Influence of pre-treatments in cerium conversion treatment of AA2024-T3 and 7075-T6 alloys

The role of pre-treatment in the formation of a cerium conversion coating is investigated for the protection of AA2024-T3 and 7075-T6 alloys. The alloys were alkaline-etched and de-smutted in nitric acid, prior to cerium treatment in Ce(NO₃)₃ at 85 °C with H₂O₂ accelerator. Potentiodynamic polarization studies in 3.5% NaCl solution revealed a large shift of  300 mV of the corrosion potential below the pitting potential for the 7075-T6 alloy, which correlated with the development of a finely-textured, uniform coating. However, the formation of a uniform coating and protection was dependent upon the time of de-smutting, with non-uniform coatings resulting from extended times of de-smutting. In contrast, non-uniform coatings developed on the 2024-T3 alloy, with pitting potential at the corrosion potential, irrespective of the time of de-smutting. Findings for the 2024-T3 alloy indicate that extended de-smutting affects the enrichment of alloying elements.     

Corrosion of environmentally friendly lanthanum conversion coating on AA2024-T3 aluminum alloy

In this work, lanthanum conversion coating applied on AA2024-T3 aluminum alloy. The coatings were analyzed by optical and scanning electron microscope. The composition and phase structure of the coating were evaluated by EDS and XRD, respectively. Corrosion properties of the coating were investigated by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization techniques. To explain the corrosion behavior of the coating, equivalent circuit was developed from the measurements. Results showed that the coating has a nanocrystaline structure with crystal size less than 50 nm which is composed of La₂O₃. Additionally, EIS measurements showed that the lanthanum conversion coating was monolayer. Measurements taken by polarization tests showed that the coating increased corrosion potential and decreased corrosion current density approximately one order of magnitude.     

Conversion coating growth on 2024-T3 Al alloy. The effect of pre-treatments

The cleaning pre-treatments, generally based on alkaline etching and acid pickling, provide a surface standard condition before the application of conversion coatings. In this work, the effect of cleaning pre-treatments on the surface morphology and activity of 2024-T3 Al alloy and on chromium-free Ce-based conversion coatings formation has been studied. Variations in the open circuit potential of the alloy of ~ 1.5 V were registered during the cleaning pre-treatment, accounting for the complex electrochemical phenomena involved. Even though, the presence and concentration of additives such as H₂O₂ was found to be essential to a fast kinetics coating formation, effective cleaning pre-treatments are demonstrated to be associated to the production of grained well structured coatings, allowing necessary redox reactions for conversion coating formation. The effect of copper-rich areas and its effect on the conversion coating growth rate and morphology were also studied using benzotriazole additions at different stages in the cleaning pre-treatments.     

Factors influencing the deposition of Ce-based conversion coatings, Part II: The role of localised reactions

In Part I it was demonstrated that the deposition of Ce-based conversion coatings onto the matrix of AA2024-T3 was well modeled by titrations of the Ce-coating solution with added Al³⁺ ions. In Part II the coating composition over the surface was imaged with Raman spectroscopy and X-ray elemental mapping. Multivariate analysis was used to interpret variance in the X-ray elemental maps. The coating composition was found to be determined by the underlying microstructure of the alloy. The microstructure is dominated by the distribution of Cu resulting from enrichment as a consequence of anodic etching of the matrix and dealloying of intermetallic particles, principally Al₂CuMg during coating. A model is presented that proposes two mechanisms for the deposition of the coating. For the matrix, coating deposition can be explained on the basis of a pH rise in the adjacent solution analogous to precipitation from solution during titration as described in Part I. For the rest of the coating, the mechanism of deposition is dictated by the heterogeneous surface of AA2024-T3.     

Influence of copper on chromate conversion coating of aluminium alloys

Abstract

The formation of chromate/fluoride conversion coatings is investigated for binary Al–2·3 at.-%Cu alloy and AA 2014-T6 aluminium alloy in order to determine the influences of copper on coating development. Copper is shown to cause detachment of the coating layer after a particular time of coating. The time of detachment is reduced with increase in the concentration of copper in solid solution in the various alloy regions. Therefore, a thicker layer of coating material can develop on the copper depleted matrix regions of AA 2014-T6 alloy compared with that on the copper rich intermetallic particles. The detachment occurs following enrichment of copper in the alloy during the initial stages of formation of the coating, with the main layer of hydrated chromia detaching from close to the alloy surface. Following detachment, subsequent thickening of the coating is restricted. It is suggested that this behaviour is related to the incorporation of nanoparticles of copper into the coating.     

