Acid Anodizing Process Research Articles

Development of Photocatalytically Active Anodized Layers by a Modified Phosphoric Acid Anodizing Process for Air Purification

One of the key urban air quality issues is pollution by nitrogen oxides (NOx). To reduce NOx, facade cladding could be provided with photocatalytic properties by incorporating titanium dioxide nanoparticles. For this purpose, a modified phosphoric acid anodizing process (MPAA) was developed for the facade alloy EN AW-5005, in which highly ordered anodized structures with a low degree of arborization and tortuosity were produced. Pore widths between 70 nm and 150 nm and layer thicknesses of about 2–3 μm were obtained. The subsequent impregnation was carried out by dip coating from water-based systems. Depending on the dip-coating parameters and the suspension used, the pores can be filled up to 60% with the TiO2 nanoparticles. Photocatalytic tests according to ISO 22197-1 certify a high photocatalytic activity was obtained with rPCE values > 8 and with rPCE > 2, achieving “photocatalytically active for air purification”. Tests on the corrosion resistance of the anodized coatings with a commercially available aluminum and facade cleaner confirm a protective effect of the anodized coatings when compared with nonanodized aluminum material, as well as with compacted anodized layers.

Effect of anodizing surface morphology on the adhesion performance of 6061 aluminum alloy

An experimental investigation on the phosphoric acid anodizing process of 6061 aluminum alloy has been carried out to enhance the adhesive bonding strength of 6061 aluminum alloy and the carbon fiber reinforced thermo-plastic. The phosphoric acid anodizing parameters of 6061 aluminum alloy were optimized by an orthogonal experiment design, and substrates with different surface morphologies were obtained. The surface free energies, surface roughness, and joint failure morphologies of samples with specific surface morphology were examined and analyzed to comprehensively evaluate the effect of surface morphology on the bonding performance of joints in high-temperature applications. The results indicate that under appropriate conditions, phosphoric acid anodization can change the fracture mode of the joint between the 6061 aluminum alloy and thermoplastic polyetheretherketone adhesive, as well as improve the performance of the joint. The appropriate number of circular structure pits on the surface of the aluminum alloy is key to improving the shear strength of the joint.

Influence of Fe-rich intermetallics and their segregation on anodising properties of Al-Si-Mg rheocast alloys

During rheocasting of Al alloys, the component's surface can be enriched in Fe-rich intermetallics due to the surface liquid segregation or the interaction with the die material. Although this precipitates enrichment affects only the surface, it can have a big impact on the quality of the surface treatments or durability. In this paper, the effect of Fe-rich intermetallics on the hardness and corrosion resistance of the anodized layer of AlSiMg alloys was examined by using nanoindentation and electrochemical impedance spectroscopy (EIS) techniques. A sulfuric acid anodising process was performed on unground and mechanical ground surfaces of TX630 substrates produced by the rheocasting process. During anodising, Fe-rich intermetallics can be dissolved, leaving voids in the oxide during its growth. The anodised samples with two different oxide layer thickness (4 and 7 μm) were tested by EIS in 3 wt-% NaCl solution for 12 h. A comparison of the EIS results of unground and mechanically ground surfaces demonstrated that the mechanical grinding successfully removed the Fe enriched layer, improving corrosion protection of the anodised oxide. Moreover, the hardness measurements indicate that the oxide layer on the mechanically ground surface with low Fe content shows higher hardness than on the unground surface with high Fe content. • Rheocasting surface can be enriched in Fe-rich intermetallic in form of Fe-rich skin. • Fe-rich intermetallic under the oxide layer facilitate the corrosion of the matrix. • The removal of Fe-rich skin makes the oxide layer harder and more resistant to corrosion.

