Acid Anodizing Research Articles

Effects of chromic acid anodizing on fatigue behavior of 7050 T7451 aluminum alloy

Abstract The effect of chromic acid anodizing (CAA) surface treatment on 7050 T7451 aluminum alloy was presented in this study in terms of fatigue behavior. CAA is a treatment against corrosion by producing aluminum oxide layer (Al2O3) at the surface. However, fatigue performance of 7050 T7451 is affected by the coating. In this study, eight different CAA processes were examined with regard to etching stage of pre-treatments by using an alkaline etchant and/or acid etchants with various immersion times. Optical microscopic examinations were applied in order to determine pitting characteristics for the selection of CAA process parameters before fatigue tests. A CAA process was selected among eight processes in terms of pitting characteristics in order to apply fatigue specimens. Four fatigue test groups were determined to investigate bare condition of 7050 T7451 and sub-stages of the CAA particularly. Constant amplitude axial fatigue tests were conducted on specimens at 91 Hz at stress ratio (R) -1 until run-out criteria, which was 106 cycles. Fatigue life reduction was determined due to pretreatments of CAA. Fracture surfaces of the specimens were examined by scanning electron microscope (SEM) to investigate morphology and crack initiation sites.

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.

Persistent reduction of boiling incipience of ethanol on biphilic porous textured surfaces

Boiling of highly wetting fluids is of great interest to thermal management of high-powered electronic devices, enhancement of which conventionally relies on functional modifications of surface shape and structure so as to promote bubble generation and growth. In this paper we attempt to combine the approach of porous texturing with a novel technique of surface wettability engineering to enhance saturated pool boiling of ethanol. Thin layers of microporous and nanoporous surface topographies were chemically deposited on an aluminum substrate by anodization in phosphoric acid and sulfuric acid, respectively. Boiling on the textured surfaces recorded over-twofold increases in terms of the heat transfer coefficient compared with that on a plain smooth surface, which can be attributed to a proliferation of active nucleation sites. The boiling incipience point on both porous surfaces, however, exhibited an interesting dependence on the initial wetting state of the surface. With the application of amphiphobic coatings of fluoropolymer modified with halloysite nanotubes, we managed to engender a biphilic (i.e., spatially alternating hydrophobicity and hydrophilicity) pattern on the porous-textured surfaces. The repeat control experiments on the hybrid surfaces showed an equally efficient mode of nucleate boiling, whose inception, by contrast, seemed to occur at consistently low superheats. The remarkable reduction (by more than 80%) of the minimum superheat at the boiling onset hints at an alternative nanobubble-related mechanism for heterogeneous bubble nucleation, which is notably less affected by incondensable gas, to the classic vapor-trapping-cavity model.

Formation of Nanoporous Gold Film on Ti by Anodization in Oxalic Acid

Abstract A nanoporous gold (NPG) film was formed on a Ti sheet or sputter-deposited thin Ti film by the anodization of a thin gold film on the Ti substrates in oxalic acid. A remarkably low anodic current of the Ti sheet compared with that of the Al sheet indicates that Ti is a stable substrate for the NPG film formed by the anodization. It is also confirmed that the preanodization of the Ti sheet significantly improves the adhesion of the NPG film to Ti. By increasing the anodic potential, NPG film was formed on sputter-deposited thin Ti film with a thickness of 130 nm.

Prediction of salt spray test results of micro arc oxidation coatings on AA2024 alloys by combination of accelerated electrochemical test and artificial neural network

Micro arc oxidation (MAO) coating was prepared on AA2024 as a substitute of sulfuric acid anodizing (SAA) with the aim to improve anticorrosion behavior. Evaluation of corrosion resistance was first performed by means of salt spray test (SST) as a conventional but long way in time method, usually more than 1000 h for aeronautical needs. In parallel, an original approach combining electrochemical characterization and Artificial Intelligence was evaluated in order to predict corrosion occurrence in SST. The electrochemical setup consists in an improvement of ISO17463 accelerated cyclic electrochemical method (ACET) with an additional linear polarization step. The whole generates up to 50 parameters that are treated by artificial neural network (ANN). After training and refinement, the average accuracy of predicted corrosion resistance and corroded surface after 200 h of SST are 99 and 90% respectively. This promising combination of ACET and ANN enables the evaluation of corrosion behavior of coating in less than 24 h.

