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P. Campestrini - One of the best experts on this subject based on the ideXlab platform.
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Chromate Conversion Coating on aluminum alloys ii effect of the microstructure
Journal of The Electrochemical Society, 2004Co-Authors: P. Campestrini, H Terryn, J VereeckenAbstract:By means of the combined use of scanning electron microscopy, energy dispersive spectroscopy, and atomic force microscopy in the Kelvin probe mode the mechanism of the Chromate Conversion Coating (CCC) formation, discussed in the first part of the paper, was extended to the case of AA2024, taking into account the effect of the intermetallics (IMCs). It was found that the cathodic IMCs play a fundamental role during the nucleation of the CCC, since they act as preferential nucleation sites. Besides, the influence of the surface preparation procedure was also considered. The use of an acid pickling treatment prior to the Chromate process was shown to strongly affect the growth of the CCC, since it results in the formation on the AA2024 surface of a copper-rich smut, which inhibits the Chromate reduction reaction. Both IMCs and copper deposits negatively affect the CCC morphology. Indeed, the former are locations of defects and the latter decreases the thickness and adherence of the film.
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Chromate Conversion Coating on aluminum alloys iii corrosion protection
Journal of The Electrochemical Society, 2004Co-Authors: P. Campestrini, H Terryn, J VereeckenAbstract:Electrochemical impedance spectroscopy (EIS) was used to investigate the corrosion resistance of the Chromate Conversion Coating (CCC) formed on two different substrates, namely bare and clad 2024 aluminum alloys, and after different surface preparation procedures, namely acid pickling with or without desmutting. The impedance data were fitted using previously published equivalent circuits, which were developed on the basis of a physical model of the CCC. The evolution of the network parameters during exposure to the chloride solution enabled the correlation of the differences in the film morphology, which are caused by the variation in the substrate microstructure, with the corrosion protection provided by the Conversion film. Specifically, the CCC corrosion resistance is strongly decreased by the presence of both intermetallic particles and copper smut on the Al surface, since the former are locations of defects in the Conversion layer and the latter inhibits the film growth and lowers its adherence to the substrate. The corrosion resistance of the CCC formed on the different substrates and after the different surface preparation treatments can be clearly differentiated using the EIS technique and fitting the data with the proposed equivalent circuits, which represent the Conversion film as a nonhomogeneous system containing large defects where the corrosion attack principally occurs.
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Chromate Conversion Coating on aluminum alloys i formation mechanism
Journal of The Electrochemical Society, 2004Co-Authors: P. Campestrini, H Terryn, G Goeminne, J VereeckenAbstract:The formation of Chromate Conversion Coatings (CCCs) on commercially pure Al (AA1050) during immersion in a chromic/ hydrofluoric acid solution has been investigated. A film formation mechanism is proposed based on the combination of open circuit potential measurements and surface analysis techniques, such as Auger electron spectroscopy and atomic force microscopy. It is suggested that the Conversion of the Al surface takes place in three distinct stages: activation of the surface, initiation of the film formation, and growth of the Conversion layer. In addition to the classical approach, a sol-gel model was considered for the initiation and growth of the film. The effect of the free fluoride and chromic acid concentration was studied using a thermodynamic model whereas the influence of the Al oxide film was investigated by forming an anodic barrier layer prior to the Conversion process. It was shown that the rate-determining step in the CCC formation is the activation of the Al surface. Therefore, the morphology and structure of the Conversion layer is determined not only by the bath composition but also by the thickness of the Al oxide film.
