Corrosion Resistance Of Anodic Films Research Articles

Effect of Barrier Layer on Corrosion Resistance of Porous-Type Anodic Films Formed on AA2055 Al–Cu–Li Alloy and Pure Aluminum

It is known that typical porous-type anodic films formed on aluminum and its alloys consist of two layers, i.e. a thick porous layer and thin barrier layer. However, the effect of the two layers on corrosion resistance of the anodic film is still not clear, which prevents a thorough understanding of the protection mechanism of the anodic film, thereby limiting the potential to further optimize the anodizing treatment. Herein, an electrochemical barrier layer thinning (EBLT) process was employed to reduce the thickness of the barrier layer of the porous-type anodic films formed on an AA2055 Al–Cu–Li alloy and pure aluminum. Then the structure and composition of the anodic films before and after immersion in a NaCl solution were studied in a comparison manner. It was found that the EBLT process evidently decreased the corrosion resistance of the anodic films formed on AA2055 alloy and pure aluminum. Thus, it was conclusively validated that the barrier layer of the porous-type anodic film played a decisive role in controlling the corrosion resistance of anodized aluminum and aluminum alloys prior to post sealing.

Effects of pore parameters on performance of anodic film on 2024 aluminum alloy

Anodic films on aluminum alloys are prepared in sulphuric/phosphoric acid electrolyte. The relationship between the pore parameters and the mechanical properties and corrosion resistance of the film is studied. With increased film porosity, the film microhardness decreases and the bonding strength increases, while for the same film porosity, larger pores decrease microhardness but increase bonding strength. As sulfuric acid concentration increases, the number of dissolved pores increases while the interpore distance and the pore roundness decrease. All these changes would promote cracking of the anodic film after heating. With changed pore parameters, the crack density increases more quickly than the crack width. After 4800 h immersion in NaCl solution, for films with porosity of 24.3% and 28.2% the barrier layer resistance decreased slowly, but for films with porosity higher than 30%, the barrier layer resistance decreased much faster. The porous layer resistance of the films shows similar variations as the barrier layer resistance. The lower film porosity, better film continuity and more regular pore structure could improve the corrosion resistance of anodic films. From the view of corrosion protection, it is suggested that the porosity of anodic films should be controlled below 30%.

Enhanced Corrosion Resistance of Anodic Films Containing Alumina Nanoparticles on as-rolled AZ31 alloy

In this work, the samples of magnesium alloy AZ31 were anodized in two kinds of electrolytes, each with and without alumina nanoparticles. The structure, morphology and composition of four kinds of films were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersal spectroscopy (EDS), respectively. The corrosion behaviors of films were studied using electrochemical impedance spectroscopy and potentidynamic polarization curves. The wear behaviors were evaluated using a ball-on-disc rotating wear test. The results showed that the alumina nanoparticles tended to gather in cracks and pores leading to increased film density and lower corrosion rates. The wear rates of films were decreased by adding nanoparticles.

Effect of Electrolyte Concentration on the Structure and Corrosion Resistance of Anodic Films Formed on Magnesium through Plasma Electrolytic Oxidation

The thickness and corrosion resistance of films formed on AZ31 magnesium alloys through plasma electrolytic oxidation for the same amount of electricity supplied increased with decreasing electrolyte concentration irrespective of the current density. For all electrolytes of phosphate, silicate, and aluminate, the initial bending/breakdown voltage increased linearly with the logarithm of the decreasing electrolyte concentration and exhibited no dependence on the current density. The chemical composition of the films was independent of all electrolysis conditions such as electrolyte concentration, current density, and anodization time. This behavior is explained by a mechanism in which anion incorporation is governed by a chemical reaction/precipitation under plasma but not by the electric field, unlike the case for conventional anodizing of valve metals without sparking. The corrosion resistance of the films tended to increase as a function of the logarithm of the film thickness, irrespective of the electrolyte species and electrolysis conditions.

Effect of SiO2 Sol Sealing on Corrosion Resistance of Anodic Films of AZ31 Mg Alloy

Silica sol was used to seal the pores of anodic oxide film of AZ31 magnesium alloy. The effect of the immersion time, the calcinating temperature and the immersion times on morphology and corrosion resistance of the film was discussed. Scanning electron microscope (SEM) was employed to characterize the surface morphology of the composite film, and potentiodynamic polarization curves were used to analyze the corrosion behaviors. The surface composition of the sealed anodic films was studied by energy dispersion spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). It has been found that the corrosion resistance of anodic film is improved by sealing by silica sol. When the dipping time is 1.5 min and the calcination temperature is 350 °C, the corrosion resistance of the film is the best. Times of dipping and calcination have a greater influence on the performance of the film. When the dipping-calcination is 7 times, the film has a good corrosion resistance. After sealing, silica sol is adsorbed on inner pores and outside pores of the anodic oxidation film.