A high-and-rapid-response capacitive humidity sensor of nanoporous anodic alumina by one-step anodizing commercial 1050 aluminum alloy and its enhancement mechanism

Anodic aluminum oxide (AAO) is a well-known nanoporous material and a template for nanotechnology application. The AAO based humidity sensor is traditionally fabricated using low-temperature (0–10 °C) two-step anodization in oxalic acid at 40 V from the expensive high-purity aluminum of small-area as well with low response and low intensity. Here, one-step anodizing commercial large-area low-purity aluminum 1050 alloy was realized for high-performance humidity sensor by hybrid-pulse anodization in 0.3 M oxalic acid at 20 V at 25 °C that merits low cost, short fabrication time and high-and-rapid capacitive response. The performance enhancement is attributed to the increased total pore area adsorption ratio and the electrode area of AAO with lower thickness to make much more water vapor molecules adsorb onto the wall surface of pores for sensing with high capacitance, response and short response-recovery time. The low-thickness pores make water molecules adsorb onto and desorb from the pores surface more quickly because of shorter diffusion path. Also, a small-area sample of traditional 40 V AAO is used as a reference for comparison. A shorter anodization time of 55 min is also fabricated for understanding the thickness effect on the response-recovery time. The large-area AAO humidity sensor of 16 cm2 at 20 V exhibits high response (above 5000 %, 3.5 times the traditional reference) and quick response-recovery time (9 s). The high-and-rapid-response enhancement mechanism of AAO capacitive humidity sensor is established.

Mechanistic study on pH-related behavior in rocking-chair capacitive deionization

Rocking-chair capacitive deionization (RCDI) is a semi-continuous desalination technology, however, suffering from issues pertaining to effluent salinity and pH. We thoroughly investigated the composition evolution and pH-related behavior in RCDI and disclosed the underlying causes for quality decay. RCDI concomitantly produces two streams: the desalted cathodic effluent becomes basic, whereas the brine anodic stream becomes acidic. These pH-related species deteriorate the outflow quality and distort conductivity profiles. Under variation in dissolved oxygen and potential distribution, we verified the carbon oxidation and oxygen evolution reactions as causes for the acidic anode and oxygen reduction as the cause for the basic cathode. Faradaic reactions and resultant pH fluctuation were exacerbated with increasing charging voltages, even at 0.9 V, whereas they were negligibly affected by the resembling potential distribution at different flowrates. To improve pH stability and water recovery, we integrated the ion exchange and flowrate adjustment procedures; consequently, water recovery increased to 71.43% and pH was stabilized at 5–6.5 for over 80 cycles, with significant adsorption capacity enhancement owing to the ion exchange with pH-related species. Thus, with a better understanding of pH-related behaviors, RCDI can be optimized and extended more specifically for demands such as acid/base production and resource enrichment.

Friction Stir Welding of AA2099-T83 and AA2060-T8E30 Aluminium Alloys with New Cr-Free Surface Treatments and Sealant Application

The feasibility for friction stir welding (FSW) surface-treated AA2099-T83 aluminium extrusions with AA2060-T8E30 aluminium sheets in the overlap configuration and using a sealant at the interface was investigated in this work. New Cr-free surface treatments such as thin film sulphuric acid anodising (TFSAA) and sol–gel were applied to the parent materials, and a sealant was applied before applying the FSW process. FSW welds were produced using several combinations of surface treatments and sealant application with no significant influence on FSW process stability and performance. The metallographic examination of the welds showed that a good protection of the crevice was achieved with some sealant accumulation at the edges of the overlapping region. The microstructural analysis showed no sealant remnants but the presence of some oxide remnants in the stir zone (SZ) of the welds, especially in the TFSAA treated parent material cases. However, these remnants did not show any significant effect in the static pull-out strength of the joints and failures at the most stressed zone of the AA2099-T83 extrusions outside the FSW weld region were consistently obtained.

