Anodic Alumina Films Research Articles

Nano-anchor effect by anodic oxidation of aluminum sheets in joining by electrodeposition

Two anodized and stacked aluminum sheets were adhered by electrodeposition of copper. The anodizing conditions were varied and the adhesion strength (interface shear strength) was investigated by lap shear tests. The interfacial shear strength depended on the conditions of the anodic oxidation and reached >60 MPa when the aluminum sheets was anodized in H3PO4 before electrodeposition of copper. An anodic aluminum oxide film was formed by anodic oxidation (anodization) of the aluminum sheets and had porous structure with straight nanopores perpendicular to the surface. Hence, anodized Al sheets were strongly adhered by Cu electrodeposition because of nano-anchor or interlocking effect between the porous structure and electrodeposited copper. The proposed joining technique through electrodeposition may be useful because it can provide electrically conductive joints at room temperature and ambient pressure.

Model of Stress Generation in Anodic Aluminum Oxide Films: Part I. Origin of Stress at the Film Interfaces

Stress builds up near the interfaces of anodic aluminum oxide layers prior to formation of self-organized arrays of pores. Pore initiation may result from a viscous flow instability driven by interfacial stress gradients. We examined the mechanism of interfacial stress generation through a model of coupled viscous flow and ionic migration during growth of planar anodic aluminum oxide layers. In the model, diffusion-induced stress is produced at the oxide-solution interface because deposition of new oxide is constrained by strong adsorption of electrolyte oxyanions. Stress distributions were calculated for barrier oxide growth in phosphoric acid, using values of ion migration parameters and oxide viscosity from independent experimental results. Both the stress layer thickness of a few nanometers and the magnitude of the interfacial stress agreed with those found experimentally. The model also correctly predicted transitions between compressive and tensile stress at the oxide-solution and metal-oxide interfaces at critical current densities. The thin stress layer at the solution interface can be viewed as effective surface tension dependent on the local current density. The current distribution on a nonplanar interface would induce surface tension gradients resulting in shear and normal stress in the oxide, and these in turn could drive oxide flow leading to self-organized porous films.

Model of Stress Generation in Anodic Aluminum Oxide Films: Part II. Surface Stress Accumulation Preceding Formation of Self-Organized Pore Arrays

Growth of self-organized porous anodic alumina films initiates after growth of the initial compact barrier oxide becomes unstable. The process of self-ordering is characterized by evolution of an irregular distribution of incipient pores into the characteristic steady-state pore array. We explore the roles of mechanical stress and oxide flow in self-ordering, through a modeling analysis of stress-driven viscous flow of oxide. To account for oxide topography evolution, the model domain is chosen to be a spherical shell of oxide representative of the film during the transition to the steady-state porous layer. The model is based on coupled viscous flow of oxide and stress- and electric field-driven migration of Al+3 and O−2 ions. The calculation results successfully depict the buildup of compressive stress within a layer of a few nanometers thickness at the oxide surface, as inferred previously from in situ stress measurements. Additionally, the model predictions are consistent with observations of tensile bulk stress in the oxide accompanying inward oxide flow during the transition to the steady-state pore array. This transition is due to competitive growth of the initially nonuniform distribution of incipient pores until a monodisperse steady-state distribution is attained.

Interconnects and Interposer in Porous Anodic Aluminum Oxide: The Fabrication Technology and Process Integration

Porous anodic aluminum oxide (AAO) features its interesting nanoporous structure which is capable of being used as a template for nanopatterning, highly anisotropic nanowire fabrication, and MEMs devices fabrication. In this work, AAO was demonstrated to be an interposer substrate for 2.5/3D packaging technology in microelectronics exploiting the unique etching characteristic of nanopores. Not only the high density vertical through via was fabricated but also metallized with Cu as void-less interconnect simply by using wet chemicals. Passive devices like inductors were embedded in AAO film after via metallization and re-distribution layer Cu electroplating which provide multifunctionalities to proposed AAO interposer. AAO with the special structural and etching properties plus its low electrical loss, thermal stability, and mechanical robustness, is a promising material for heterogeneous integration and advanced microelectronic packaging technology.