Mechanistic study of thin film sulfuric acid anodizing rate difference between Al2024 T3 and Al6061 T6

Why different aluminum alloys anodizing at very different rate has been a puzzle for more than half a century. In this study, two high strength aluminum alloys (Al2024 and Al6061) were quantitatively compared for thin film sulfuric acid anodizing (TFSAA) process, on anodizing current density, anodized coating thickness, coating weight, and anodizing efficiency, to understand the underlying mechanism for the different anodizing rates. In-situ electrochemical impedance (EIS) measurement was carried out to deconvolute the different factors that affect the anodizing process. Anodized coating thickness and weight growth were correlated to different charge transfer resistances from the equivalent circuit model. The higher Cu content in Al2024 led to slower anodizing rate due to enhanced charge transfer resistance, and to lower anodizing efficiency through its catalytic effect for O2 evolution side reaction. A new schematic mechanism of the anodizing process was proposed taking into consideration of the electrolyte chemistry. O2 generation from water oxidation in the porous anodic oxide layer during anodizing was elaborated as the main factor for anodizing efficiency loss.

Nanoparticle-based impregnation of chromate-free anodizing layers for corrosion protection and adhesive bonding

The present work suggests a new approach of chromate-free surface pretreatment of aluminum alloy AA 1050 by using a modified phosphoric acid anodizing process, followed by an impregnation with a commercial SiO2-nanoparticle dispersion (Levasil® 300/30%) by dip-coating technique. The nanoparticles constitute an additional barrier layer against corrosion without closing the pores completely. The open porous oxide layer is a suitable promoter of adhesive bonding. The influence of withdrawing speed, the dipping time, solid content, temperature and pH-value in respect to the filling height in the pores of the anodized oxide layer were investigated. Surface morphology of the impregnated anodic oxide layers as well as their cross-section microstructure were investigated by scanning electron microscopy (SEM). Corrosion properties were investigated by electrochemical impedance spectroscopy (EIS) and adhesion was evaluated by single lap shear tests.

Morphological and chemical characterization of highly ordered conical-pore anodic alumina prepared by multistep citric acid anodizing and chemical etching process

Highly ordered, conical-pore anodic alumina (AAO) membranes with interpore distance (Dc) between ca. 530 and 620 nm and thickness ranging between 2.4 and 7.8 μm, were produced. In the fabrication process aluminum surface was first pre-patterned by the anodization in etidronic acid solution. Then, the regular arrays of Al concaves were used as nucleation sites to grow AAO during the second anodization, which was carried out in highly concentrated citric acid solution (20 wt%) and at relatively high temperature (33–35 °C). The conical pore shape was engineered by a multistep process combining anodization in the citric acid electrolyte and the subsequent chemical pore broadening in phosphoric acid solution. The morphological analyses has revealed that the geometrical parameters of the Al concaves were successfully transferred to the AAO membranes. Furthermore, FTIR spectra analysis confirmed that the electrolyte species, such as phosphonate and citric ions, are being embedded into the AAO framework during the anodization. The graded-index structure formed in AAO can be used for a production of antireflective coatings operating in a broad spectral range.

Coupled effects of substrate microstructure and sulphuric acid anodizing on fatigue life of a 2017A aluminum alloy

The present study has been conducted in order to investigate the coupled effects of substrate microstructure and the sulphuric acid anodizing process, on the fatigue life of a 2017A aluminum alloy, by means of plane bending fatigue tests (R = − 1). The effect of two different tempers, namely artificially aging (T6) and recovery annealing, prior to anodizing, was studied. The microstructural analysis of the substrate and anodic films was carried out by means of different techniques. It has been determined that the anodic film gives rise to a decrease in the fatigue properties of the heat treated alloy at all stress levels. However, at low stresses, the reduction in fatigue life is more pronounced for the annealed material. The effect of substrate microstructure on the morphology of the anodic layer and on the reduction of the fatigue life of the alloy is also discussed.