Scratch Cell Test: A Simple,Cost Effective Screening Tool to Evaluate Self-Healing in Anti-Corrosion Coatings

A quick and simple scratch cell set up to evaluate the self-healing of an hybrid sol-gel (ormosil) coating was fabricated. This methacrylate-based anti-corrosion coating was applied on the aerospace aluminium alloy AA2024-T3, and cured at room temperature. This technique of evaluation requires minimum instrumentation. The inhibitors cerium nitrate, benzotriazole and 8-hydroxy quinoline (8-HQ) were used in the study. The self-healing ability of the inhibitors decreased in the following order: 8-HQ, BTZ and Ce. 8-HQ showed the highest self-healing ability and was comparable to the commercial hexavalent chromium conversion coating—Alodine. Spectroscopic analysis of the electrolyte and EDX of the coatings indicated the movement of the inhibitor from the coating to the site of damage, thereby effecting self-healing. It was observed that an increased inhibitor concentration in the coatings did not accelerate the healing process. Inhibitor release was slower in the coatings doped with inhibitor-loaded nano-containers, when compared to inhibitor-spiked coatings. This property of controlled release is desirable in self-healing coatings. Electro impedance studies further confirmed self-healing efficiency of the coatings. The scratch cell study reported here is the first of its kind with the ormosil under study on AA2024-T3 aluminium alloy. The results are encouraging and warranty a quick and simple qualitative screening of the self-healing potential of the inhibitors with minimum instrumentation.     

Factors influencing the deposition of Ce-based conversion coatings, part I: The role of Al ions

The mechanism of deposition of cerium-based conversion coatings and the influence of Al³⁺ ions was modeled using titrations of cerium-based conversion coating solutions with and without added Al³⁺ ions. Precipitates resulting from these titrations were characterised by Raman spectroscopy and thermogravimetric analysis. Cerium peroxo species and precipitated aluminium compounds were detected in the precipitates. The titrations indicated that cerium and aluminium compounds precipitate independently. The coating deposited onto AA2024-T3 was studied using Raman spectroscopy and optical and scanning electron microscopy. Similar species were observed to those in the precipitates. Changes to the properties of the coating during ageing were monitored using contact angle measurements and Raman spectroscopy. A model is presented that proposes a mechanism for the deposition of the coating over the matrix of AA2024-T3.     

The high throughput assessment of aluminium alloy corrosion using fluorometric methods. Part I - Development of a fluorometric method to quantify aluminium ion concentration

This paper examines a new method developed for the rapid assessment of corrosion inhibitors and their combinations for aluminium alloy 2024-T3 (AA2024-T3). Two fluorometric probes, lumogallion and morin, were used to rapidly assess the concentration of aluminium ions in solution through the use of a spectrophotometric plate reader. Fourteen potential inhibitor compounds and other assay constituents were screened for fluoroprobe interference: sodium chromate, sodium metavanadate, cerium chloride, lanthanum chloride, europium chloride, gadolinium chloride, neodymium chloride, yttrium chloride, barium metaborate, sodium metatungstate, potassium phosphate, sodium metasilicate, sodium phosphate, and sodium molybdate. Calibration curves were established for the two probes.     

Direct-to-metal UV-cured hybrid coating for the corrosion protection of aircraft aluminium alloy

Finding eco-efficient and environmentally viable alternatives to chromate coatings represents a fundamental milestone in the aerospace industry. Here, we show a chromate-free approach to protective hybrid coatings on aluminium alloy (AA2024-T3) departing from photoinduced sol–gel and cationic polymerizations. Beginning with a film of n-alkyltrimethoxysilane and diepoxy monomer, we rely on photogenerated superacids to induce the single step formation of two inorganic and organic barrier networks. Such system combines the unique aspects of photopolymerization including fast reactions, temporal control, solvent-free composition and temperature independence. Used without chemical conversion coating or anodizing, some films have passed 2000 h of salt spray testing.     

The high throughput assessment of aluminium alloy corrosion using fluorometric methods. Part II - A combinatorial study of corrosion inhibitors and synergistic combinations

The discovery of new corrosion inhibition systems remains dependent on experimental methods for screening and characterization. This paper examines a new method to rapidly assess aluminium alloy corrosion and inhibitor performance through quantification of aluminium ion concentration using fluorometric probes. The fluorometric probes, lumogallion and morin, were used in conjunction with a plate reader to rapidly assess fourteen corrosion inhibitors and their combinations for AA2024-T3. AA2024-T3 wire electrodes were exposed to 3.4 mM total inhibitor concentration in 0.6 M NaCl adjusted to pH 2, 4, 7, 10, and 12 for 1-7 days. Corrosion inhibition provided by mixtures of lanthanum and molybdate, cerium and molybdate, and metavanadate with phosphate was found to be superior to the equivalent concentration of sodium chromate.     