Rapid Fabrication of Porous Anodic Alumina Films with Large Interpore Distances in an Ethylene Glycol and Ammonium Dihydrogen Phosphate-Based Electrolyte

Porous anodic alumina (PAA) films have become popular nanotemplates due to the ordered hexagonal pore array structure. The fabrication of well-ordered PAA films with larger interpore distances is still a challenge to date. Here, we develop a unique electrolyte system based on the reaction products of ethylene glycol (EG) with ammonium dihydrogen phosphate (ADP). In the novel electrolytes, stable high-voltage anodization with extremely high current densities (the peak value is close to 300 mA cm−2) can be realized at 400 V or even higher without breakdown. In 5 wt% EG-ADP-based electrolyte containing phosphoric acid, a well-ordered PAA film with an interpore distance of ∼610 nm can be fabricated at 250 V at a high growth rate of 1.3 μm·min−1.

Fabrication of Crystallized Porous Anodic Aluminum Oxide under Ultra-High Anodization Voltage

Porous anodic aluminum oxide (PAAO) films have attracted increasing attention due to their widespread applications as porous templates. In fact, PAAO films fabricated using high-voltage anodization have apparent advantages which include large-range tunable interpore distance, less impurities, and diverse microstructures. Finding the proper methods to explore the maximum anodization voltage and to analyze related mechanisms is important. Here, crystallized PAAO films with novel microstructures have been fabricated under ultra-high anodization voltage of 700–1000 V in ethanol-oxalic electrolyte system. The unique microstructure and composition of the prepared PAAO films have been studied by using characterization methods of scanniong electron microscopy and X-ray diffraction. Some important phenomena become more obvious under extreme experimental conditions, thus a better understanding of related intrinsic mechanisms of aluminum anodization can be achieved.

Effect of Joule Heating on Formation of Porous Structure of Thin Oxalic Acid Anodic Alumina Films

The effect of electrolyte temperature and anodizing voltage on the surface morphology of thin oxalic acid anodic alumina films formed on the SiO2/Si and glass-ceramic substrate was studied. It was shown when anodizing voltage increased from 10 to 20 V pore diameter increased from 12.2 to 15.5 nm and from 14.5 to 17.7 nm for the films formed on SiO2/Si and glass ceramic substrates, respectively. For the films formed at anodizing voltage (Ua) 30 and 40 V, pore diameter was 20.5 and 25.5 nm, respectively, and it was independent of the substrate type. It was demonstrated that when thin aluminum films were oxidized in oxalic acid, anodizing voltage of 30 V was a turning point at which the mechanism of the formation of anodic porous structure changed. By performing the experiments on the substrates with different thermal conductivity, we showed that at Ua < 30 V generated in the barrier layer Joule heating was absorbed by the substrate, but at Ua > 30 V the heat warmed up the electrolyte at the pore bottom that resulted in the changes of morphology of the films. We also suggested the empiric equation that connected pore diameter, anodizing current and generated Joule heating.

Self-organization of porous anodic alumina films studied in situ by grazing-incidence transmission small-angle X-ray scattering.

Self-ordered porous anodic alumina (PAA) films are studied extensively due to a large number of possible applications in nanotechnology and low cost of production. Whereas empirical relationships between growth conditions and produced oxides have been established, fundamental aspects regarding pore formation and self-organization are still under debate. We present in situ structural studies of PAA films using grazing-incidence transmission small-angle X-ray scattering. We have considered the two most used recipes where the pores self-organize: 0.3 M H2SO4 at 25 V and 0.3 M C2H2O4 at 40 V. During anodization we have followed the evolution of the structural parameters: average interpore distance, length of ordered pores domains, and thickness of the porous oxide layer. Compared to the extensively used ex situ investigations, our approach gives an unprecedented temporal accuracy in determination of the parameters. By using of Al(100), Al(110) and Al(111) surfaces, the influence of surface orientation on the structural evolution was studied, and no significant differences in the interpore distance and domain length could be observed. However, the rate of oxide growth in 0.3 M C2H2O4 at 40 V was significantly influenced by the surface orientation, where the slowest growth occurs for Al(111). In 0.3 M H2SO4 at 25 V, the growth rates were higher, but the influence of surface orientation was not obvious. The structural evolution was also studied on pre-patterned aluminum surfaces. These studies show that although the initial structures of the oxides are governed by pre-patterning geometry, the final structures are dictated by the anodization conditions.

