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.     

The corrosion resistance of hot dip galvanised steel and AA2024-T3 pre-treated with bis-[triethoxysilylpropyl] tetrasulfide solutions doped with Ce(NO3)3

The present work aims at evaluating the anti-corrosion behaviour of a novel pre-treatment based on bis-[triethoxysilylpropyl] tetrasulfide (BTESPT) doped with cerium nitrate for application on hot dip galvanised steel and AA2024-T3 substrates. The corrosion resistance was evaluated by electrochemical impedance spectroscopy (EIS) and by the scanning vibrating electrode technique (SVET), during immersion in NaCl solutions. The electrochemical results showed that the pre-treatment provides excellent corrosion protection to the substrates. Furthermore, the results evidenced improved protection comparatively to the use of undoped BTESPT pre-treatments, both for galvanised steel and AA2024-T3. This improvement is most likely due to enhanced barrier properties of the film and additional active corrosion protection originated from the inhibiting action of the cerium-based inhibitor impregnated in the silane matrix.     

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.     

Effect of the cerium (III) chloride heptahydrate on the corrosion inhibition of aluminum alloy

In the present work, the corrosion protection of aluminum alloy AA2024-T3 has been studied in NaCl solution, with and without the addition of cerium (III) chloride heptahydrate. The corrosion inhibitor efficiency after immersion into 10 mM NaCl, with or without 3 mM of CeCl₃·7H₂O at 20°C, 40°C, and 60°C was investigated. The performed quantitative tests include electrochemical techniques, such as the method of quasipotentiostatic polarization (Tafel extrapolation), cyclic polarization, and electrochemical impedance s pectroscopy to determine corrosion rate (v_corr), inhibition efficiency (η %), protective ability (γ), degree of coverage (ϑ), and pitting nucleation resistance. The samples were analyzed with scanning electron microscopy and energy dispersive X-ray analysis to evaluate and characterize the precipitates formed on the surface of aluminum samples and to determine dominant type of corrosion. The formation of Ce³⁺ precipitates occurred on cathodic intermetallic sites and the surface, in general, resulting in improved corrosion resistance. Tested cerium (III) chloride heptahydrate proved to be an effective inorganic corrosion inhibitor for AA2024-T3 in chloride solution, which, by the action of cerium ions, reduced corrosion on the surface of the studied aluminum alloy.     

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.     

Investigation of pretreatment effects on the formation of lanthanum based conversion coating on AA2024-T3

The effects of a pretreatment process on the formation and properties of lanthanum based conversion coatings on AA2024-T3 was investigated using optical and scanning electron microscopy (SEM), X-ray diffraction (XRD) and open circuit potential (OCP) measurements. The results indicate that the conventional alkaline activation pretreatment with acidic desmutting could not create a suitable condition to produce lanthanum based conversion coating on AA2024-T3. Therefore, in this study, a new approach to produce lanthanum based conversion coating on AA2024-T3 by acidic pretreatment is proposed. The pretreatment of the alloy in acidic and chloride environments (90 s at 30 vol % HCl) creates more cathodic sites on AA2024-T3 compared to alkaline activation with acidic desmutting. Additionally, it was found that the formation stages of lanthanum based conversion coating is similar to those of cerium based conversion coating on AA2024-T3 except lanthanum ions require more localized pH increase to deposit on the alloy.     

Corrosion protection of aluminium alloy 2024-T3 in 0.05 M NaCl by cerium cinnamate

The corrosion protection of AA 2024-T3 in 0.05 M NaCl by cerium cinnamate has been studied. Polarization measurements demonstrate that cerium cinnamate is markedly effective for suppressing anodic process of the alloy corrosion during the initial 72 h of immersion. The protection mechanism during the immersion period appears to involve two stages: the deposition of cerium cinnamate, and then hydrolysis of cerium ions forming a cerium oxide/hydroxide, prevailing over the foregoing deposition. This study also elucidates the inhibition effect of cerium cinnamate on the early corrosion attack occurred from the second phase particles by X-ray photoelectron spectroscopy and electron-probe microanalysis.     

Evolution of corrosion protection performance of hybrid silica based sol–gel nanocoatings by doping inorganic inhibitor

Silica based hybrid sol–gel coatings were developed to protect AA2024 alloy from corrosion. In order to have an active protection, cerium nitrate corrosion inhibitor was introduced into the coating system. The anti‐corrosion properties of the coatings were evaluated by electrochemical impedance spectroscopy (EIS) and potentiodynamic scanning (PDS) methods. The structure of the films was studied by scanning electron microscopy (SEM) after corrosion. The results indicate that the improvement of the protection properties of the films occurred with immersion time. This would imply that cerium ions could reach the defects, hindering the corrosion reactions and thus reduces the corrosion rate.     