Self-Ordering Behavior of Anodic Porous Alumina via Selenic Acid Anodizing

The self-ordering behavior of anodic porous alumina that was formed by anodizing in selenic acid electrolyte (H2SeO4) at various concentrations and voltages was investigated with SEM and AFM imaging. A high purity aluminum foil was anodized in 0.1-3.0M selenic acid solutions at 273K and at constant cell voltages in the range of 37 to 51V. The regularity of the cell arrangement increased with increasing anodizing voltage and selenic acid concentration under conditions of steady oxide growth without burning. Anodizing at 42-46V in 3.0M selenic acid produced highly ordered porous alumina. By selective dissolution of the anodic porous alumina, highly ordered convex nanostructures of aluminum with diameters of 20nm and heights of 40nm were exposed at the apexes of each hexagonal dimple array. Highly ordered anodic porous alumina with a cell size of 102nm from top to bottom can be fabricated by a two-step selenic acid anodizing process, that includes the first anodizing step, the selective oxide dissolution, and the second anodizing step.

Optimization of tartaric/sulphuric acid anodizing process using Doehlert design

The aim of this study was to investigate the influence of tartaric/sulphuric acid anodizing parameters (bath temperature, anodic current density, sulphuric acid and tartaric acid concentrations) on the anodic oxide layer properties. A Doehlert design was implemented to optimize this process. Four responses were retained, namely: growth rate, microhardness, dissolution rate and weight loss after abrasion of the anodic oxide layer. A multicriteria optimization using desirability function was performed in order to find a compromise between maximization of growth rate and microhardness and minimization of dissolution rate and weight loss after abrasion.Under the determined optimal anodizing conditions (Ctart=23.1gL−1, 8.6°C, 2.1Adm−2, Csul=151gL−1), the estimated response values were 0.59μmmin−1, 3.64gm−2min−1, 446HV and 70gm−2 for growth rate, dissolution rate, microhardness and weight loss after abrasion, respectively. The morphology and the composition of the anodic oxide layer were examined by scanning electron microscopy, atomic force microscopy and glow-discharge optical emission spectroscopy.

Fatigue and fracture assessment of toxic metal replacement coatings for aerospace applications

The use of corrosion resistant and adhesion promoting films and coatings is established industrial practice for many fatigue sensitive components and structures. However, recent environmental legislation restricting the use of a range of toxic heavy metals and their derivative processes, such as chromic acid anodising (CAA), has meant that a number of new coatings systems and pretreatments are currently being developed to replace the traditional processes still in use. Typical of these new systems are the boric–sulphuric acid anodising (BSAA) process, which can be modified to provide excellent adhesive bonding properties, the sulphuric acid anodising process, which includes an additional electrolytic phosphoric acid deoxidising stage (EPAD) to produce a duplex oxide layer, and the recently patented ACDC sulphuric acid anodising process which produces a two layered oxide film which can be tailored to produce different porosity volume fractions within each layer. This communication reports the preliminary findings of a study carried out to assess the fatigue response of Al2618:T6 aluminium alloys to these new processes. In contrast to CAA anodising, the initial results indicate that the EPAD and ACDC processes do not appear to have a significant effect on fatigue.

A Novel Micro-Arc Anodising Process for Aluminium Alloys Containing High Silicon

Although the anodising process has been widely used in surface treatment of Al alloys, the well known sulphuric acid anodising process produces inhomogeneous and highly porous anodised coatings on Al alloys containing high Si due to the inertness of the silicon particles. The present work reports a novel micro-arc anodising process for such high-silicon aluminium alloys. Uniform and thick anodising coatings can be obtained through an anodising process in an alkaline electrolyte under high voltage and low current density conditions. The microstructure examination in SEM indicates that most pores in the coatings have been sealed during the anodising process, thus post-treatment that is normally needed for the currently used anodising process can be eliminated. The coating is very promising in the improvement of corrosion resistance for cast high-silicon aluminium alloys.

Role of Tartaric Acid on the Anodizing and Corrosion Behavior of AA 2024 T3 Aluminum Alloy

Tartaric acid is added to sulfuric acid anodizing baths to generate porous anodic film that provides corrosion resistance to practical aerospace alloys and reduces the environmental impact of the traditional chromic acid anodizing process. Here, a fundamental study on the effects of the addition of tartaric acid to the sulfuric acid anodizing electrolyte has been undertaken. During anodizing, it was evident that tartaric acid does not significantly affect the mechanism of porous film growth, but it reduces the growth rate of the porous anodic film. After anodizing, in acidic environments, it may reduce the dissolution rate of a previously formed oxide. Furthermore, it was found that in a nearly neutral, chloride-rich environment, tartaric acid limits the anodic reaction of aluminum dissolution at concentrations in the hundreds of ppm range. The previous suggests that the good anticorrosion performance of alloys anodized in the presence of tartaric acid is due to residues of tartaric acid in the pore solution.