Research on the Morphologies and Corrosion Resistance of Anodic Alumina Films Influenced by Additives in Sulfuric Electrolyte

The surface morphologies, chemical composition, phase composition, compactness, thickness and the corrosion resistance of porous anodic films prepared in the sulfuric electrolyte with different additives at 35°C were investigated by XRD, FESEM, EDS, drop method in this paper. The additives are including organic acids, polyhydric alcohols and rare earth (REE) salts. It is indicated that a porously anodic film with about 20 nm holes in diameter can be obtained by additives into the sulfuric electrolyte. The main chemical compositions of the anodic films are Al, O elements, and a small amount of S. The film after boiling water sealing is composed of boehmite phase (Al2O3·H2O) and alumina (Al2O3). The corrosion resistance of the anodic film can be improved by the coupling effect of the additives. The mechanism is discussed.

Enhancing aluminum corrosion resistance by two-step anodizing process

Two-step anodizing process can be seen as an effective way to form an adhesive and continued layer on aluminum to enhance its corrosion resistance. Anodic alumina was produced by two types of anodization process, namely single-step and two-step anodizing. We have then evaluated their protection effectiveness against aluminum corrosion in aggressive medium (NaCl 3%). In both cases, X-ray diffraction reveals an amorphous nature of porous oxide films. The electrochemical impedance spectroscopy (EIS) results revealed a strong correlation between the corrosion performance and the morphology of the development anodic films. Also, these results indicated that the morphology and the corrosion resistance of anodic film formed by two-step anodizing process are very improved if compared to single one.

Effects of boric acid on microstructure and corrosion resistance of boric/sulfuric acid anodic film on 7050 aluminum alloy

The microstructure and corrosion resistance of different boric/sulfuric acid anodic (BSAA) films on 7050 aluminum alloy were studied by atomic force microscopy (AFM), electrochemical impedance spectroscopy (EIS) and scanning Kelvin probe (SKP). The results show that boric acid does not change the structure of barrier layer of anodic film, but will significantly affect the structure of porous layer, consequently affect the corrosion resistance of anodic film. As the content of boric acid in electrolyte increases from 0 to 8 g/L, the resistance of porous layer (Rp) of BSAA film increases, the capacitance of porous layer (CPEp) decreases, the surface potential moves positively, the pore size lessens, and the corrosion resistance improves. However, the Rp, CPEp and surface potential will change towards opposite direction when the content of boric acid is over 8 g/L.

Corrosion Behavior of Anodic Films Formed on Aluminium 7A04 Alloy in Boric/Sulphuric Mixed Acid and in Chromic Acid

The corrosion behavior of the anodic oxide film on aluminium 7A04 alloy is investigated in current paper, where the different anodic oxide films are prepared. Morphology observation on different anodic films is carried out by using laser confocal microscopy (LCM) and copper sulfate test. Meanwhile, the electrochemical properties are evaluated through using electrochemical impedance spectroscopy (EIS). The LCM observation shows that the anodic oxide film formed in boric/sulphuric mixed acid is uniform with porous appearance, while the anodic oxide film formed in chromic aid is nonporous one but defective. The copper sulfate test results show that the corrosion regions almost developed from micropores in strong corrosive media. The evaluation of EIS in NaCl solution demonstrates that the corrosion resistance of specimen with anodic oxide film is much better than that without film. Furthermore, the corrosion resistance of anodic film formed in boric/sulphuric mixed acid is better than that formed in chromic acid.

Optimisation of anodising electrolyte for magnesium alloy AZ31 and characteristics of anodic film

An environmentally friendly anodising electrolyte for the corrosion protection of magnesium alloy AZ31 was optimised using orthogonal experimental design of four factors with three levels. Electrochemical tests were conducted to determine the corrosion resistance of anodic film. The microstructure and phase composition of the optimised film were characterised by scanning electron microscope and X-ray diffraction respectively. The results show that the optimised electrolyte is composed of 60 g L−1 potassium hydroxide, 20 g L−1 sodium silicate, 30 g L−1 sodium tetraborate and 10 g L−1 sodium phosphate. The significance of electrolyte factors affecting the corrosion resistance of anodic film can be ranked as sodium phosphate>sodium silicate>sodium tetraborate>potassium hydroxide. The optimised film is porous like the basic film, but it is more compact and therefore more corrosion resistant than the basic film. The optimised film is mainly composed of MgO, Mg2SiO4 and MgP4O11.