Formation of subsurface crevices in aluminum alloy 2024-T3 during deposition of cerium-based conversion coatings

The processing variables that contributed to the formation of subsurface crevices under cerium-based conversion coatings on AA 2024-T3 were investigated. Focused ion beam milling revealed the presence of subsurface crevices underneath a small fraction (∼ 10%) of coated areas, typically in areas with large cracks through the coatings. A solution of sodium chloride and H₂O₂ etched AA 2024-T3 and produced features similar to subsurface crevices, which confirmed that crevices formed during deposition due to the composition of the coating solution. Using sodium nitrate in place of sodium chloride resulted in no etching of the substrate. Thus, coatings free of subsurface crevices could be produced by using cerium nitrate instead of cerium chloride in the coating solution. Electrodeposited coatings, even those deposited from solutions containing chloride ions and H₂O₂, were also free of subsurface crevices. As a result, subsurface crevices are not inherent to cerium-based conversion coatings, but rather were formed due to certain process parameters, specifically the presence of chloride ions and hydrogen peroxide in the coating solution.     

基于晶格储存能的航空铝合金腐蚀行为研究

采用多种电子显微技术对AA2024-T3航空铝合金的局部腐蚀行为进行研究。结果表明:AA2024-T3铝合金的局部腐蚀主要以晶间腐蚀的形式出现,相对于晶界沉淀相,晶格储存能更为显著地影响晶间腐蚀的发展,促使其优先发生在晶格储存能更高的晶粒周围。研究结果有助于提高对航空铝合金晶间腐蚀机理的认识,并为航空铝合金的设计提供理论指导,对保障航空安全具有重要意义。     

A morphological study of filiform corrosive attack on chromated and alkaline-cleaned AA2024-T351 aluminium alloy

In order to characterise filiform corrosion on a commercial AA2024-T351 aluminium alloy, a detailed microscopical study using SEM and EDS was performed. One set of AA2024-T351 aluminium alloy samples was alkaline-cleaned and deoxidised and chromate conversion coated. Another set was alkaline-cleaned only. Both samples were similarly spray coated with a 42 μm clear polyurethane topcoat. Filaments were subjected to a range of specimen preparation techniques. Sections and top views examined by SEM revealed varying degrees of attack ranging from generalised etching without local attack to severe local attack in the form of pitting, resulting in grain etchout, grain boundary attack and subsurface etchout. EDS revealed the presence of chloride deep into the pits and the subsurface etchout.     

2024-T3铝合金搅拌摩擦焊接中搅拌头尺寸和搅拌针形状影响的数值模拟

采用完全热力耦合模型研究了搅拌针直径、轴肩直径以及搅拌针锥角对2024-T3铝合金搅拌摩擦焊接过程中热生成、材料变形和能量历史的影响。结果表明：相比搅拌针接触面,轴肩接触面对搅拌摩擦焊接的热生成起主要作用。增加轴肩直径和减小搅拌针直径均能增加焊接温度,但是轴肩尺寸变化的影响更为明显。与6061-T6铝合金的搅拌摩擦焊接过程相比,2024-T3铝合金搅拌摩擦焊接的能量输入明显增加,同时塑性耗散与摩擦耗散的能量比减小。     

Corrosion pathways in aluminium alloys

The corrosion pathways in AA2024-T3, AA5083-O and AA6082-T6 alloys have been investigated. The objective of the investigation is to further the understanding of the complex localised corrosion mechanism in aluminium alloys. The investigation was carried out by examining the corroded surfaces of the alloys after potentiodynamic polarization tests in a 3.5% NaCl solution with the aid of a scanning electron microscope, and by analysing the flow of anolyte solution using the scanning vibrating electrode technique. The results revealed that the overall corrosion pathways in the alloys are distinctively different and are influenced by the flow of anolyte solution. Also revealed, was the fact that corrosion propagates in two ways (particularly in the AA5083-O and AA6082-T6 alloys): an overall pathway in the corrosion front (filiform-like pathway in the AA5083 alloy and organized linear pathways in AA6082 alloy); and the crystallographic channelling along the 〈100〉 directions. These are dependent on the grain distinct features of the AA5083-O and AA6082-T6 alloys and are not influenced by the presence of coarse second phase particles in these alloys, compared with the AA2024-T3 alloy, where the corrosion pathways are more dependent on the presence of second phase particles and grain boundary character.     