An Analysis of the Electrolyte Resistivity Effect on the Pore Diameter and Pore Density of Anodic Aluminium Oxide (AAO) Films Produced by Single-Step Anodization

An Analysis of the Electrolyte Resistivity Effect on the Pore Diameter and Pore Density of Anodic Aluminium Oxide (AAO) Films Produced by Single-Step Anodization

Controlling the Color and Effective Refractive Index of Metal-Anodic Aluminum Oxide (AAO)–Al Nanostructures: Morphology of AAO

Metal-anodic aluminum oxide (AAO)–Al nanostructures have been deposited by using sputtering for the metal layer deposition and a two-step anodization process in different electrolytes to produce self-ordered anodic aluminum oxide films. The effect of the morphological parameters of AAO films (such as thickness, pore diameter, interpore distance, and porosity) on the optical properties was studied. The UV–vis reflectance properties as a function of the thickness for the different electrolytes of metal-AAO–Al films were analyzed in order to obtain the color diagrams and the effective refractive indexes of the films. The effective refractive index was found to depend on the thickness and porosity of AAO films. The change in color observed in metal-AAO–Al nanostructures is due to the thickness and porosity of the AAO films. In order to verify the experimental results, UV–vis reflectance spectra of AAO–Al films were simulated using commercially available finite element simulation software.

Morphology evolution of porous anodic alumina in mixed H3PO4/NH4F electrolytes

Most anodizations of Al to obtain porous anodic alumina (PAA) are conducted in acid solution including sulfuric acid, oxalic acid and phosphoric acid solutions. It is well known that fluoride-containing electrolyte is generally used to fabricate anodic TiO2 nanotubes (ATNTs). Nevertheless, there are few articles reporting anodization processes of Al in NH4F electrolyte and mixed H3PO4/NH4F/ethylene glycol electrolyte. Herein, in order to explore different morphology of anodic alumina film during a series of constant current anodization processes, NH4F solutions, H3PO4 solutions of different concentrations (2wt%, 4wt% and 6wt%), and their mixed electrolytes containing different concentrations of NH4F and H3PO4 were employed. It is the first time for Al to be performed anodization in NH4F solution under constant current. PAA with thick pore wall, thick barrier oxide layer and narrow pores were obtained. The results show that three stages of voltage-time curve are not the requirement of obtaining PAA. Which anodic oxide (porous anodic oxide or compact anodic oxide) occurs only depends on the size of the electronic current. As long as the electronic current is high enough, porous anodic oxide is easily obtained. While the concentration of NH4F increases to 0.6wt%, the barrier oxide is much thicker than that in H3PO4 electrolyte due to the high ionic current and low electronic current.

Growth of Porous Anodic Alumina on Low-Index Surfaces of Al Single Crystals

The pseudoepitaxial growth of amorphous anodic alumina with ordered porous structure within single crystal grains of aluminum substrates is an amazing feature of the self-organization process, which occurs during anodization. Here, we used single crystal Al(100), Al(110), and Al(111) substrates to inspect the effect of aluminum crystallography on anodization rates and the morphology of the resulting alumina films grown under different anodization conditions. The difference in the kinetics of porous film growth on various substrates is described in terms of the activation barrier of aluminum atom release from the metal surface to the oxide layer. Scanning electron microscopy and small-angle X-ray scattering are applied for quantitative characterization of different kinds of ordering in anodic alumina films. The highest number of straight channels was found in porous anodic alumina grown on Al(100) substrates, whereas Al(111) was proved to induce the best orientational order in anodic alumina with the forma...

Advances of the research evolution on aluminum electrochemical anodic oxidation technology

This article gives an overview on the development of aluminum anodization technique in terms of fundamental aspects and practical applications in the past decades. Besides, the formation mechanism and structural characteristics of anodic alumina films as well as the factors affected the formation of porous anodic alumina films are also discussed. Anodic aluminum oxide (AAO) prepared by the anodization method can be divided into two categories: dense anodic alumina (DAA) and porous anodic alumina (PAA). This article also summarizes the optical properties, magnetic properties, solar absorption properties, and catalytic properties of porous anodic alumina film and its applications in nanomaterials, optical materials, magnetic materials, biosensors, solar cells, and so on. In addition, future developmental trend of porous anodic alumina film is covered.

The effect of propylene glycol on the optical properties of iridescent porous anodic alumina films

Porous anodic alumina films with structural colors were successfully prepared in the mixed electrolyte which was the phosphoric acid of 0.6 M/L added with propylene glycol at 100 V and − 5 °C. In the experiment, by changing the electrolyte composition, the anodization rate of the film was controlled, and thus a precise adjustment to the structural colors of the films was achieved and, as a result, more intermediate colors were obtained. This study shows that the pore growth rate of the films during anodization can be greatly decreased by adding propylene glycol, and the reasons would be given in this paper. It was also discovered that the Au-coated thin layer helps to enhance the saturation of the film colors. These films show potential in many areas including decoration, display, anti-counterfeiting and so on.

