V) samples were altered on a micro and nano scale by femtosecond laser irradiation and subsequent electrochemical anodization. The produced micro-cones and nano-ripples covered by the additional oxide layer were characterised and their influence on osteoblast-like Saos-2 cell growth was studied. Surface topography and morphology were studied by Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). From the wettability experiments, it was found that the contact angle vs. roughness follows the Wenzel equation and that the wettability increases after the anodization. The composition of the formed oxide layer was determined by X-ray photoelectron spectroscopy (XPS) and oxide incorporation of phosphate ions could be confirmed. Moreover, estimated electrochemical surface area (ECSA) of the femtosecond laser treated sample was 14 times larger compared to a polished sample. The anodic oxide film formation factors for different electrolytes were determined and were in the range of 1.3 nm V−1–2.0 nm V−1. Finally, the samples anodized with 0.1 M H2SO4 and 3 M H3PO4 showed decreased Saos-2 cell growth.

Formation of hard anodic films on the 7075-T6 aluminum alloy by anodization in sulfuric acid and ethylene glycol

The 7075-T6 aluminum alloy was anodized in 1.5 mol·dm−3 sulfuric acid alone for 60 min, and small horizontal pores, along with the typical straight pores, were formed inside the pore wall, attributed to the chemical dissolution of the anodic film during the anodization. Conversely, after adding 50 vol% ethylene glycol to the sulfuric acid, the porous film almost maintained its initial structure with a low porosity even with the same anodization period of 60 min. The coating ratio and Martens hardness of the film formed in the mixture of sulfuric acid and ethylene glycol increased by 1.35-fold and 3.4-fold, respectively, compared with the reference values of the film formed in sulfuric acid alone. The properties of the film strongly reflect the dissolution behavior of not only the alloying elements in the anodic film but also the alumina, which constitutes the cell structure.

Changes in the Structure and Corrosion Protection Ability of Porous Anodic Oxide Films on Pure Al and Al Alloys by Pore Sealing Treatment

It is well known that corrosion protection of pure Al is enormously improved by the formation of porous anodic oxide films and by pore sealing treatment. However, the effects of anodizing and pore sealing on corrosion protection for Al alloys are unclear, because the alloying elements included in Al alloys affect the structure of anodic oxide films. In the present study, porous anodic oxide films are formed on pure Al, 1050-, 3003- and 5052-Al alloys, and pore sealing was carried out in boiling water. Changes in the structure and corrosion protection ability of porous anodic oxide films on pure Al and the Al alloys by pore sealing, were examined by scanning electron microscopy (SEM) and electrochemical impedance spectroscopy (EIS). SEM observation showed that anodic oxide films formed on pure Al have a smooth surface after pore sealing, and that cracks are formed in anodic oxide films on 1050-, 3003- and 5052-aluminum alloys, after pore sealing. Corrosion protection after pore sealing increased with anodizing time on pure Al, but only slightly increased with anodizing time on the Al alloys.

Effect of the Passive Oxide Films Structure and Surface Temperature on the Rate of Anodic Dissolution of Chromium-Nickel and Titanium Alloys in Electrolytes for Their Electrochemical Machining. Part 2. Anodic Dissolution of Titanium Alloys in Nitrate and Chloride Solutions

An experimental study of the anodic dissolution of titanium and its alloys, including pulsed dissolution (up to 100 A/cm2) under control of hydrodynamic conditions and surface temperature in nitrate and chloride solutions, showed that the process is carried out through the electrochemical formation of anodic oxide films (AOF) which can be chemically dissolved. An AOF has a bilayered structure, i.e. two barrier films on the interface with a metal and a solution. Its model is PDM-III (Point Defect Model). At certain conditions, it is possible to achieve a steady state in which the film growth rate is compensated by the rate of its chemical dissolution (observed at pulsed conditions). In this case, over 100% current efficiency based on the ionization of titanium in the oxidation state is achieved. During the conducted experiments, when treated with the direct current, the rate of the electrochemical formation of an AOF exceeded the rate of its chemical dissolution, which led to a decrease in to current efficiency which did not exceed 75%. An increase of the dissolution rate takes place with an increase in the electrolyte flow rate. Upon reaching the thermokinetic instability (TKI) of the AOF (thermal explosion due to a feedback rate of the electrochemical reaction-surface tem-perature-rate of an electrochemical reaction), the interaction of electrolyte components with the surface free of the film takes place, and, as a result, an “anomalous” anodic dissolution is observed with the current efficiency greater than 100%. Regardless the nature of the electrolyte, TKI is achieved at a current density of 1 A/cm2. It is shown that in nitrate solutions with certain parameters of pulsed processing (duty factor 2, dc 50%), the dissolution rate, and, in the case of electrochemical machining, the feed rate of a cathode-tool, can be more than twice higher than the rate of machining with the direct current of the same density.

