Formation Of Chromate Conversion Coatings Research Articles

Formation of Chromate Conversion Coatings on Aluminum and Its Alloys: An In Situ XANES Study

In situ X-ray adsorption near-edge structure (XANES) has been used to investigate the formation of chromate conversion coatings on pure Al, commercial Al alloys (AA 1100, AA2024, and AA7075), and a series of binary Al–Cu alloys. The method employed cells designed to determine the growth of the total chromium [Cr(total)] and hexavalent chromate [Cr(VI)] in the chromate conversion coating (CCC) as a function of exposure time to a chromate solution. Three sets of data were obtained, where (i) the Al was exposed to only a limited amount of solution; (ii) the chromate solution was excluded after short periods of repeated exposures to the solution; and (iii) the Al was exposed continuously to the chromate solution. All the results showed a very rapid initial growth within the first seconds, followed by a continued increase in thickness for exposures up to . Measurements with Al–Cu binary alloys demonstrated that the difference observed in AA2024 and AA1100 may not be due to Cu alloying. The proportion of Cr(VI) in the coatings becomes approximately constant after of exposure for all the specimens examined, even though the coatings continued to grow.

The role of hexafluorozirconate in the formation of chromate conversion coatings on aluminum alloys

Aluminum based surfaces are routinely coated with a chromate based layer that provides unparalleled corrosion protection. Widely used conversion coating treatment formulations contain hexafluorozirconate as a major constituent besides chromate, ferricyanide, fluoride, and fluoborate. The function of hexafluorozirconate is the subject of this study as its function is still largely unknown. Hydrophobicity, surface morphology, and the chemistry of the surface, resulting from treatment with hexafluorozirconate, were studied using contact angle measurements, scanning electron microscopy, and energy dispersive spectroscopy, respectively. X-ray photoelectron spectroscopy was extensively utilized to determine the chemistry of the surface resulting from the hexafluorozirconate pretreatment. Our results indicate that fluoride ion containing hexafluorozirconate complex does not attack the oxide film in a manner that uncomplexed simple fluoride ion does. Hexafluorozirconate is involved in the formation of an Al-Zr-O-F based hydrated layer that increases the hydrophilicity of the surface, activates the surface, and lowers the corrosion resistance. These factors enhance the interaction of chromate with the alloy surface to result in the formation of a uniform conversion coating. Based on these results, a new model has been proposed for the formation of chromate conversion coatings.

Surface Pretreatments of Aluminum Alloy AA2024-T3 and Formation of Chromate Conversion Coatings : I. Composition and Electrochemical Behavior of the Oxide Film

Aluminum alloy AA2024-T3 has been subjected to various pretreatments including those routinely carried out in the industry prior to conversion coating of the alloy. The surface chemistry of the samples after pretreatment was analyzed using variable angle X-ray photoelectron spectroscopy (VAXPS). Electrochemical techniques were used to evaluate the corrosion behavior of the samples in quiescent 0.05 M NaCl solution. The results were compared with those obtained from an untreated sample used in as-received condition that acted as a control. The surface of the alloy was observed to be enriched in copper and magnesium after certain pretreatments. Composition of the surface oxide film as well as the electrochemical behavior was found to vary significantly depending on the pretreatment. In all cases, the overlayer formed on the alloy was less than the sampling depth of the XPS (∼90 Å). Pretreatment of the alloy using Alconox® was found to result in the formation of a highly protective aluminum oxide surface film. The corrosion current of the samples was observed to be directly proportional to the aluminum oxyhydroxide content of the film, while the polarization resistance of the sample was inversely proportional to the surface copper content. © 2004 The Electrochemical Society. All rights reserved.