Formation of Porous Anodic Alumina under Unstable Electroconvection Flow Regimes: A Case Study of Tartronic Acid Electrolyte

This work focuses on the systematic study of porous anodic alumina films grown in novel moderately concentrated tartronic (2-hydroxymalonic) acid electrolyte. The morphological analysis enabled us to reveal new details of the alumina pore formation mechanism and to discover the processes responsible for such frequently occurring defects as branched pores and multiple pore openings within a single cell. The qualitatively new model developed herein explains satellite nanochannel formation from a colloidal electroconvection standpoint—namely, by the temporary change of the existing nanoconvective flows, which give rise to the cooperative cell pattern in porous alumina, from laminar to turbulent mode (such flow regimes are generally termed “transitional” or in some recent literature also “laminar unstable”). Our theoretical discussion demonstrates that the morphological parameters of self-organized layers are strongly related to such properties of the liquid phase as the dielectric constant and the Debye leng...

Influence of the anodizing process variables on the acidic properties of anodic alumina films

Abstract In the present work, the effect of the different variables involved in the process of aluminum anodizing on the total surface acidity of the samples obtained was studied. Aluminum foils were treated by the electro-chemical process of anodic anodizing within the following variable ranges: concentration = 1.5-2.5 M; temperature = 303-323 K; voltage = 10-20 V; time = 30-90 min. The total acidity of the samples was characterized by two different methods: acid-base titration using Hammett indicators and potentiometric titration. The results showed that anodizing time, temperature and concentration were the main variables that determined the surface acid properties of the samples, and to a lesser extent voltage. Acidity increased with increasing concentration of the electrolytic bath, whereas the rest of the variables had the opposite effect. The results obtained provide a novel tool for variable selection in order to use synthetized materials as catalytic supports, adding to previous research based on the morphology of alumina layers.

Formation Mechanism of Alumina Nanotubes By Anodization of Aluminum Alloy

Anodic porous alumina film, a typical self-ordered nanohole material formed by anodizing aluminum in an appropriate acidic solution, is used as a starting material of nanofabrication of functional devices besides a corrosion resistant surface oxide. In contrast to anodic titania nanotubes, anodic porous alumina is generally compact without spacing between the cell boundary. The tubular shape of anodic titania is resulted in enrichment of fluoride between cells which is originated form electrolyte and more soluble than anodic titania. In the case of aluminum, it is reported that enrichment of only sulfate or ethylene glycol originated form electrolytes produces alumina nanotubes by anodization. In the present paper, the formation mechanism of anodic alumina nanotubes will be discussed. STEM image of vertical cross section prepared by ion-milling of the anodic film, which was formed on 99.99 % pure aluminum in oxalic acid at 160V, indicated close-packed hexagonal cell arrays with small voids of 15-20 nm at the triple cell points. Dark field STEM image clearly revealed a bright particle in the each void suggesting the presence of a heavy metal because of Z-contrast. By EDX point analysis, Cu enriched particles were found in the voids at the triple points. Nevertheless, in the case of anodic film formed at the similar condition on AC8A cast alloy including 0.97 % of Cu, tubular cells with the space of approximately 50 nm between cell boundary were formed unlike the case of pure aluminum including 60 ppm of Cu. Numerous branching of pores were also observed. EDX mapping revealed that Si was enriched in the outer layer of the cells and Cu was enriched at the area near cell boundaries. Therefore, it is deduced that enrichment of Cu at the cell boundaries produces the tubular cells by accelerating oxide dissolution similar to the cases of enrichment of sulfate and ethylene glycol.

Effect of Electric Field Strength on Barrier Layer Thickness and Anion Incorporation of Anodic Porous Alumina