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Study of the formation of Chromate Conversion Coatings on Alclad 2024 aluminum alloy using spectroscopic ellipsometry
Thin Solid Films, 2002Co-Authors: P. Campestrini, Siva Bohm, T. Schram, Herman TerrynAbstract:The influence of pH of a Chromate bath on the morphological and chemical properties of a Chromate Conversion Coating, formed on Alclad 2024-T3 aluminum alloy, has been investigated by variable angle spectroscopic ellipsometry in the visible and infra-red regions. Other techniques such as glow discharge optical emission spectroscopy, transmission and scanning electron microscopy, atomic force microscopy and Auger electron spectroscopy have been used in order to confirm and sustain the results obtained with this technique. The combination of different analytical methods showed a decrease in thickness together with changes in the morphology and chemical composition of the Chromate film when the pH is increased from 1.2 to 2.4. Although the complexity of the Chromate system and the pronounced roughness of commercial rolled aluminum limit the accuracy of spectroscopic ellipsometry, it is demonstrated that this technique can be usefully applied to the study of thin Chromate films formed on industrially relevant aluminum products.
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investigation of the Chromate Conversion Coating on alclad 2024 aluminium alloy effect of the ph of the Chromate bath
Electrochimica Acta, 2002Co-Authors: P. Campestrini, E P M Van Westing, A HovestadAbstract:The parameters of the Chromate bath, like temperature, pH, and fluoride content, strongly affect the morphology and chemical composition of the Chromate Conversion Coating and as a consequence have a large influence on its corrosion performance. In this paper, electrochemical impedance spectroscopy (EIS) was used in combination with other techniques to investigate the role played by the pH of the Chromate bath on the properties of the Chromate film formed on Alclad 2024 aluminium alloy. Scanning electron microscopy (SEM), atomic force microscopy (AFM) and spectroscopic ellipsometry (SE) have shown the formation of a thicker and less dense Chromate layer when the pH of the Chromate bath is changed from 2.4 to 1.2. The analysis of the EIS spectra have highlighted that this change in pH leads to the formation of more protective and more resistant Chromate corrosion products (CCP) inside the defects of the Chromate film. When a thin, dense and protective layer of CCP is formed in the defects, the corrosion behaviour of the Chromate Conversion Coating improves for two main reason: (a) further attack of the defects is avoided or delayed; (b) the change in volume caused by the formation of the CCP is limited resulting in a low level of stress in the film, which as a consequence is not detached from the aluminium substrate.
Gary P Halada - One of the best experts on this subject based on the ideXlab platform.
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On the nature of the Chromate Conversion Coating formed on intermetallic constituents of AA2024‐T3
Surface and Interface Analysis, 2020Co-Authors: M. Jaime Vasquez, Gary P Halada, Clive R. Clayton, Jon P LongtinAbstract:The objective of the research reported in this paper was to refine an existing model for Chromate Conversion Coatings (CCC) formed on the surface of AA2024-T3, an Al–Cu aircraft alloy, by considering the composition and structure of the CCC formed on constituent intermetallic compounds (IMCs). To achieve this aim it was necessary to develop large-area samples composed of compositionally homogeneous thin films of the various IMCs found on the AA2024-T3 surface, which were galvanically attached to thin films of Al–4.2wt.%Cu (representative of the AA2024-T3 matrix). This was performed in a two-step process: disks of IMC compositions were formed by reactive arc melting (RAM), followed by femtosecond laser ablation of the RAM IMCs, resulting in the formation of homogeneous thin films. These thin films were used to analyze the formation of CCC on IMCs, the AA2024-T3 matrix analog and matrix–IMC galvanic couples. Secondary ion mass spectrometry depth profiling revealed significant variations in CCC film thickness and composition related to the underlying IMCs. The SIMS results indicated that the CCC formed on the matrix analog had the same average thickness as the average CCC formed on AA2024-T3, whereas those formed on individual θ and S phases were only 9% and 12% as thick, respectively, when uncoupled to the matrix and 11% and 14% as thick when coupled to the matrix. The CCC thickness on the Al20Cu2(MnFe)3 was found to be variable, having an islanded structure. Topographical maps indicate that the IMC/matrix boundaries are covered with CCC of a thickness approaching that of the matrix. Inhibition of CCC growth is related to the high copper content (and therefore insufficient aluminum) present in the IMCs compared to the matrix, which provides a more accurate model of CCC formation on AA2024-T3. Copyright © 2002 John Wiley & Sons, Ltd.