Surface Pretreatments of Aluminum Alloy AA2024-T3 and Formation of Chromate Conversion Coatings : II. Composition and Electrochemical Behavior of the Chromate Conversion Coating

Aluminum alloy AA2024-T3 samples, subjected to various pretreatments, were conversion coated using Alodine 1200S. The ratio of hexavalent chromium to total chromium in the chromate conversion coating (CCC) was determined by X-ray absorption near-edge spectroscopy. The hexavalent chromium content of the CCC formed on AA2024-T3 varied with the method of pretreatment. The coated surfaces were analyzed by X-ray photoelectron spectroscopy to determine the surface composition of the coatings. Cr(VI) forms the major constituent (46-74% of total chromium) on the surface. Protective passive films of aluminum oxide and aluminum phosphate inhibit the formation of CCCs. Corrosion behavior of the CCCs were studied using open-circuit potential measurements, potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS). Conversion coated alloys exhibited up to 40-fold higher corrosion resistance when compared to bare pretreated alloys. EIS behavior was modeled using a seven-element equivalent circuit. EIS results indicated industrially used bromate pretreatment to be best suited for CCC formation. Coupling the AA2024-T3 alloy with platinum during bromate pretreatment resulted in lowering the amount of copper intermetallics on the surface. This led to an increase in the corrosion resistance of the subsequently formed CCC by over an order of magnitude as compared to CCCs formed following alternative pretreatments. Therefore, this simple modification of coupling this alloy with platinum is recommended. © 2004 The Electrochemical Society. All rights reserved.

Chromate Conversion Coating on Aluminum Alloys: I. Formation Mechanism

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. © 2004 The Electrochemical Society. All rights reserved.

Characterization of chromate conversion coating on AA7075‐T6 aluminum alloy

AbstractChromate conversion coatings (CCCs) on AA7075‐T6 were characterized using scanning electron microscopy, focused ion beam sectioning and scanning transmission electron microscopy with nano‐electron dispersive spectroscopy line profiling. The thickness and composition of the CCC was different at different locations on the heterogeneous microstructure of AA7075‐T6. The coating formed on the matrix phase was much thicker than that formed on the coarse Al–Cu–Mg, Al–Fe–Cu and Mg–Si intermetallic particles. Nano‐electron dispersive spectroscopy line profiling indicated that the coating on the Al–Fe–Cu particles was similar to the CCC formed on the phase matrix, primarily a chromium oxide. However, the coatings on the Al–Cu–Mg and Mg–Si particles were mixed Al/Mg/Cr oxide and Mg oxide, respectively. The growth of CCC followed a linear‐logarithmic kinetic rate law. The observations of this study support the sol‐gel model of CCC formation. Copyright © 2004 John Wiley & Sons, Ltd.

Dehydration-Induced Loss of Corrosion Protection Properties in Chromate Conversion Coatings on Aluminum Alloy 2024-T3

It is well known that chromate conversion coatings (CCCs) exhibit a prompt loss in corrosion resistance when exposed to moderately elevated temperatures (60-100°C). They also suffer a gradual loss in corrosion resistance due to ambient temperature exposure. To better understand the origins of losses in corrosion resistance, CCCs were formed on 99.99% Al, 1100 Al [Al-1.0(Fe, Si, Cu)], and 2024-T3 (Al-4.4Cu-1.5Mg-0.6Mn), exposed to elevated and ambient temperature exposures for various lengths of time, and characterized using several different methods. The Cr(VI):total Cr ratio in CCCs was estimated by analysis of data derived from X-ray absorption spectroscopy. Corrosion resistance was measured by electrochemical impedance spectroscopy. The evolution of shrinkage cracking was examined by environmental scanning electron microscopy. Cr(VI) leaching experiments were also carried out to characterize the effect of thermal exposure on Cr(VI) release. Thermal analysis and X-ray diffraction were conducted to characterize the temperature-dependent changes in CCCs. Results show that CCCs degrade in several distinct steps over the temperature range of 20 to 500°C. Our findings support the longstanding notion that dehydration is the root cause for losses in corrosion resistance due to aging at ambient and low temperatures Extended X-ray fine structure measurements indicate a shortening in Cr(III)-Cr(III) nearest neighbor distances upon dehydration. This is interpreted as a consolidation in the backbone of the CCC which leads to shrinkage cracking and immobilization of Cr(VI). This finding is important because it links older observations of the effects of aging and heating on CCCs to newer interpretations of CCC formation based on inorganic polymerization. Overall, these results reveal CCCs to be highly dynamic coatings with corrosion resistance properties that vary considerably in both the short term and long term. © 2003 The Electrochemical Society. All rights reserved.