Anodic porous alumina films having a closely packed hexagonal array of cells have attracted much attention as a starting material in the fabrication of nano-devices because of the high controllability of porous structures (e.g., cell size, pore size, pore interval, and pore depth) on dimensions ranging from the submicron to nanometer range. In recent years, the mechanism of self-ordering of cell arrangement of anodic porous alumina has been intensely studied. We proposed anodizing under high electric field strength as a key factor for self-ordering of cell arrangement [1, 2]. However, only a few reports are found on the irregularities of barrier layer thickness and morphology of each cell of anodic porous alumina. In this study, we evaluated the influence of electrolytic factors (e.g., current density, applied voltage, and anodizing time) on cell morphology of anodic porous alumina. Especially, we focused on the effect of the electric field strength on the barrier layer thickness and anion incorporation behavior. High purity aluminum sheets were anodized in typical electrolytes of sulfuric acid, oxalic acid, and phosphoric acid at different voltage. After detachment of the porous alumina films from the substrate in saturated mercuric chloride solution, the specimens were immersed in phosphoric acid to dissolve the barrier layer from the backside. By SEM observation of structural change of the cells during chemical etching, the homogeneity of the barrier layer thickness could be evaluated. Although the thickness of the barrier layer of porous alumina formed at constant voltage has been believed to be uniform, we confirmed that the thickness of the barrier layer of each cell was not uniform; the thickness of the barrier layer of the smaller cells was thinner than that of the larger cells. Concerning anion incorporation behavior, we evaluated the depth of anion incorporation into the cell wall by the difference in the strength of contrast of the SEM image of rear surface of porous alumina films because the dissolution rate of the anion-free layer was slower than that of the anion-incorporated layer. The details of the thickness inhomogeneity of the barrier layer and anion incorporation behavior will be discussed from a standpoint of the high field theory.

Impact of Interconnections, Dynamic Conductivity, Pore Size on the Performance of V2O5 Cathode for Lithium Ion Batteries

It is well known that a nanostructured electrodes have superior performance over their bulk counter parts. It has been pursued by the battery community to explore different nanostructures to discover critical parameters for designing next generation batteries. Here, we describe a systematic study from 1D array electrode to 3D electrode enabled by nanostructured scaffolds and atomic layer deposition. Nanostructured scaffolds have been used as templates for the fabrication of nanostructured materials for wide range of applications. One of the commonly used scaffolding material is porous anodized aluminum oxide films (AAO). AAO are used for their hexagonally ordered nanoporous structures and the ease of controlling the structural parameters such as the pore diameter and the pore length by altering the anodization conditions. Here we use voltage control to fabricate not only the traditional straight porous AAO templates, for 1D nanotube array, but also interconnected porous AAO templates, 3D interconnected nanotube array. From previous work, by deviating from a constant voltage, which results in straight pores, to lower voltage and back to the initial voltage cause the pores to deviate from their straight structure to interconnecting layer and back to their straight structure allowing controllable insertion of the interconnecting layers throughout the film. Using atomic layer deposition, V2O5 was conformally coated along the walls of these high aspect ratio AAO with sputtering of gold current collector at the top. Using AAO as a template which can be tuned systematically, the resulting nanostructured V2O5 electrodes were examined for Li ion batteries as it is one of the well characterized cathode material. V2O5 can reversibly accommodate up to two Li ions. From the previous study, when V2O5 was cycled within the one Li insertion window, the 3D interconnected nanotubular structure showed superior power capabilities over the 1D nanotubular array. This was due to the interconnections allowed more of the materials closer to the top current collector. As faradaic reactions are much more sensitive to the ohmic loss, this close proximity to the current collector allowed more efficient utilization of the overall electrode. With the two Li ion insertion window, an opposite trend was observed, with interconnected electrode performing worse. As the structure between the one and two Li insertion windows were kept constant, we hypothesized two possibilities. One, because we have more material within the interconnected electrodes, it will require more Li ions than the straight electrodes. However the pore opening is the same which could lead to ion starvation in the interconnected electrodes because the diffusion of the Li ions into these interconnected electrodes could not keep up with the demand. Two, V2O5 with the second lithiation causes a dramatic change the electrochemical properties more severely in the interconnected electrode. To test these hypothesis, different pore diameter electrodes were used. With the larger pores, the electrodes overall performed better but the difference between the interconnected and the straight electrodes could not be explained. Literatures show the electronic conductivity of V2O5 varies dramatically depending on the degree of lithiation. Up to one lithiation, the conductivity of the material slightly increases whereas with the second lithiation the conductivity dramatically drops by two orders of magnitude due to the disruption in the electron conduction mechanism. This could explain the performance difference between the straight and the interconnected electrodes. When the current density is low, the electrons have enough time to travel through the electrode regardless of the structure. When the current density is high, now the conductivity matters. In the interconnected electrode, with one Li insertion, the conductivity of the electrode near the current collector increases with the degree of lithiation allowing better usage of the material farther away from the current collector due to lower ohmic loss of the overpotential needed. However, with two Li insertion the conductivity drops near the current collector which prevents the utilization of the material farther away resulting in lower power performance. Comsol simulations are in progress to better understand the overpotentials and ion depletion in these straight and interconnected electrodes. This work is able to show the need to match the properties of the electroactive material with the properties of the nanostructure for high performance electrodes.