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failure of navy Coating systems 2 failure pathways of artificially weathered navy Coating systems applied to Chromate Conversion coated aa2024 t3 substrates
Progress in Organic Coatings, 2005Co-Authors: Lionel T Keene, Jaime M Vasquez, C R Clayton, Gary P HaladaAbstract:Dual layer Coating systems consisting of volatile organic compound (VOC)-based aliphatic polyurethane topcoats and epoxy primers applied to AA2024-T3 substrates pretreated with Chromate Conversion Coating (CCC) are the current mainstay of the U.S. Navy. Government mandates preclude their future use, however, and they are being phased out of service in favor of new VOC-free topcoat and primer high-solids reformulations. The durability of this new formulation to artificial weathering protocol GM9540P was evaluated using transmission-mode and diffuse reflectance Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS) imaging, optical and scanning electron microscopy energy dispersive spectroscopy (SEM-EDS) and laser-confocal topography. Extensive blistering of the Coating system was observed. Depth-resolved FTIR spectra show a lack of chemical modification of the topcoat layer. Data collected show localized corrosion of the substrate material in the form of pitting, as well as chemical inhomogeneities within the primer layer manifest as regional undercuring of the epoxy matrix. It is hypothesized that barrier failure of the Coating/pretreatment system is the result of a cascading series of component-specific failures, notably an apparent shift in the target polydispersity of the prepolymer components of the epoxy primer.
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surface pretreatments of aluminum alloy aa2024 t3 and formation of Chromate Conversion Coatings ii composition and electrochemical behavior of the Chromate Conversion Coating
Journal of The Electrochemical Society, 2004Co-Authors: Devicharan Chidambaram, C R Clayton, Martin W Kendig, Gary P HaladaAbstract:Rockwell Scientific Company, Surface and Electrochemical Processes Department, Thousand Oaks,California 91360, USAAluminum alloy AA2024-T3 samples, subjected to various pretreatments, were Conversion coated using Alodine 1200S. The ratioof hexavalent chromium to total chromium in the Chromate Conversion Coating ~CCC! was determined by X-ray absorptionnear-edge spectroscopy. The hexavalent chromium content of the CCC formed on AA2024-T3 varied with the method of pre-treatment. The coated surfaces were analyzed by X-ray photoelectron spectroscopy to determine the surface composition of theCoatings. Cr~VI! forms the major constituent ~46-74% of total chromium! on the surface. Protective passive films of aluminumoxide and aluminum phosphate inhibit the formation of CCCs. Corrosion behavior of the CCCs were studied using open-circuitpotential measurements, potentiodynamic polarization, and electrochemical impedance spectroscopy ~EIS!. Conversion coatedalloys exhibited up to 40-fold higher corrosion resistance when compared to bare pretreated alloys. EIS behavior was modeledusing a seven-element equivalent circuit. EIS results indicated industrially used bromate pretreatment to be best suited for CCCformation. Coupling the AA2024-T3 alloy with platinum during bromate pretreatment resulted in lowering the amount of copperintermetallics on the surface. This led to an increase in the corrosion resistance of the subsequently formed CCC by over an orderof magnitude as compared to CCCs formed following alternative pretreatments. Therefore, this simple modification of couplingthis alloy with platinum is recommended.© 2004 The Electrochemical Society. @DOI: 10.1149/1.1806393# All rights reserved.Manuscript submitted December 1, 2003; revised manuscript received May 11, 2004. Available electronically October 27, 2004.