Corrosion inhibition performance of polypyrrole Al flake composite coatings for Al alloys

Polypyrrole (PPy) was deposited on Al flakes in the presence of oxyanions (molybdate, phosphate or vanadate) as inhibiting dopants and the PPy modified Al flakes were incorporated into an epoxy primer as protection pigments for AA 2024-T3. The protection performance of the PPy Al flake coating was assessed by electrochemical evaluations including Scanning Vibrating Electrode Technique (SVET), Electrochemical Impedance Spectroscopy (EIS) and salt spray test. The oxyanion doped PPy Al flake coating shows enhanced protection performance for AA 2024-T3. An oxygen scavenger protection mechanism is suggested for PPy in the composite coating, and the roles of Al flake and oxyanions in the formulation are discussed.     

Corrosion protection of AA 2024-T3 by bis-[3-(triethoxysilyl)propyl]tetrasulfide in neutral sodium chloride solution. Part 1: corrosion of AA 2024-T3

This study consists of two parts. In the first part, the corrosion of 2024-T3 aluminum alloy (AA 2024-T3) was studied using scanning electron microscopy and energy-dispersive X-ray spectroscopy. The results showed that the anodic S phase (Al₂CuMg) particles dealloyed Al and Mg during the 3.5 h of immersion in a neutral 0.6 M sodium chloride (NaCl) solution; with the dealloying of Mg being the most severe. Simultaneously, a heavy dissolution was also observed for the surrounding Al matrix of the S phase particles. This Al dissolution is likely to be caused by a local alkalization resulting from the coupled cathodic reaction (water and/or oxygen reduction). Such corrosion in AA 2024-T3, however, can be inhibited efficiently after the treatment of bis-[3-(triethoxysilyl)propyl]tetrasulfide (bis-sulfur silane). The associated studies on bis-sulfur silane treated AA 2024-T3 will be presented in the second part.     

Synergistic effect of pH and oxalate concentration on corrosion of aluminium alloy 2024-T3

This work aims at obtaining comprehensive information on the corrosion behaviour of AA2024-T3 in oxalic acid solutions under different concentrations (0.007–0.2?M) and pH (2–6). Various tests to characterise the corrosion behaviour are performed, including weight loss and electrochemical tests. Coupon surfaces are examined using infinite-focus microscopy, scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy, Raman spectroscopy and diffuse reflectance infrared Fourier transform spectroscopy. Corrosion rates, anodic and cathodic reaction kinetics are detailed to provide a fundamental understanding of the electrochemical behaviour of AA2024-T3 as a function of pH and oxalate concentration. The synergistic effect of pH and oxalate concentration on corrosion of AA2024-T3 is evaluated for both AA2024-T3 matrix and main intermetallics (i.e. Al–Cu–Mg and Al–Cu–Fe–Mn) and the electrochemical behaviour of each constituent element in AA2024-T3 is systematically studied.     

Effect of polystyrenesulphonate and electrolyte concentration for electrical properties of polypyrrole film on aluminium alloy using conductive AFM

The electrochemical synthesis of polystyrenesulphonate (PSS) doped polypyrrole (PPy) film onto aluminium alloy (AA 2024-T3) under galvanostatic conditions at current densities of 1 mA cm^?2 was studied. In this study, conductive atomic force microscopy (C-AFM) was performed to investigate the electrical properties of PPy film on AA 2024-T3 depending on the concentration of PSS as dopant and nitric acid as electrolyte during electrochemical synthesis. The addition of HNO₃ to aqueous electrolyte solution is found to allow the electrochemical synthesis of well adhering homogeneous PPy film in the presence of PSS on AA 2023-T3. The PPy film in the presence of nitric acid alone can be synthesised, although this film showed the poor quality of the electrical properties of PPy film. According to C-AFM, the study confirmed that the conductivity of PPy film is significantly increased with increasing the PSS, nitric acid concentration and electrochemical deposition time.     

Statistical evaluation of EIS and ENM data collected for monitoring corrosion barrier properties of organic coatings on Al-2024-T3

This paper describes the statistical treatment of electrochemical noise (electrochemical noise methods, ENM) and impedance (electrochemical impedance spectroscopy, EIS) data collected for corrosion monitoring of epoxy coated aluminum 2024-T3. The epoxy was applied using electrodeposition to Al-2024-T3 panels treated with one of three surface pretreatments. Six coating systems were prepared by varying the electrodeposition parameters and the pretreatment. Linear regression analysis of the data was used to detect the individual contributions relating to the analysis technique, topcoat application and pretreatment type. As a result of the statistical treatment it was found that EIS data has significantly less variance compared to ENM data, that EIS data can be used to differentiate the effects generated from the coating and pretreatment, and that ENM data cannot be used to differentiate the effects generated from the coating and pretreatment due to the high variance in data values associated with ENM. None of this information could be quantified without the statistical treatment.