Improving the photoelectrochemical performance of porous anodic SnO x films by adjusting electrosynthesis conditions

A comprehensive characterization of photoelectrochemical (PEC) properties of the nanoporous tin oxide (SnOx) films of different thicknesses and complex internal morphology is presented. Nanoporous SnOx layers were obtained by one-step anodic oxidation (anodization) of Sn foil in 1 M NaOH. Semiconducting films with a uniform diameter of ca. 50 nm were grown if the potential of 4 V was applied during anodization, and the layers with different thicknesses were synthesized by changing the duration of the process. Switching the potential between 2 and 4 V allows the formation of anodic films with segments of different channel diameters. The optimal anodizing duration for layers with uniform channels was found to be 50 minutes, which corresponds to the thickness of SnOx film of slightly above 6 μm. In the case of layers with segments of different channel diameters, the segment arrangement was found to be crucial for optimal PEC performance. Layers with the segment of narrower channels being in the contact with the current collector were found to exhibit enhanced photoelectrochemical performance compared to their counterparts with an opposite segment arrangement. This was mainly attributed to the favorable band arrangement facilitating the transfer of the photogenerated electrons to the external circuit as well as enhanced adhesion of the anodic SnOx layer to the current collector. Highlights Nanoporous SnOx films were synthesized electrochemically. Layers with various morphologies were studied as photoanodes. The optimal thickness of anodic SnOx was found to be 6 μm. Enhanced PEC performance was observed for multisegment anodic films. The segment arrangement is crucial for optimal PEC performance.

Formation of smooth anodic nanoporous iron oxide film for enhancing photocathodic protection on plain carbon steel

It is essential to fabricate smooth nanoporous iron oxide film with fewer defects on plain carbon steel (CS) to improve photocathodic protection of the film in corrosive media. For that, high temperature annealing heat treatment was chosen to manipulate the metallographic structure of CS in the muffle furnace at 900 °C with the duration of 30 min. Following that, the annealed CS was anodized in a mixture of aqueous ammonium fluoride solution and ethylene glycol to fabricate smooth nanoporous oxide film on CS. Finally, as-anodized oxide film was calcinated at 450 °C for 4 h in nitrogen atmosphere to form crystalline iron oxide film. Several electrochemical techniques were applied to analyze the photoelectron property and photocathodic protection performance of nanoporous iron oxide films. On the contrary, rough nanoporous iron oxide film with more striking defects was fabricated on untreated (no annealed) CS by using the same anodization and calcination processes. The results show that the metallographic structure of annealed CS behaved more uniform ferrite grains and fewer pearlite precipitate than untreated CS. Thereby, a smooth nanoporous iron oxide film with fewer defects formed on annealed CS. On the contrary, a rough nanoporous iron oxide film with more defects, such as dimples and cracks were fabricated on untreated CS. Both of iron oxide films were composed of hematite as the main phase and magnetite as the secondary phase. Therefore, the smooth iron oxide film displayed comparatively better conductivity and higher donor density under the condition of illumination than the rough film. On account of those, smooth film exhibits comparatively higher anodic and cathodic reaction activities and more excellent and stable photocathodic protection to the counter.