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Spectroscopic Elucidation of the Repassivation of Active Sites on Aluminum by Chromate Conversion Coating
Electrochemical and Solid State Letters, 2004Co-Authors: Devicharan Chidambaram, Gary P Halada, Clive R. ClaytonAbstract:It has long been hypothesized and recently shown that hexavalent chromium ions in the Chromate Conversion Coating (CCC) migrate to active sites. This repassivation behavior of Chromate Conversion Coatings leads to protection from corrosion resulting from mechanical damage. The nature of this repassivation has been explored here using Raman spectroscopy and synchrotron X-ray absorption near-edge structure (XANES) spectroscopy. Raman spectroscopic analysis of a mechanically damaged area on the CCC supports the theory of chromium ion migration and formation of an Al(III)-Cr(VI) complex at the active sites. Results of an investigation performed using XANES corroborate the formation of the complex at active sites such as pits/valleys/scratches even under other chromating conditions. This study therefore provides unambiguous evidence for the theory of migration of Chromates.
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interactions of the components of Chromate Conversion Coating with the constituents of aluminum alloy aa2024 t3
Journal of The Electrochemical Society, 2004Co-Authors: Devicharan Chidambaram, C R Clayton, Gary P HaladaAbstract:Electrochemical techniques were employed to study the nature of individual interactions between the chemicals constituting a widely used commercial Chromate Conversion-Coating treatment and the main constituents of AA2024-T3 alloy, namely, Al (99.999%), Al 2 Cu, Al-Al 2 Cu galvanic couple, and AA2024-T3 alloy. The samples were pretreated using Chromate, ferricyanide, fluoride, tetrafluoroborate, and hexafluorozirconate, prior to electrochemical corrosion tests. The results were compared with untreated (control) samples. For the first time, the chemical interaction of aluminum with hexafluorozirconate has been studied. Maximum activation was observed in the case of hexafluorozirconate pretreatment, which also decreased the interfacial tension and increased surface wetting. The electrochemistry of the control and pretreated (except for hexafluorozirconate pretreatment) AA2024-T3 and Al 2 Cu were found to be similar. The cathodic electron transfer reaction rate for oxygen reduction was found to be enhanced by the presence of copper in the systems. The enhancement was proportional to the copper content. The cathodic reaction on all systems was inhibited by Chromate. Aluminum was observed to undergo cathodic corrosion, leading to an enrichment of the surface copper content and subsequent enhancement of the cathodic electron transfer reaction.
Richard L Mccreery - One of the best experts on this subject based on the ideXlab platform.
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chemistry of a Chromate Conversion Coating on aluminum alloy aa2024 t3 probed by vibrational spectroscopy
Journal of The Electrochemical Society, 1998Co-Authors: Richard L MccreeryAbstract:Infrared and Raman spectroscopy were used to characterize a Chromate Conversion Coating (CCC) on 2024-T3 aluminum aircraft alloy with a long range objective of determining the anticorrosion mechanism of the CCC. Spectra were compared to those from synthetic mixed oxides of aluminum, Cr(III), and Cr(VI) made by treating pure reagents with NaOH. The Fourier transform infrared (FTIR) and Raman spectra of the CCC showed similar behavior to the chromium III/VI mixed oxide for both the initial materials and after heat-treatment. Analysis of the CCC and chromium mixed oxide by UV-vis spectroscopy indicated that both have a 3:1 ratio of Cr(III) to Cr(VI). When the chromium mixed oxide was immersed in pH 4 HNO 3 , the ratio of released H + to released Cr(VI) ranged from 0.98 to 1.07. In addition, a compound with a Raman spectrum very similar to that of a CCC was formed by a reaction of Cr(III) hydroxide with Cr 2 O 7 -2 , CrO 4 -2 , or the Alodine chromating bath. The results indicate a strong structural similarity between the Cr-mixed oxide and a major component of the CCC. A likely structure for this common material involves covalent bonding between polymeric Cr(III) hydroxide and Cr 2 O 7 -2 or CrO 4 2 , This mixed oxide structure may hydrolyze to release H + and soluble Chromate.