Chromate replacement in coatings for corrosion protection of aerospace aluminium alloys

Mixed rare earth organophosphates have been investigated as potential corrosion inhibitors for AA2024‐T3 with the aim of replacing chromate‐based technologies. Cerium diphenyl phosphate (Ce(dpp)₃) and mischmetal diphenyl phosphate (Mm(dpp)₃) were added to epoxy coatings applied to AA2024‐T3 panels and they were effective in reducing the amount and rate of filiform corrosion in high humidity conditions. Ce(dpp)₃ was the most effective and characterisation of the coating formulations showed approximately a factor of 5 reduction in both the number of corrosion filaments initiated as well as the length of these. Mm(dpp)₃ appeared to reduce the corrosion growth rate by a factor of 2 although it was the more effective inhibitor in solution studies. Spectroscopic characterisation of the coatings indicated that the cerium based inhibitor may disrupt network formation in the epoxy thus resulting in a coating that absorbed more water and allowed greater solubilisation of the corrosion inhibiting compound.     

Multifunctional epoxy coatings combining a mixture of traps and inhibitor loaded nanocontainers for corrosion protection of AA2024-T3

This work aims at investigating the corrosion protection effectiveness of multifunctional epoxy coatings modified with pigments such as ceramic nanocontainers loaded with corrosion inhibitor, chloride and water traps, applied on AA2024-T3. Characterizations on the morphology, composition and structure of the coatings were conducted. The corrosion resistance was studied by electrochemical impedance spectroscopy, localized electrochemical impedance spectroscopy and scanning vibrating electrode technique. The mechanical behaviour of the coatings was examined through nanoindentation and nanoscratching tests. Electrochemical and nanomechanical testing results, evidenced the improvement of the corrosion protective properties and mechanical behaviour of the coatings in the presence of the various pigments.     

Novel hybrid organo-silicate corrosion resistant coatings based on hyperbranched polymers

Corrosion protective sol-gel coatings were developed on AA2024-T3 through an aqueous sol-gel process (Modified Self-Assembled Nanophase Particle method) which includes in-situ formation of a dense silica network from hydrolyzed 3-glycidoxy-propyltri-methoxysilane and tetraethoxysilane by proper cross-linking. In the present study, hyperbranched poly(ethylene imine) of two different molecular weights was investigated as a cross-linking agent since this polymer is able to solubilize non-water-soluble organic molecules. In this respect, two organic corrosion inhibitors (2-mercaptobenzothiazole and 2-mercaptobenzimidazole) were combined with poly(ethylene imine) to induce self healing properties. The as developed coatings were also compared with Si/Zr containing coatings developed by a solvent based sol-gel technique. The potentiodynamic scan and the electrochemical impedance spectroscopy methods were employed to evaluate the corrosion protection performance, whereas the chemical structure, morphology and integrity of the coatings were evaluated by Fourier transformed infrared spectroscopy and scanning electron microscopy studies. Formulations that contain poly(ethylene imine) demonstrate better corrosion barrier properties compared to formulations cross-linked with the simple molecule of diethylenetriamine in all cases and especially when combined with the organic inhibitors.     

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.     

Corrosion inhibition of 7000 series aluminium alloys with cerium diphenyl phosphate

Cerium diphenyl phosphate (Ce(dpp)₃) has previously been shown to be a strong corrosion inhibitor for aluminium-copper magnesium alloy AA2024-T3 and AA7075 in chloride solutions. Surface characterisation including SEM and ToF-SIMS coupled with electrochemical impedance spectroscopy (EIS) measurements are used to propose a mechanism of corrosion inhibition which appears to involve the formation of a complex oxide film of aluminium and cerium also incorporating the organophosphate component. The formation of a thin complex film consisting of hydrolysis products of the Ce(dpp)₃ compound and aluminium oxide is proposed to lead to the observed inhibition. SEM analysis shows that some intermetallics favour the creation of thicker deposits predominantly containing cerium oxide compounds.     

An electrochemical and superficial assessment of the corrosion behavior of AA 2024-T3 treated with metacryloxypropylmethoxysilane and cerium nitrate

The present work aims at studying the corrosion behavior of treatments based on the deposition of layers of metacryloxypropylmethoxysilane (MAOS) and/or cerium nitrate on aluminum alloy 2024 T-3 (AA2024-T3). The corrosion resistance was evaluated by electrochemical impedance spectroscopy (EIS) during immersion in 0.1 M Na₂SO₄ and NaCl solutions. Atomic force microscopy (AFM), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were also used to perform a surface analysis before and after the treatments. The electrochemical results show that cerium nitrate, when present between two layers of MAOS (sandwich-type deposited layer), improves the corrosion resistance. This can be attributed to the presence of an internal layer rich in silicium and cerium and another external MAOS layer, which further improves the barrier effect of the layer.     