Analysis for anodized film blackening of aluminum alloy components

Sulfuric acid anodic oxidation is an important means to improve the corrosion resistance of aluminum alloy components. The blackening phenomenon of aluminum alloy components after sulfuric acid anodizing is analyzed by chemical composition test, mechanical property test, process parameters analysis, metallographic examination and other tests. The analysis results are compared with normal color components after anodized. The result shows that the unsufficient solid solution heat treatment is the main reason for anodized film blackening of the aluminum alloy components, which result in the abnormal microstructures. At last, the measure of secondary heat treatment is taken to solve this problem effectively.

Corrosion Behavior of AA 1100 Anodized in Gallic-Sulfuric Acid Solution

Sulfuric acid anodization is one of the common methods used to improve corrosion resistance of aluminum alloys. Organic acids can be added to the sulfuric acid electrolyte in order to improve the properties of the anodized aluminum produced. In this study, the use of gallic acid as an additive to the sulfuric acid anodization of AA1100 was explored. The effect of varying anodization current density and gallic acid concentration on the properties of anodized aluminum samples was observed using electrochemical impedance spectroscopy, linear polarization, and scanning electron microscopy. It was observed that the corrosion resistance of samples anodized in gallic-sulfuric acid solution at 10 mA·cm−2 is lower than samples anodized in sulfuric acid. It was also observed that higher anodization current density can lead to lower corrosion resistances for aluminum samples anodized in gallic-sulfuric acid solution. However, samples anodized at 5 mA·cm−2 and at a gallic acid concentration of 5 g·L−1 showed better corrosion performance than the samples anodized in sulfuric acid only. This suggests that the use of low amounts of gallic acid as an additive for sulfuric acid anodization can lead to better corrosion resistances for anodized aluminum.

Effect of Heat Treatment and Sulfuric Acid Anodization on Corrosion Resistance of Aluminum Alloy (AA7075)

This paper, studied the effect of heat treatment and anodization on corrosion resistance of aluminum alloy 7075 (AA7075), with a view to improving its corrosion resistance. Microstructure and micro hardness of the anodic film of the samples were studied with the aid of optical metallurgical microscope and automated micro hardness testing machine. Linear polarization methods were used to assess the corrosion behaviour of the alloy in 0.5M HCl. The microstructure of the annealed sample showed formation of dendrites while precipitation hardened samples in palm kernel oil and SAE 40 engine oil showed precipitates of MgZn2. The SEMS result showed pores and micro cracks on the surfaces of the anodized samples, with the as cast and anodized sample in sulfuric acid exhibiting most compact with few pores. The as cast and sulfuric acid anodized sample shows highest micro hardness value of 205.33 HV, while the least value of 150.67 HV was recorded in sample precipitation hardened in SAE 40 engine oil and anodized in sulfuric acid. Analysis of the potentiodynamic polarization data and curves showed a linear relationship (decrease in icorr, decreases the corrosion rate) between current density and the corrosion rate in all the samples. Higher polarization resistance of 15.093 Ω/cm2 was recorded by the as cast and Sulfuric acid (SA) anodized sample while the precipitation treated in SAE 40 engine oil plus SA anodized sample recorded lowest polarization resistance of 5.2311 Ω/cm2. Heat treatment alone improves corrosion resistance of AA 7075 in 0.5 M HCl solution but heat treatment plus SA anodization does not improve corrosion resistance in the same environment.

Role of grain boundary on the growth behavior of anodic film on spark plasma sintered AA6061

To explore the influence of grain boundary on the growth behavior of anodic film, the grain boundary density of aluminum alloy is adjusted by spark plasma sintering (SPS) with different grain sizes and investigated by techniques including SEM, TEM, and SKP. The results indicated that a suitable grain boundary density effectively uniforms the anodizing current distribution and improves the thickness of the anodic film. In 30 min sulfuric-adipic acid anodizing, the growth rate of the film on the sintering AA6061 reaches 0.47 μm min−1, which is much higher than that of commercial AA6061 under the same conditions. However, excessive grain refinement results in the formation of fine-crystal clusters and a surge of grain boundary density. It enhances field-assistant dissolution in initial anodizing and has a direct weaken effect on the final anodic film thickness.