FESEM and EIS Study of Sealed AA2024 T3 Anodized in Sulfuric Acid Electrolytes: Influence of Tartaric Acid

The present study investigates the effect of tartaric acid on anodic film morphology and on corrosion resistance of hydrothermally sealed anodized AA2024. Anodizing treatment was performed in dilute sulfuric acid electrolyte with or without addition of tartaric acid. Hydrothermal sealing was carried out in boiling water for each anodized specimen. The morphology of the sealed anodic films was examined using field-emission scanning electron microscopy (FESEM). Electrochemical impedance spectroscopy (EIS) was performed to assess sealing quality and corrosion resistance of the anodic films. It was shown that the sealing was more efficient in the case of anodic films formed in tartaric acid electrolyte mainly due to their lower porosity. It was also observed that the properties of the barrier layer were higher when sealing was performed on specimens anodized in the presence of tartaric acid. This suggests a specific role of the species on the barrier layer, which contributes to the enhancement of the performance in terms of corrosion resistance of the sealed anodic films. The present study clearly validates the beneficial role of tartaric acid in anodizing baths for the corrosion protection of AA2024.

The electrochemical behavior of anodic alumina films implanted with Ni ions

The motivation of this study is to investigate the possibility of improving corrosion resistance of anodic films on aluminum by Ni ions implantation. The implantation of Ni ions was carried out using a MEVVA source at an energy of 40 keV and a dose of 5.0 × 10 17 ions cm −2. Electrochemical impedance spectroscopy (EIS) was used to study the influence of the ions implantation on electrochemical behavior of anodic films in basic NaCl solution. The results show that Ni ions implantation strengthens the self-sealing process of the anodic film in solutions. The resistance of the porous layer ( R p) increases and the capacitance of the porous layer ( C p) decreases after implantation of Ni ions. For the anodic layer without implantation of Ni ions, as the immersion time increases, changes of C p and R p are slower. A more rapid change is observed for the anodic films implanted with Ni ions. XPS results reveal that Ni is present on the anodic film surface as metallic Ni and nickel oxide (NiO). AFM results show that the implantation of Ni ions made the surface structure and morphology of anodic films more uniform and less rough. The semi conductive property of anodic films in basic NaCl solution with and without Ni ions implantation was studied by Mott–Schottky analysis. For anodic films implanted with Ni ions, the profile is characteristic of p-type semiconductors from −1.0 V to −0.5 V. By expanding the M–S plots at high potentials to more positive potential than −0.5 V, the anodic film shows n-type characteristic. Possible reasons for the observed changes in surface properties are discussed.

Electrochemical behaviors of anodic alumina sealed by Ce-Mo in NaCl solutions

The elimination of toxic materials in sealing methods for anodic films on 1070 aluminum alloy was studied. The new process uses chemical treatments in cerium solution and an electrochemical treatment in a molybdate solution. Potentiodynamic polarization and electrochemical impedance spectroscopy(EIS) were used to study the influences of sealing methods on the corrosion behavior of anodic films in NaCl solutions. The results show that the Ce-Mo sealing makes the surface structure and morphology of anodic films uniform and compact. Ce and Mo produce a cooperative effect to improve the corrosion resistance of anodic films. Anodic films sealed by Ce-Mo provide high corrosion resistance both in acidic and basic solutions.

The improved corrosion resistance of anodic films on aluminum by nickel ions implantation

The influence of nickel ions implantation on the corrosion resistance of porous anodic films on aluminum was studied. Nickel ions were implanted into anodic films on 1070 aluminum using a MEVVA source at an energy of 40 keV with doses of 1.0×1017 and 5.0×1017 ions/cm2, respectively. Atomic force microscope (AFM) observations showed that the implantation of nickel ions made the surface structure of the anodic films more uniform. XPS analysis revealed that nickel is present in the states of metallic nickel and NiO on the surface of the anodic film. Electrochemical tests showed that anodic films with nickel ion implantation provided relatively higher corrosion resistance both in acidic and in basic solutions. Nickel ion implantation mainly affects the property of porous layer, which is characterized by the capacitance Cp and the resistance Rp. After nickel ions implantation, Rp values increased and Cp values decreased, which shows a more difficult penetration of the aggressive electrolyte into the anodic film. The concentration of nickel and formation of NiO on the surface of anodic film, which block the pores in the film, should be the main reason for better corrosion resistance of the implanted samples.

Microstructure and Corrosion Resistance of Anodic Films Formed on Magnesium

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EIS characterisation of anodic films formed on 2024 aluminium alloy, in sulphuric acid containing molybdate or permanganate species

Electrolytes composed of sulphuric acid and corrosion inhibitors (molybdate or permanganate species) were proposed in order to replace chromic acid for the anodising of 2024 aluminium alloy. The electrochemical impedance spectroscopy (EIS) method was used to visualise the correlation between the corrosion performance in NaCl and the morphology of these new anodic layers. From an appropriate equivalent circuit, EIS parameters concerning the porous and barrier layers were detected. Their evolution during corrosion tests was discussed. The results indicate that the morphology and the corrosion resistance of anodic films formed in acid sulphuric with molybdate species remain unchanged. On the contrary, morphological properties of anodic films formed in presence of permanganate species are modified, favouring their corrosion performance. EIS analyses were completed with SEM technique.