Novel methods, incorporating pre- and post-anodising steps, for the replacement of the Bengough–Stuart chromic acid anodising process in structural bonding applications

The present investigation focuses on novel anodising processes which are being developed for the replacement of the hexavalent chromium containing 40/50V Bengough–Stuart process, with particular emphasis on their resultant performance in structurally bonded systems used in the demanding and harsh environments encountered on operational aircraft. An electrolytic phosphoric acid-based deoxidiser (EPAD) has been studied in combination with a standard sulphuric acid anodise. It has been shown that the EPAD provides an open porous structure in order to enhance adhesion to the modified sulphuric acid anodised (SAA) surface. Additionally, a post-anodising (PAD) treatment has been used to aid structural adhesion in combination with the SAA processes.As a control, the standard 40/50V Bengough–Stuart chromic acid anodising (CAA) has been used as a baseline performance indicator in adhesion tests. Single-lap shear (SLS) and modified Boeing wedge tests were used to determine adhesion performance. SLS tests were used to determine initial, dry joint strengths whilst wedge test joints immersed in deionised water for up to 100h gave a measure of joint durability. Overall, excellent initial joint strengths and durability have been found with both EPAD plus SAA and PAD plus SAA processes suggesting that these environmentally benign treatments may be used as possible drop-in replacements for the currently used hexavalent chromium process. Electron microscopy has been used to investigate the topographical changes introduced to the surface by the various surface pretreatments under investigation to provide an explanation for the observed adhesion test results.

The bonding strength and corrosion resistance of aluminum alloy by anodizing treatment in a phosphoric acid modified boric acid/sulfuric acid bath

The process of phosphoric/boric/sulfuric acids anodizing was studied as a new pre-treatment for adhesive bonding of aluminum alloys. The microstructure and topography of the anodic films were examined using SEM and AFM, and the adhesive strength and corrosion behavior were studied with lap-shear test, wedge test and electrochemical technology. The results showed that by the process of phosphoric/boric/sulfuric acids anodizing a thicker film with high porosity and big pores can be obtained. The porous film was beneficial to improve the durability and lap-shear strength of the bonding joints. The thicker film can also provide better corrosion resistance. Compared with the films by boric/sulfuric acids anodizing and phosphoric acid anodizing, under humid and hot environments the phosphoric/boric/sulfuric acids anodic film showed better corrosion resistance, higher bonding strength and durability, and is a promising pre-treatment for adhesive bonding of aluminum alloys instead of the chromic acid anodizing process.

Strategies for the replacement of chromic acid anodising for the structural bonding of aluminium alloys

The 40/50 V Bengough–Stuart chromic acid anodise process is widely used in demanding applications as a prebond treatment. This process has a number of disadvantages and its replacement is the subject of much interest in the aerospace, automotive and defence sectors, amongst others. This paper details a number of modifications to the standard boric–sulphuric acid anodising (BSAA) process specifically to achieve satisfactory structural bond performance. These included: variations in the deoxidiser and anodising parameters; and the use of a post-anodising dip. It has been demonstrated in these studies that there are three possible methods of providing excellent durability using a variation of the standard BSAA process: the use of an electrolytic phosphoric acid deoxidiser; a high temperature anodise at 35 °C; and the use of a post-anodise phosphoric acid dip.

Anodising of Al 2024-T3 in a modified sulphuric acid/boric acid bath for aeronautical applications

A novel borate/boric/sulphuric acid anodising process is studied. The results show that the physical structure of the films is influenced not only by the bath used, but also and mainly by the substrate, i.e., Al 2024-T3 or Al. The corrosion resistance of the anodised specimens is satisfactory for practical applications and the fatigue resistance is not significantly different from that obtained with the traditional chromic acid anodising.