(Invited) Role of Stress Induced By Anion Incorporation in Formation of Porous Anodic Alumina Films

Procedures to grow porous anodic alumina films with ordered and controllable arrangements of nanoscale pores have led to extensive research on PAA films as functional materials. Experiments and calculations show that transport in the oxide is assisted by plastic flow driven by mechanical stress (1-3). Pores initiate as the result of a morphological instability at the oxide-solution interface of the initially formed barrier oxide (4). Stress distribution measurements revealed high near-surface compressive stress levels, which may be associated with locally elevated concentrations of incorporated electrolyte anions. The instability leading to pore formation may be influenced by flow driven by anion incorporation-induced stress (5). Direct evidence for anion-induced stress in aluminum oxide was sought through stress measurements during open-circuit immersion of Al in phosphoric and sulfuric acids. The stress is compared with elastic anion incorporation-induced stress calculated from X-ray photoelectron spectroscopy (XPS) measurements of film composition. Al sheet samples of 99.998% purity were exposed at open circuit in either 0.4 M H3PO4 or 0.4 M H2SO4 solutions. In situ stress measurements during acid dissolution were carried out by phase-shifting curvature interferometry (6). According to the thin-film Stoney approximation, the detected sample curvature changes are directly related to the surface force change per sample width. The force per width is the biaxial in-plane stress integrated through the sample thickness. XPS was performed with a K-alpha instrument (Thermo Scientific) (7). The relative concentrations of P, S, Al, and O in the oxide were determined from areas of P2s, S2s, Al2p, O1s peaks. The oxide thickness and the anion concentration per area were determined from the relative areas of the Al2p peak contributions from oxide and metal. Fig. 1 shows that upon immersion of Al in both acids, force per width increased abruptly in the compressive direction to 2 min, and then relaxed slowly to constant levels. The initial compressive increases are not due to mechanical disturbances, as they significantly exceed the stress change in distilled water. The hypothetical anion incorporation-induced stress was calculated from the anion concentration per area by assuming that all the detected anions are absorbed within the oxide. The elastic force per width accompanying incorporation is then FwA = -(2/9) W A EoxNA /(1 - n ox ) where WA is the anion molar volume, Eox and n ox the elastic modulus and Poisson's ratio of alumina, and NA the per-area anion concentration from XPS. Fig. 1 shows that for dissolution in H2SO4, FwA is in excellent agreement with the measured force at all times. For H3PO4, agreement during the initial compressive increase is found but thereafter phosphate adsorption causes the calculation to diverge. Fig. 1 is strong evidence that anion incorporation in alumina films induces compressive stress. The stress of ~ 2 GPa inferred from the figure are comparable to near-surface stress levels measured earlier in anodic films (5). Additionally, we discuss evidence for anion-incorporated surface layers in anodic alumina from high-voltage cyclic voltammetry (CV) experiments (8). At high scan rates, the current-voltage dependence in the cathodic-direction scan reveals the conduction characteristics of the anodic film. By varying the anodic potential limit of CV, the dependences of conduction on both film thickness and electric field are assessed. High-resistivity surface layers are revealed in anodic films formed in phosphoric, sulfuric and oxalic acids. An instability mechanism for anodic alumina is described in which both the high resistivity and elevated stress level of the anion-containing near-surface layer play important roles. ACKNOWLEDGMENT Financial support was provided by JFE Steel Corporation and China Scholarship Council. REFERENCES 1. S. J. Garcia-Vergara, et al., Electrochim. Acta. 52, 681 (2006). 2. J. E. Houser, K. R. Hebert, Nature Mater. 8, 415 (2009). 3. J. Oh, C. V. Thompson, Electrochim. Acta, 56, 4044 (2011). 4. P. Mishra, K. R. Hebert, Electrochim. Acta, 222, 1186-1190 (2016). 5. Ö. Ö. Çapraz, et al., Electrochim. Acta 167, 404-411 (2015). 6. Ö. Ö. Çapraz, K. R. Hebert, P. Shrotriya, J. Electrochem. Soc., 160, D501-516 (2013). 7. Ö. Ö. Çapraz et al., Electrochim. Acta, 238, 368-374 (2017). 8. S. Ide, P. Mishra and K. R. Hebert, Electrochim. Acta, 221, 1-7 (2016). Figure 1