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Chemistry of a Chromate Conversion Coating on Aluminum Alloy AA2024‐T3 Probed by Vibrational Spectroscopy
Journal of The Electrochemical Society, 1998Co-Authors: Richard L MccreeryAbstract:Infrared and Raman spectroscopy were used to characterize a Chromate Conversion Coating (CCC) on 2024-T3 aluminum aircraft alloy with a long range objective of determining the anticorrosion mechanism of the CCC. Spectra were compared to those from synthetic mixed oxides of aluminum, Cr(III), and Cr(VI) made by treating pure reagents with NaOH. The Fourier transform infrared (FTIR) and Raman spectra of the CCC showed similar behavior to the chromium III/VI mixed oxide for both the initial materials and after heat-treatment. Analysis of the CCC and chromium mixed oxide by UV-vis spectroscopy indicated that both have a 3:1 ratio of Cr(III) to Cr(VI). When the chromium mixed oxide was immersed in pH 4 HNO 3 , the ratio of released H + to released Cr(VI) ranged from 0.98 to 1.07. In addition, a compound with a Raman spectrum very similar to that of a CCC was formed by a reaction of Cr(III) hydroxide with Cr 2 O 7 -2 , CrO 4 -2 , or the Alodine chromating bath. The results indicate a strong structural similarity between the Cr-mixed oxide and a major component of the CCC. A likely structure for this common material involves covalent bonding between polymeric Cr(III) hydroxide and Cr 2 O 7 -2 or CrO 4 2 , This mixed oxide structure may hydrolyze to release H + and soluble Chromate.
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corrosion protection of untreated aa 2024 t3 in chloride solution by a Chromate Conversion Coating monitored with raman spectroscopy
Journal of The Electrochemical Society, 1998Co-Authors: Jun Zhao, G S Frankel, Richard L MccreeryAbstract:The behavior of Chromate Conversion Coatings (CCCs) on the aluminum aircraft alloy AA 2024-T3 was examined by several types of experiments, using Raman spectroscopy as a primary technique. First, Raman spectra of the CCC film made from a commercial process revealed a Raman feature characteristic of Cr(VI) which was distinct from Raman bands of pure CrO4 − 2 or Cr2O7 − 2 . Second, Raman spectroscopy was used to monitor migration of Chromate species from a CCC film to an initially untreated alloy sample. The release of Chromate from a CCC was demonstrated, as was redeposition of a Chromate film on the fresh alloy surface. Formation of a Raman-observable Cr(VI)-containing deposit was more rapid in or near pits in the untreated alloy sample, and the deposit was spectroscopically very similar to the original CCC film. The initially untreated alloy became much less active toward corrosion after migration of Chromate from the nearby CCC film, with the polarization resistance increasing by at least two orders of magnitude and the pitting potential increasing by 60 mV. The results clarify the mechanism of self-healing exhibited by CCC films, in which Chromate species released from the CCC migrate to an actively corroding region and stop aluminum dissolution. The migrating Chromate is selectively deposited at active corrosion sites, either by forming an insoluble Al/Chromate is selectively deposited at active corrosion sites, either by forming an insoluble Al/Chromate precipitate or by adsorption by previously formed corrosion
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Corrosion Protection of Untreated AA‐2024‐T3 in Chloride Solution by a Chromate Conversion Coating Monitored with Raman Spectroscopy
Journal of The Electrochemical Society, 1998Co-Authors: Jun Zhao, G S Frankel, Richard L MccreeryAbstract:The behavior of Chromate Conversion Coatings (CCCs) on the aluminum aircraft alloy AA 2024-T3 was examined by several types of experiments, using Raman spectroscopy as a primary technique. First, Raman spectra of the CCC film made from a commercial process revealed a Raman feature characteristic of Cr(VI) which was distinct from Raman bands of pure CrO4 − 2 or Cr2O7 − 2 . Second, Raman spectroscopy was used to monitor migration of Chromate species from a CCC film to an initially untreated alloy sample. The release of Chromate from a CCC was demonstrated, as was redeposition of a Chromate film on the fresh alloy surface. Formation of a Raman-observable Cr(VI)-containing deposit was more rapid in or near pits in the untreated alloy sample, and the deposit was spectroscopically very similar to the original CCC film. The initially untreated alloy became much less active toward corrosion after migration of Chromate from the nearby CCC film, with the polarization resistance increasing by at least two orders of magnitude and the pitting potential increasing by 60 mV. The results clarify the mechanism of self-healing exhibited by CCC films, in which Chromate species released from the CCC migrate to an actively corroding region and stop aluminum dissolution. The migrating Chromate is selectively deposited at active corrosion sites, either by forming an insoluble Al/Chromate is selectively deposited at active corrosion sites, either by forming an insoluble Al/Chromate precipitate or by adsorption by previously formed corrosion
J.h.w. De Wit - One of the best experts on this subject based on the ideXlab platform.