Nanostructure of Porous Si and Anodic SiO2 Surface Passivation for Improved Efficiency Porous Si Solar Cells.

The photovoltaic effect in the anodic formation of silicon dioxide (SiO2) on porous silicon (PS) surfaces was investigated toward developing a potential passivation technique to achieve high efficiency nanostructured Si solar cells. The PS layers were prepared by electrochemical anodization in hydrofluoric acid (HF) containing electrolyte. An anodic SiO2 layer was formed on the PS surface via a bottom-up anodization mechanism in HCl/H2O solution at room temperature. The thickness of the oxide layer for surface passivation was precisely controlled by adjusting the anodizing current density and the passivation time, for optimal oxidation on the PS layer while maintaining its original nanostructure. HRTEM characterization of the microstructure of the PS layer confirms an atomic lattice matching at the PS/Si interface. The dependence of photovoltaic performance, series resistance, and shunt resistance on passivation time was examined. Due to sufficient passivation on the PS surface, a sample with anodization duration of 30 s achieved the best conversion efficiency of 10.7%. The external quantum efficiency (EQE) and internal quantum efficiency (IQE) indicate a significant decrease in reflectivity due to the PS anti-reflection property and indicate superior performance due to SiO2 surface passivation. In conclusion, the surface of PS solar cells could be successfully passivated by electrochemical anodization.

Porous Alumina Films Fabricated by Reduced Temperature Sulfuric Acid Anodizing: Morphology, Composition and Volumetric Growth.

The volumetric growth, composition, and morphology of porous alumina films fabricated by reduced temperature 280 K galvanostatic anodizing of aluminum foil in 0.4, 1.0, and 2.0 M aqueous sulfuric acid with 0.5–10 mA·cm−2 current densities were investigated. It appeared that an increase in the solution concentration from 0.4 to 2 M has no significant effect on the anodizing rate, but leads to an increase in the porous alumina film growth. The volumetric growth coefficient increases from 1.26 to 1.67 with increasing current density from 0.5 to 10 mA·cm−2 and decreases with increasing solution concentration from 0.4 to 2.0 M. In addition, in the anodized samples, metallic aluminum phases are identified, and a tendency towards a decrease in the aluminum content with an increase in solution concentration is observed. Anodizing at 0.5 mA·cm−2 in 2.0 M sulfuric acid leads to formation of a non-typical nanostructured porous alumina film, consisting of ordered hemispheres containing radially diverging pores.

Development and certification of chromic acid-free anodizing process for aircraft grade aluminium alloys

Chromic acid (Cr 6+ ) anodization process is widely used for the corrosion protection of aircraft aluminium alloys. Hexavalent chromium being toxic in nature need to be phased out by eco-friendly alternatives. In the present study modified tartaric-sulphuric acid (TSA) process has been developed followed by sealing in permanganate based bath to obtain 4 to 6 µm thick anodic oxide layer on 2024-T3, 6061-T6 and 7075-T6 aluminium alloys. The process was carried out using a pilot scale anodizing plant. The anodized specimens were characterized for visual observation, thickness, adhesion, electrical breakdown voltage, corrosion resistance and tensile behaviour. All the tests were carried out as per MIL-A-8625F specifications. The specimens were also subjected for about 800 hrs to real time corrosion testing, 200 metres away from sea shore at Mandapam Camp, Rameshwaram, India. The performance of the permanganate sealed TSA anodized aluminium alloys are comparable with that of the conventional chromic acid anodized coatings. This chromic acid-free anodization process has been qualified to airworthiness regulating standards by Indian military certification authorities. Efforts are in progress to commercialize this technology for use on aero platforms.