Functionalized plasma polymer coatings for improved durability of aluminium-epoxy adhesive joints: fractography

The effectiveness of carboxylic acid functionalized plasma polymers (PPs) as adhesion promotion pretreatments for aluminium is investigated. Plasma polymers of acrylic acid have been deposited onto AA6016 aluminium alloy and subsequently bonded with an amine-hardened epoxy-based adhesive formulation to make single lap shear (SLS) joints. Bond durability was investigated by testing joints after storage in water at 60 °C for 1 week. A combination of tensile testing of the SLS joints and locus of failure determination, by transmission electron microscopy and imaging XPS, was used to assess the PP pretreatment performance relative to the commercially applied chromic acid anodizing process. All non-aged joints failed in the adhesive, but after ageing of the joints in water a reduced failure stress was measured and areas of failure near to the aluminium oxide surface were visible. The locus of failure was determined through comparison of transmission electron micrographs, of ultramicrotomed sections transverse to the fracture surface, and XPS images of the surface. Failure was identified at both the PP/aluminium oxide and the PP/adhesive interfaces. It is anticipated that further application of this fractography approach will facilitate the optimization of PP adhesion promotion pretreatments to improve on the promising results obtained in this initial study.

ホウ酸硫酸混合電解液を用いるアルミニウムの陽極酸化とホウ素含有量

The boric/sulfuric acid anodizing process has been developed in the U. S. as a replacement for chromic acid anodizing to meet stricter environmental regulations. A comparative study has been conducted on the microstructures of boric/sulfuric acid films and sulfuric acid films. The sulfur and boron contents of the oxide films were determined by prompt gamma ray analysis. The type of V-t and I-t curves, and the morphology of pores were not appreciably influenced by the presence of boric acid in the anodizing bath, but the rate of formation of the boric/sulfuric acid films increased slightly and the rate of their disolution in phosphoric acid solution decreased slightly. The concentration of boric acid did not influence the amount of surfur incorporated into the oxide films. After the pores were sealed, surfur content decreased significantly due to the dissolution of sulfur compounds to the sealing solution.The boron content of porous type films was negligible, in the order of ppm. This is in remarkable contrast to the barrier type films formed in borate solutions, where boron contents of the order of several per cent have been reported. The boron content increased with increasing concentration of boric acid in the electrolytes.

Airborne Particulate Emissions from a Chromic Acid Anodizing Process Tank

Particulate mass concentration, particle size distribution, and particle chemical composition measurements have been conducted on the gases exhausting from a chromic acid anodizing process tank. Particle mass concentrations in the 200 to 20,000 μg/m3 range were measured using open-faced filters (47 mm diameter) adjacent to the process tank liquid and with closed filters (90 mm diameter) in the exhaust duct. Particle size distributions, measured using University of Washington Mark 3 and Mark 20 Cascade Impactors, showed the particle aerodynamic mass median diameter was about 3 microns. Chemical analysis of the particle samples obtained by the Modified EPA Method 5 sampling train, the Mark 20 UW Cascade Impactors, and by the 47 mm and 90 mm diameter filters showed Cr+6 concentrations in the 20 to 1,500 μg/m3 range with over 99 percent of the chromium in particles larger than 1.0 microns diameter. An integrating nephelometer was used to measure the light scattering coefficient of the exhaust gases upstream of the wet scrubber. The light scattering coefficient increased by a factor of about 2–3 over the background level during the 40 minute time period while a part was being anodized. The bscat values ranged from 3 × 10−5 to 3 × 10−4 meters−1 for the aerosol particles less than about 6 microns aerodynamic diameter.

Paste Forms of a Cold Chromate Etch Treatment for Aluminium

Abstract While the usual etching treatments in baths of solution are convenient and satisfactory for comparatively small components in conditions of factory manufacture, they are less adequate for large components and conditions which are less well controlled. To meet these conditions various modifications to produce etching treatments in a paste form have been explored. Many of these have involved the phosphoric acid anodising process but there have been reports of a paste formulation of the FPL chromic acid treatment although the details have not always been very precise.