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Influence of surface preparation on performance of Chromate Conversion Coatings on Alclad 2024 aluminium alloy
Electrochimica Acta, 2001Co-Authors: P. Campestrini, E.p.m Van Westing, J.h.w. De WitAbstract:The microstructure and morphology of the surface of aluminium alloys strongly affect the nucleation, growth, and protective properties of Chromate Conversion Coatings. Samples of AA 2024-T3 cladded with AA1230 were pretreated with two different procedures before they underwent a chromating process, which consisted of dipping in a commercial Alodine 1200 solution. In one procedure, an extra step, consisting of immersion in nitric–hydrofluoric acid solution for 30 s (desmutting bath), was carried out after immersion in sulfuric –phosphoric acid solution (acid pickling bath). Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM) and electrochemical investigations highlighted the fact that this extra step in the pretreatment route gave rise to a more etched, but less reactive surface, due to the removal of a large amount of cathodic sites, mainly related to Al---Fe---Si precipitates present in the clad layer. A similar effect, but on a smaller scale, was caused also by the increase of the time of immersion in sulfuric –phosphoric acid solution. The decrease of galvanic coupling gives rise to a more homogeneous nucleation of the Chromate Conversion Coating on the surface of the Alclad 2024-T3. Moreover, the Chromate film tends to be denser and with fewer defects.
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Influence of surface preparation on performance of Chromate Conversion Coatings on Alclad 2024 aluminium alloy - Part I: Nucleation and growth
Electrochimica Acta, 2001Co-Authors: P. Campestrini, E.p.m Van Westing, J.h.w. De WitAbstract:The microstructure and morphology of the surface of aluminium alloys strongly affect the nucleation, growth, and protective properties of Chromate Conversion Coatings. Samples of AA 2024-T3 cladded with AA1230 were pretreated with two different procedures before they underwent a chromating process, which consisted of dipping in a commercial Alodine 1200 solution. In one procedure, an extra step, consisting of immersion in nitric-hydrofluoric acid solution for 30 s (desmutting bath), was carried out after immersion in sulfuric -phosphoric acid solution (acid pickling bath). Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM) and electrochemical investigations highlighted the fact that this extra step in the pretreatment route gave rise to a more etched, but less reactive surface, due to the removal of a large amount of cathodic sites, mainly related to Al-Fe-Si precipitates present in the clad layer. A similar effect, but on a smaller scale, was caused also by the increase of the time of immersion in sulfuric -phosphoric acid solution. The decrease of galvanic coupling gives rise to a more homogeneous nucleation of the Chromate Conversion Coating on the surface of the Alclad 2024-T3. Moreover, the Chromate film tends to be denser and with fewer defects. © 2001 Elsevier Science Ltd.
C R Clayton - One of the best experts on this subject based on the ideXlab platform.