Corrosion Inhibition of Aluminum and Aluminum Alloys by Soluble Chromates, Chromate Coatings, and Chromate-Free Coatings

Abstract This review covers the aqueous chemistry of chromium and the origins of chromium toxicity. Evidence from older and more recent studies is presented showing that inhibition of Al corrosion is derived from both inhibition of oxygen reduction and inhibition of metal dissolution reactions mainly due to a delay in the onset of pitting. Inhibition of corrosion by chromates appears to be closely linked to their ability to irreversibly adsorb on to metal and oxide surfaces. With respect to chromate conversion coatings (CCCs), the evolution of modern coating formulations is presented with a focus on key advances that led to process simplification and improved coating performance. The current understanding of CCC formation, protection, and aging is presented. In the last section of the paper, processing and properties of selected chromate-free conversion coating chemistries is discussed. This discussion focuses on coatings that offer particularly attractive attributes such as self-healing and excellent adh...

Chromate Conversion Coatings Formation on Zinc Studied by Electrochemical and Electrohydrodynamical Impedances

The formation of chromate conversion coatings on zinc was studied by chronopotentiometric, electrochemical, and electrohydrodynamic impedances, and interfacial pH measurements. The electrochemical experiments were performed with a rotating disk electrode of pure zinc, and the pH measurements were obtained with a zinc deposit on a gold grid electrode in a submerged impinging jet cell. The electrolyte was an industrial chromate bath. The experimental results were achieved for different immersion times, temperatures, and rotation speeds. Kinetic reactions and physical model for the chromate layer formation on zinc were proposed, and the electrochemical and electrohydrodynamic impedances were well simulated. © 2002 The Electrochemical Society. All rights reserved.

On the nature of the chromate conversion coating formed on intermetallic constituents of AA2024‐T3

AbstractThe 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.

The application of synchrotron-based spectroscopic techniques to the study of chromate conversion coatings

This work utilized synchrotron spectroscopic techniques such as extended X-ray absorption fine structure (EXAFS), X-ray absorption near edge structure (XANES) and synchrotron infrared microspectroscopy (SIRMS) to understand the chemistry, formation, structure and aging of protective chromate conversion coatings (CCC). We shall discuss issues related CCC formed on aluminum–copper alloys. SIRMS combined with argon ion sputtering demonstrated spatial and depth chemical variations of the CCC formed on AA2024-T3. XANES indicated that in the first stages of CCC formation the thickness depends on the Cu content of the substrate and that thinner CCC appears to contain less Cr 6+ for protection. Micro-XANES shows that even monolayer coverage of chromate can protect the alloy surface. EXAFS and XANES measurements were used to monitor the structural changes of the CCC with time. It was observed that a slight decrease in the Cr(VI)Cr(III) ratio occurred during the first 24 h following coating. The structure of the CCC is in close agreement with a Cr mixed oxide. EXAFS data appeared to indicate an increase in ordering in the CCC structure with time.

Studying the formation process of chromate conversion coatings on aluminium using continuous electrochemical noise resistance measurements

A unique electrochemical technique, namely continuous noise resistance calculation (CNRC), was used to obtain electrochemical kinetic information from the formation process of chromate conversion coatings (CCC) on aluminium electrodes. It was found that the noise resistance ( R n) of aluminium electrodes remained almost unchanged during electrodes' immersion in a chromate containing acidic solution where the CCC films were supposed to form rapidly. This result indicates that the formation of CCC was associated with continuous corrosion of the aluminium electrodes and that the CCC films formed on aluminium surface were not intact barrier films, but most likely porous layers. The CCCs became protective only after they were aged in the environment. Based on these findings, the formation and inhibition mechanisms of CCC have been discussed.