Anodizing—The pore makes the difference

AbstractThe presentation gives an insight into the fascinating world of anodizing as corrosion protection but also goes far beyond. Starting with an introduction to the formation of anodic oxide films and their characteristic features, the authors briefly outline the subject of pore nucleation and growth. The number, size, and shape of the pores essentially determine the properties and play a crucial role in the final application of the anodizing layers. The tailoring of pore design, considering the final application as well as the use of the porous oxide layers for subsequent functional modifications, is demonstrated by examples, exclusively developed in the working groups of the respective authors or in joint research projects, which were in part professionally accompanied and supported by the German Corrosion Society GfKORR e.V.

Electrochemically driven one-pot oxidative conversion of arylhydrazines into aromatic iodides

The efficient metal-free electrosynthesis of 2,4-dinitrophenyl iodide is here reported starting from 2,4-dinitrophenylhydrazine. Surprisingly this dinitrated arylhydrazine minimizes, under the applied experimental conditions, any anodic multilayered film formation. This sustainable iodide-mediated oxidative dehydrazination enables coupling reaction of electrogenerated iodine with aryl radicals from electron-deficient arylhydrazines employing electricity as the driving force and an inexpensive halogen source. A mechanistic proposal explaining the formation of aryl iodides is presented and discussed.

Sulfosalicylic/oxalic acid anodizing process of 5854 aluminum-magnesium alloy: Influence of sealing time and corrosion tendency

In this paper, anodizing process of 5854 aluminum-magnesium alloy (5854Al-Mg) in oxalic acid (OXA) was investigated in the absence and presence of 5-sulfosalicylic acid (SSA). Electrochemical and surface morphological techniques were used to study the corrosion resistance and film characterizations. Furthermore, the influence of sealing time on corrosion rate was addressed. It was observed that the corrosion current density decreased with the addition of SSA and increasing with sealing time. Maximum coating efficiency was 90.72% in the presence of 0.09 M SSA at 50 min of sealing period. The open circuit potential (OCP) was shifted to a more positive direction indicating the formation of passive film. While, the values of polarization resistance (Rp) of aluminum alloy were higher in the presence of SSA. Scanning electron microscopy (SEM) showed that the addition of SSA improved the surface properties. The anodizing of 5854 Al alloy in the presence of SSA resulted in the formation of anodic films of higher hardness, less roughness, and high film thickness as compared with OXA alone. Reaction kinetics models were proposed. The best fitting was obtained with zero-order reaction kinetics. Half-life time was increased from 9.05 to 123.3 min in case of OXA/SSA anodizing process. In addition, four mathematical equations were suggested to represent the corrosion current density as a function of time. Power model was the most significant one as compared with linear, second order, and exponential models.

Investigation of the features of the porous morphology of anodic alumina films at the initial stage of anodization

The work is devoted to the study of the porous structure formation of anodic alumina films at the initial stage of aluminium anodizing. SEM images of the surface morphology of the oxalic acid anodic films were analyzed. It was shown that at the initial stage, both major and minor pores are formed, the diameter ratio of which is about 1.16 and does not depend on the anodizing voltage. The results obtained indicate that the minor pores in the anodic films are located inside hexagonal cells composed of the major pores.

Passivity of Holmium Studied Theoretically by Potential-pH Diagrams for Selection of Electrolytes and Experimental Proof of the Formation of Ultra-Thin Anodic Films

We present an efficient strategy to identify anodizing electrolytes by coupling conventional Eh–pH diagrams with first-principles density functional theory calculations. Herein, the growth of ultra-thin films having a thickness of 10–20 nm is successfully demonstrated on thermally evaporated holmium. Considering only thermodynamic basis, simulated Eh-pH diagrams, and solubility analysis, electrolytes having different compositions and pH values are suggested for the efficient growth of anodic films. The Eh–pH diagrams are modified by incorporation of oxide–forming species in such a way to appreciably extend the stability domain. The predicted diagrams showed a strong agreement with the experimental observations and provide a better understanding of Ho–H2O based aqueous systems and can serve as a guide in other rare-earth elements anodizing. Grown film properties are investigated by using electrochemical impedance studies which disclose a linear increase in inverse capacitance with formation voltage indicating a uniform growth of anodic films irrespective of electrolyte selection. Formation factor, k, of anodic films grown up to 10 V varies from 1.16 to 1.95 nm V −1 in selected electrolytes. The k values greater than unity may contribute to the uniform film growth at different pH values.