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failure of navy Coating systems 2 failure pathways of artificially weathered navy Coating systems applied to Chromate Conversion coated aa2024 t3 substrates
Progress in Organic Coatings, 2005Co-Authors: Lionel T Keene, Jaime M Vasquez, C R Clayton, Gary P HaladaAbstract:Dual layer Coating systems consisting of volatile organic compound (VOC)-based aliphatic polyurethane topcoats and epoxy primers applied to AA2024-T3 substrates pretreated with Chromate Conversion Coating (CCC) are the current mainstay of the U.S. Navy. Government mandates preclude their future use, however, and they are being phased out of service in favor of new VOC-free topcoat and primer high-solids reformulations. The durability of this new formulation to artificial weathering protocol GM9540P was evaluated using transmission-mode and diffuse reflectance Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS) imaging, optical and scanning electron microscopy energy dispersive spectroscopy (SEM-EDS) and laser-confocal topography. Extensive blistering of the Coating system was observed. Depth-resolved FTIR spectra show a lack of chemical modification of the topcoat layer. Data collected show localized corrosion of the substrate material in the form of pitting, as well as chemical inhomogeneities within the primer layer manifest as regional undercuring of the epoxy matrix. It is hypothesized that barrier failure of the Coating/pretreatment system is the result of a cascading series of component-specific failures, notably an apparent shift in the target polydispersity of the prepolymer components of the epoxy primer.
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surface pretreatments of aluminum alloy aa2024 t3 and formation of Chromate Conversion Coatings ii composition and electrochemical behavior of the Chromate Conversion Coating
Journal of The Electrochemical Society, 2004Co-Authors: Devicharan Chidambaram, C R Clayton, Martin W Kendig, Gary P HaladaAbstract:Rockwell Scientific Company, Surface and Electrochemical Processes Department, Thousand Oaks,California 91360, USAAluminum alloy AA2024-T3 samples, subjected to various pretreatments, were Conversion coated using Alodine 1200S. The ratioof hexavalent chromium to total chromium in the Chromate Conversion Coating ~CCC! was determined by X-ray absorptionnear-edge spectroscopy. The hexavalent chromium content of the CCC formed on AA2024-T3 varied with the method of pre-treatment. The coated surfaces were analyzed by X-ray photoelectron spectroscopy to determine the surface composition of theCoatings. Cr~VI! forms the major constituent ~46-74% of total chromium! on the surface. Protective passive films of aluminumoxide and aluminum phosphate inhibit the formation of CCCs. Corrosion behavior of the CCCs were studied using open-circuitpotential measurements, potentiodynamic polarization, and electrochemical impedance spectroscopy ~EIS!. Conversion coatedalloys exhibited up to 40-fold higher corrosion resistance when compared to bare pretreated alloys. EIS behavior was modeledusing a seven-element equivalent circuit. EIS results indicated industrially used bromate pretreatment to be best suited for CCCformation. Coupling the AA2024-T3 alloy with platinum during bromate pretreatment resulted in lowering the amount of copperintermetallics on the surface. This led to an increase in the corrosion resistance of the subsequently formed CCC by over an orderof magnitude as compared to CCCs formed following alternative pretreatments. Therefore, this simple modification of couplingthis alloy with platinum is recommended.© 2004 The Electrochemical Society. @DOI: 10.1149/1.1806393# All rights reserved.Manuscript submitted December 1, 2003; revised manuscript received May 11, 2004. Available electronically October 27, 2004.
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interactions of the components of Chromate Conversion Coating with the constituents of aluminum alloy aa2024 t3
Journal of The Electrochemical Society, 2004Co-Authors: Devicharan Chidambaram, C R Clayton, Gary P HaladaAbstract:Electrochemical techniques were employed to study the nature of individual interactions between the chemicals constituting a widely used commercial Chromate Conversion-Coating treatment and the main constituents of AA2024-T3 alloy, namely, Al (99.999%), Al 2 Cu, Al-Al 2 Cu galvanic couple, and AA2024-T3 alloy. The samples were pretreated using Chromate, ferricyanide, fluoride, tetrafluoroborate, and hexafluorozirconate, prior to electrochemical corrosion tests. The results were compared with untreated (control) samples. For the first time, the chemical interaction of aluminum with hexafluorozirconate has been studied. Maximum activation was observed in the case of hexafluorozirconate pretreatment, which also decreased the interfacial tension and increased surface wetting. The electrochemistry of the control and pretreated (except for hexafluorozirconate pretreatment) AA2024-T3 and Al 2 Cu were found to be similar. The cathodic electron transfer reaction rate for oxygen reduction was found to be enhanced by the presence of copper in the systems. The enhancement was proportional to the copper content. The cathodic reaction on all systems was inhibited by Chromate. Aluminum was observed to undergo cathodic corrosion, leading to an enrichment of the surface copper content and subsequent enhancement of the cathodic electron transfer reaction.