Improvement of corrosion properties of plasma in an aluminum alloy 6061-T6 by phytic acid anodization temperature

We report on the formation of anodic aluminum oxide (AAO) film using phytic acid (C6H18O24P6), a naturally obtainable and non-toxic organic acid electrolyte. When the temperature of the phytic acid electrolytes changes from 0 °C to 20 °C, the pore size increases, and the AAO film becomes less dense because the growth rate of the AAO film increases. In particular, when the temperature of the phytic acid electrolyte is above 15 °C, the AAO film changes dramatically. According to XRD analysis, the alpha-alumina intensity was relatively higher in the AAO film grown at low-temperatures (0, 5, 10 °C) than high-temperatures (15, 20 °C). It was possible to obtain a result that the properties were better. The microstructural change of the AAO film according to the temperature of the phytic acid electrolytes affected the breakdown voltage and the number of contamination particles that are generated, which are important properties for plasma corrosion protection coating materials. The AAO film grown in the low-temperature has the most suitable properties for use as a plasma corrosion protection coating material. Notably, the breakdown voltage of the AAO film is 0.45 kV at 14.9 μm thickness, with no observable crack on its surface after the plasma corrosion test. These findings provide significant evidence to support the application of phytic acid as an electrolyte to grow AAOfilms, and the AAOfilm grown in a phytic acid bath can be applied as a plasma corrosion protection coating material.

Properties and characterization of novel 3D jute reinforced natural fibre aluminium laminates

Fibre metal laminates (FML) are being used in automotive, aerospace and naval applications due to their light weight and superior performance. The FMLs are made by sandwiching composite with metal. The environmental concerns due to non-biodegradability of such structures, lead to the development of FML containing natural fibre composites. Natural fibres composite, despite having good damping properties have overall poor mechanical properties. However, this aspect can be improved by weaving the fibres in 3 D pattern. In literature, FML made using 3 D woven jute composites is never reported. Furthermore, no literature is found on adhesion of natural fibre composite-metal bonding. In this paper, development of novel 3 D Jute Reinforced natural fibre Aluminium Laminates (JuRALs) is reported. Furthermore, the effect of 3 D weaving pattern and metal-composite bonding on mechanical properties and failure mechanism of the developed samples is also discussed in detail. The four-layered 3 D woven Jute fabric reinforcement was made using four interlocking patterns. The composites and JuRALs were fabricated using epoxy resin by vacuum infusion technique. The surface of aluminium was treated using phosphoric acid anodizing. Tensile, flexural and T-peel tests were performed according to ASTM testing method using Z100 All-round, Zwick Roell. The results showed that out of four types of used reinforcements, the through-thickness composites had better tensile properties while layer-to-layer composite had better flexural properties. The tensile and flexural properties of JuRALs made with through-thickness interlock reinforcement were better as compared to layer-to-layer interlock reinforcement. The T-peel results depicted that the constituent materials influenced the metal-composite adhesion properties, rather the type of 3 D structure.

(Keynote) Anodized Oxide Films As an Important Interface in Different Aluminum Applications