Observations on chromate conversion coatings from a sol–gel perspective

Chromate conversion coatings (CCCs) have been applied to aluminum substrates for many years to improve corrosion resistance and paint adhesion. In recent years the so-called sol–gel chemistry based coating systems have been examined as possible replacements for CCCs on aluminum. A mechanism is proposed for the formation of CCC on aluminum that is consistent with sol–gel chemistry principles. The conclusions illustrate important considerations for developing non-CCCs for aluminum.

Formation of Chromate Conversion Coatings on Al‐Cu‐Mg Intermetallic Compounds and Alloys

Chromium conversion coating (CCC) formation was monitored on copper‐containing aluminum alloys AA2024-T3 and a specially cast ingot using Raman spectroscopy of the band. The ingot's different phases were compositionally analyzed with energy‐dispersive spectroscopy (EDS) and Auger electron spectroscopy (AES) before and after treatments using commercial Alodine™ 1200S chromium solution. Profilometry and Raman intensity of the band were used to gauge CCC film thickness on AA-2024-T3 alloy. A linear relationship between CCC thickness and the integrated band was shown to exist on AA-2024 alloy. Raman intensity of the band was also integrated and spatially mapped using commercially available software. These results were verified with AES depth profiling using an ion sputter. CCC formation was found to be slower on the phases with higher copper content. Similarly, in 2024-T3, CCC formation was suppressed on S‐phase and “depressed” intermetallics of 2024 aluminum alloy. adsorption on the ingot phases, Cu, Al, Mg, and AA2024-T3 were characterized by Raman in the range. © 2000 The Electrochemical Society. All rights reserved.

Structure and Function of Ferricyanide in the Formation of Chromate Conversion Coatings on Aluminum Aircraft Alloy

Raman and infrared spectroscopy were used to determine the structure of and its reaction products in chromate conversion coatings (CCCs) on AA 2024-T3 aluminum aircraft alloy. In addition, Raman spectroscopy was used to monitor CCC growth rates and their dependence on coating bath composition. The IR and Raman spectra of the air‐dried CCC corresponded to those of Berlin green, a polymer, and physisorbed on . No other cyano‐containing products were observed. When was excluded from the coating bath, CCC formation rate greatly decreased. In addition, it was observed that could rapidly oxidize AA 2024-T3, and rapidly reduced Cr(VI) in bath conditions. These results indicate a redox mediation action for , which greatly increases the reduction of Cr(VI) to Cr(III) by the alloy. This process is normally quite slow, and redox mediation by is critical to CCC formation. could substitute for to produce a chromate film with properties very similar to a conventional CCC. The results establish redox mediation as the mechanism of acceleration of CCC formation, but provide no evidence for any additional role of in corrosion protection. © 1999 The Electrochemical Society. All rights reserved.

Formation of black chromate conversion coatings on pure and zinc alloy electrolytic deposits: role of the main constituents

Preparation of electrolytic zinc coatings covered by a chromate conversion coating is of great interest for automobile companies. Apart from the protective properties required, the most important factor in this field is the aethetic aspect. Principally, black conversion films are now used. The purpose of this study is to understand and explain the mechanism of formation of chromate conversion coatings and how the black colour is obtained. Electrochemical measures showed parameters that influence the kinetic formation of the chromate layers. The use of surface analysis methods (SEM, ESCA, GDOS) showed the role of anions in a chromate electrolyte and the effect of the transition elements in zinc electrodeposits. The chromate film is made of two layers superimposed: a very thin black oxide layer at the zinc/chromate coating interface and a gel type layer at the surface. The oxide layer containing the transition elements is the essential factor for the formation of the black colour.