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studies on the repassivation behavior of aluminum and aluminum alloy exposed to Chromate solutions
Surface and Interface Analysis, 2003Co-Authors: Devicharan Chidambaram, Jaime M Vasquez, Gary P Halada, C R ClaytonAbstract:This study explores the repassivation ability of a scratch in a Chromate Conversion Coating (CCC) on aluminum alloy, AA2024-T3, and hence evaluates the theory of migration of hexavalent chromium ions from the protected surface of the aluminum alloy to the exposed surface. To confirm that protection was indeed restored by hexavalent chromium ions, the repassivation of a scratch on pure aluminum exposed to a diChromate solution was studied. This forms the simplest subsystem model of the CCC on the alloy in which the CCC is replaced by pure hexavalent chromium and alloy with pure aluminum. Open-circuit potential measurements, synchrotron infrared microspectroscopy (SIRMS) and secondary ion mass spectroscopy (SIMS) have been used judiciously to evaluate the repassivation behavior. Results indicate that the diChromate ions have high mobility. The slow migration of Cr(VI) ions from the protected surface to the scratch is observed to result in repassivation, as seen from the steady increase in the potential in 0.05 M NaCl solution. The results obtained from SIMS and SIRMS confirm the migration of the oxyanions from the protected region to the metal surface exposed by the scratch. The SIRMS results indicate the formation of an Al(III)–Cr(VI) complex, proposed and shown to be formed in the pits. Copyright © 2003 John Wiley & Sons, Ltd.
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on the nature of the Chromate Conversion Coating formed on intermetallic constituents of aa2024 t3
Surface and Interface Analysis, 2002Co-Authors: Jaime M Vasquez, Gary P Halada, C R Clayton, Jon P LongtinAbstract:The objective of the research reported in this paper was to refine an existing model for Chromate Conversion Coatings (CCC) formed on the surface of AA2024-T3, an Al–Cu aircraft alloy, by considering the composition and structure of the CCC formed on constituent intermetallic compounds (IMCs). To achieve this aim it was necessary to develop large-area samples composed of compositionally homogeneous thin films of the various IMCs found on the AA2024-T3 surface, which were galvanically attached to thin films of Al–4.2wt.%Cu (representative of the AA2024-T3 matrix). This was performed in a two-step process: disks of IMC compositions were formed by reactive arc melting (RAM), followed by femtosecond laser ablation of the RAM IMCs, resulting in the formation of homogeneous thin films. These thin films were used to analyze the formation of CCC on IMCs, the AA2024-T3 matrix analog and matrix–IMC galvanic couples. Secondary ion mass spectrometry depth profiling revealed significant variations in CCC film thickness and composition related to the underlying IMCs. The SIMS results indicated that the CCC formed on the matrix analog had the same average thickness as the average CCC formed on AA2024-T3, whereas those formed on individual θ and S phases were only 9% and 12% as thick, respectively, when uncoupled to the matrix and 11% and 14% as thick when coupled to the matrix. The CCC thickness on the Al20Cu2(MnFe)3 was found to be variable, having an islanded structure. Topographical maps indicate that the IMC/matrix boundaries are covered with CCC of a thickness approaching that of the matrix. Inhibition of CCC growth is related to the high copper content (and therefore insufficient aluminum) present in the IMCs compared to the matrix, which provides a more accurate model of CCC formation on AA2024-T3. Copyright © 2002 John Wiley & Sons, Ltd.