In several applications such as aerospace, automotive, lithography, porous anodized aluminium substrates are coated with organic coatings. Interfacial interactions between PAO and organic coatings are mainly determining the durability of the entire organic/inorganic hybrid systems. However, analyzing this solid/solid interface is challenging. Since this region is covered by a µm-thick polymer layer on one side and a porous aluminium oxide matrix on the other. The use of conventional surface analysis techniques to probe this region is often hindered and is therefore often referred to as the buried interface. In a first part we introduce model interfacial interactions of several polymeric films on modified aluminum oxides discussing in situ analytical methods for the characterization of a realistic model interface exposed to environmental. The use of Ambient Pressure X-ray Photoelectron Spectroscopy (APXPS) is employed to describe the behavior of interfacial interactions in the presence of water vapor [1]. Furthermore, combined ATR-FTIR Kretschmann with EIS is utilized to obtain a near-interface infrared spectrum while simultaneously, the influence of an above-the-polymer electrolyte (such as water) on the interface is characterized. The local deposition of organic molecules we try to probe by Nano IR or TOF-SIMS/AFM approaches. The relevance of these approaches is demonstrated in the case of Cr(VI)-free anodizing for adhesive bonding applications. It is well known that two fundamental characteristics are critical for bonding: the oxide chemistry and morphology. To separate between these two contributions, either barrier-type or porous-type oxide specimens were applied. The investigated Cr(VI)-free anodizing electrolytes are phosphoric- (PAA), sulfuric- (SAA) and mixtures of phosphoric- and sulfuric acid anodizing (PSA). Experimental results show that the incorporation of PO43- and SO42- species from the electrolytes significantly changes the oxide surface composition [2]. Bond performance using an epoxy resin is highly affected by the ingress of water, with its strength linearly increasing with the hydroxyl fraction, as quantified by X-ray photoelectron spectroscopy (XPS) on barrier-type anodic oxides [3]. The next part explores the effect of porous oxide morphology. Mechanical tests show that the anodizing conditions such as the electrolyte combination and temperature, rather than the pore size and oxide layer thickness are critical for moisture-resistant adhesion [4]. In the last part is will be shown that it is possible to modify both the chemical and morphological oxide film properties even further by applying a post-treatment. S. Pletincx, L. Trotochaud, L.L. Fockaert, J.C.M. Mol, A.R. Head, O. Karslıoğlu, H. Bluhm, H. Terryn. T. Hauffman, Scientific Reports. 7, 2017, p45123. 2.S.T. Abrahami, T. Hauffman, J.M.M. de Kok, J.M.C. Mol, H.A. Terryn. Journal of Physical Chemistry C 119 (34), 2015, p19967.3.S.T. Abrahami, T. Hauffman, J.M.M. de Kok, J.M.C. Mol, H. Terryn. Journal of Physical Chemistry C, 120, 2016, p19670.4.S.T. Abrahami, J.M.M. de Kok, V.C. Gudla, R. Ambat, H. Terryn, J.M.C. Mol npj Materials Degradation, 1(1),2017 p8.

A Review on Anodizing of Aerospace Aluminum Alloys for Corrosion Protection

Aluminum alloys used for aerospace applications provide good strength to weight ratio at a reasonable cost but exhibit only limited corrosion resistance. Therefore, a durable and effective corrosion protection system is required to fulfil structural integrity. Typically, an aerospace corrosion protection system consists of a multi-layered scheme employing an anodic oxide with good barrier properties and a porous surface, a corrosion inhibited organic primer, and an organic topcoat. The present review covers published research on the anodic oxide protection layer principles and requirements for aerospace application, the effect of the anodizing process parameters, as well as the importance of process steps taking place before and after anodizing. Moreover, the challenges of chromic acid anodizing (CAA) substitution are discussed and tartaric-sulfuric acid anodizing (TSA) is especially highlighted among the environmentally friendly alternatives.

Towards Self-Organized Anodization of Aluminum in Malic Acid Solutions-New Aspects of Anodization in the Organic Acid.

In this work, aluminum (Al) anodization in malic acid electrolytes of different concentrations (0.15 M, 0.25 M, and 0.5 M) was studied. The close-packed hexagonal pore structure was obtained for the first time in this organic acid in a 0.5 M solution, at 250 V and temperature of 5 °C. Moreover, the process was investigated as a function of the number of cycles carried out in the same electrolyte. A repetition of anodization under seemingly the same external electrochemical parameters (applied voltage, temperature, etc.) induced serious changes in the electrolyte. The changes were reflected in the current density vs. time curves and were most evident in the higher concentrated electrolytes. This phenomenon was tentatively explained by a massive incorporation of malate anions into anodic alumina (AAO) framework. The impoverishment of the electrolyte of the malate anions changed internal electrochemical conditions making easier the attraction of the anions to the Al anode and thus the AAO formation. The electrolyte modification was advantageous in terms of pore organization: In a 0.25 M solution, already after the second anodization, the pore arrangement transformed from irregular towards regular, hexagonal close-packed structure. To the best of our knowledge, this is the first observation of this kind.