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.     

Electrochemical study of inhibitor-containing organic-inorganic hybrid coatings on AA2024

Use of organic-inorganic hybrid sol-gel thin films is recently reported among the most promising approaches for the replacement of chromate based pre-treatment methods for high strength structural aluminium parts.To be applied in the aerospace industry, pre-treatments have to meet the demanding requirements and must be compatible to state of the art paint systems. The addition of inhibitive species is desirable for the active corrosion protection and the prevention of sub-surface migration at defect sites of the coating.In the present work hybrid films were produced by the controllable sol-gel route. Corrosion inhibitors were incorporated to the coatings in order to provide active protective effect. The barrier properties and the active corrosion protection on AA2024-T3 have been studied by Electrochemical Impedance Spectroscopy and Scanning Vibrating Electrode Technique. The obtained results were proven by conventional accelerated tests such as neutral salt spray test confirming validity of the mentioned electrochemical techniques for the evaluation of protection efficiency of hybrid sol-gel films.The addition of inhibitors to sol-gel coatings in most cases leads to an improvement of the active corrosion protection but on the other hand it may negatively influences the barrier properties of the films. Therefore, encapsulation of the inhibiting compounds is beneficial in order to combine the good barrier function of the coating with the corrosion mitigating effect of the inhibitors.     

A room temperature cured sol–gel anticorrosion pre-treatment for Al 2024-T3 alloys

The inherent reactivity of the Al–Cu alloys is such that their use for structural, marine, and aerospace components and structures would not be possible without prior application of a corrosion protection system. Historically these corrosion protection systems have been based upon the use of chemicals containing Cr(VI) compounds. Organic–inorganic hybrid silane coatings are of increasing interest in industry due to their potential application for the replacement of current toxic hexavalent chromate based treatments. In the present study, a hybrid epoxy–silica–alumina coating with or without doped cerium nitrate has been prepared using a sol–gel method. The hybrid coatings were applied by a dip-technique to an Al–Cu alloy, Al 2024-T3, and subsequently cured at room temperature. The anticorrosion properties of the coatings within 3.5% NaCl were studied using electrochemical impedance spectroscopy (EIS), and conventional DC polarisation. An exfoliation test method involving immersion in a solution of 4 M NaCl, 0.5 M KNO₃ and 0.1 M HNO₃ was also used. The cerium nitrate doped sol–gel coating exhibited excellent anticorrosion properties providing an adherent protection film on the Al 2024-T3 substrate. The resistance to corrosion of the sol–gel coating was also evaluated by analysing the morphology of the coating before and after corrosion testing using scanning electron microscopy.     

Mechanisms of corrosion inhibition of AA2024-T3 by vanadates

The mechanisms of corrosion inhibition of AA2024-T3 by vanadates were studied using chronoamperometry, polarization curves and adsorption isotherms. The electrochemical behaviour of clear solutions containing metavanadates and orange solutions containing decavanadates was clearly distinctive. Metavanadates reduced the kinetics of oxygen reduction to an extent similar to chromates. Corrosion inhibition of AA2024-T3 by metavanadates was very rapid and it might occur by the formation of an adsorbed layer. Reduction of clear metavanadate solution was very slow. Approximately 35 min were required to develop a monolayer of a reduced vanadate species. The adsorption of the inhibitor likely blocked reactive sites on intermetallic particles, discouraging the oxygen reduction reaction (ORR). Adsorption of the inhibitor on the Al matrix could also displace Cl⁻ ions, increasing the stability of the passive film and reducing the breakdown of S-phase particles. In contrast, decavanadates were shown to be poor inhibitors of the ORR. A sharp current spike was observed after injection of decavanadates for both Cu and AA2024-T3 at various applied cathodic potentials. Integration of the current peaks suggested the formation of several monolayers of a reduced vanadate species. The formation of several monolayers was in line with the poor performance of decavanadates as inhibitors of AA2024-T3 corrosion.     

Unravelling the corrosion inhibition mechanisms of bi-functional inhibitors by EIS and SEM–EDS

In this work the corrosion inhibition effect of aluminium alloy AA2024-T3 in aqueous solution by an inorganic cerium salt (i.e. cerium chloride) and an organic one (i.e. cerium dibutyl phosphate) is presented. The research involved long term immersion tests (up to 8 d) followed up by EIS. The analysis was complemented by a systematic selection of the most probable equivalent circuits and scanning electron microscope analysis combined with X-ray energy dispersive spectroscopy. This study highlights the different corrosion protection mechanisms of organic and inorganic ions. A model of protection is suggested based on the combination of techniques.     

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.