Corrosion behavior of copper, tin, and bronze CuSn14 in acid rain solution

AbstractThe corrosion behavior of copper, tin, and bronze CuSn14 is studied in simulated acid rain (pH 4.5) by electrochemical techniques, cyclic voltammetry and electrochemical impedance spectroscopy. The potentiodynamic formation of anodic oxide film on copper and tin is described in terms of high‐field model. Cyclic voltammetry shows that dissolution of bronze is higher than of pure copper metal in acid rain. Electrochemical impedance spectroscopy data reveal that oxide films formed on copper and tin have a higher resistance and suppressed diffusion process through the surface layer than the oxide film formed on bronze CuSn14 at the same conditions.

Effect of Mn2+ on the anodic film and corrosion behavior of Pb-Ca-Sn alloy anode in copper electrowinning

In this paper, the effect of Mn2+ (≤160 mg/L) on the anodic film properties and corrosion behavior of Pb-0.07%Ca-1.25%Sn anodes in acidic CuSO4 electrolyte is clarified. The anode was polarized in an electrolyte containing different concentrations of Mn2+ ions for 48 h to obtain a stable oxide film on the surface. The phase composition and morphology of the oxide layer formed on the Pb-0.07%Ca-1.25%Sn anode after polarization were characterized by the XRD, SEM, and EDS techniques. The influence of Mn2+ (≤160 mg/L) on the electrochemical behavior of the Pb-0.07%Ca-1.25%Sn anode was investigated in 45 g/L Cu2+ and 180 g/L H2SO4 solution via cyclic voltammetry, galvanostatic polarization, Tafel, and EIS measurements. In addition, the anodic weight loss, film formation, and corrosion mechanism of the anode are also proposed. The results reveal that the film formed on the Pb-0.07%Ca-1.25%Sn anode exhibited a gradient Pb|PbSO4|α-PbO2|α-PbO2-β-MnO2 layer in the Mn2+-containing electrolytes. Then, during the oxygen evolution reaction (OER), the gradient layer was oxidized to MnO2, which was irregularly attached to the surface of the substrate and increased the roughness of the film. The presence of Mn2+ accelerated the corrosion of the anode in the electrolyte and reduced the anodic oxygen evolution potential. When the concentration of Mn2+ ions reached 160 mg/L, the corrosion rate was increased by 62.3% despite the anode potential decreasing by 30 mV.

Influences of pulse frequency on formation and properties of composite anodic oxide films on Ti-10V-2Fe-3Al alloy

A thick composite anodic oxide film was fabricated in an environmentally friendly malic acid electrolyte containing PolyTetraFluoroEthylene (PTFE) nanoparticles on Ti-10V-2Fe-3Al alloys. The influence of pulse frequency on the morphology, microstructure and composition of composite anodic oxide films containing PTFE nanoparticles was investigated using Field Emission Scanning Electron Microscopy (FE-SEM) equipped with Energy Dispersive Spectroscopy (EDS), Atomic Force Microscopy (AFM) and Raman spectroscopy. The tribological properties in terms of the friction coefficient, wear loss and morphology of worn surfaces were measured by ball-on-disc tests. The electrochemical property was evaluated by potentiodynamic polarization. The results indicated that the titanium dioxide of composite anodic oxide films transformed from anatase to rutile with the change of pulse frequency, which could result from the electrochemical dynamic equilibrium. The combination of PTFE nanoparticles and malic acid electrolyte molecules can influence the energy fluctuation of electrochemical equilibrium and formation of composite anodic oxide films. Moreover, composite anodic oxide films fabricated under the condition of 1.0–2.0 Hz exhibited the best wear resistance and corrosion property. The schematic diagram of the film formation and PTFE nanoparticles spreading process under different frequencies was elucidated.