Transient Formation of Chromate in Trivalent Chromium Process (TCP) Coatings on AA2024 as Probed by Raman Spectroscopy

Li, Liangliang; Kim, Doo Young; Swain, Greg M.

doi:10.1149/2.019208jes

Cited by 64 publications

(127 citation statements)

References 37 publications

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“…7d) Notably, a Raman peak at 780-904 cm −1 , characteristic of Cr(VI), was not resolved in any Raman spectra in Figure 7. 12 The region of the Cr(VI) peak is highlighted in Figure 8 for a Raman spectrum recorded from the TCC coated aluminum matrix after conversion treatment for The absence of a detectable amount of Cr(VI) species in the present coatings is proposed to be related to reduced generation of hydrogen peroxide during conversion treatment of aluminum in the modified solutions, which is supported by the following measurements.…”

Section: Resultssupporting

confidence: 65%

“…Cr(VI) species were reported to be present in the freshly-formed TCC coatings, but were undetectable in the case of a fresh coating on AA2024 alloy. 4,12 A generally accepted hypothesis for the formation of Cr(VI) species is that hydrogen peroxide created by the oxygen reduction reaction can oxidize Cr(III) species in the coating. 12,13 Furthermore, the presence of hydrogen peroxide during the conversion treatment of aluminum * Electrochemical Society Fellow.…”

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Influence of Copper on Trivalent Chromium Conversion Coating Formation on Aluminum

Miao

Wang

et al. 2017

J. Electrochem. Soc.

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In the present work, modified copper-containing trivalent chromium conversion (TCC) coating processes for aluminum were investigated. The copper addition to the TCC bath was made for the purposes of reducing the generation of hydrogen peroxide and chromium (VI) species during the coating growth. The morphologies and compositions of the coatings were examined using highresolution electron microscopy, energy-dispersive X-ray spectroscopy and Raman spectroscopy. UV photometric measurements were employed to determine the amount of hydrogen peroxide in the TCC solution. The resultant coatings contained zirconium oxides, chromium hydroxide and fluorides and sulfate constituents, as well as copper oxides and copper-rich deposits that were preferred cathodic sites for oxygen reduction. Of most significance, no chromium (VI) species were detected in the coatings by Raman spectra. It is suggested that this results from reduced generation of hydrogen peroxide, as disclosed by photometric measurements, at the cathodic copper-rich particles, due to favoring of the four electron oxygen reduction reaction. Trivalent chromium conversion (TCC) coatings are promising ecofriendly alternatives to chromate conversion coatings due to the lower toxicity of trivalent chromium species compared with hexavalent chromium.1 The TCC coating solution can be regarded as a modified Zr-based conversion coating solution with addition of small amounts of trivalent chromium salts.2,3 Our previous work used scanning electron microscopy and atomic force microscopy to reveal cracking and spalling of the coating formed on superpure aluminum, especially after a prolonged conversion treatment. 4 This was associated with the fast kinetics of aluminum dissolution due to the attack by fluorine ions in the reaction solution and the stress in the coating. 5 In contrast, the TCC coating formed on AA2024 alloy suppressed the oxygen reduction reaction, due to the physical barrier created by the Zr-/Cr-rich coatings, 1,6,7 and the coating displayed improved adhesion, although the coating was significantly thinner than that formed on superpure aluminum.With respect to the effect of copper alloying, George et al. 8 investigated binary alloys containing 1, 5, and 25 at.% Cu prepared by magnetron sputtering and revealed a decrease in the coating growth rate of zirconium-based coatings with increase of the Cu/Al ratio. This was suggested to be due to the copper species present in the coating impeding the cation transport to the coating base. Furthermore, a layer of corrosion products formed at the coating base. Cerezo et al.9-11 modified the Zr-based conversion coating solution by adding a small amount of copper salts (30-50 ppm). The copper components, which had a high deposition tendency, formed copper and/or copper oxide agglomerates on the substrate, which created local alkalinity that supported the coating formation. As a consequence, the coating thickness on the multi-metal substrate (AA6014, cold rolled steel and hot dip galvanized steel) was increased.One conc...

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Section: Resultssupporting

confidence: 65%

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confidence: 99%

Influence of Copper on Trivalent Chromium Conversion Coating Formation on Aluminum

Miao

Wang

et al. 2017

J. Electrochem. Soc.

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“…The data were interpreted using RUMP software. The carbon and oxygen and contents of the specimens were assessed by NRA using the 12 • to the direction of the incident beam. The oxygen contents were quantified to an accuracy of ∼3% using a reference specimen of anodized tantalum.…”

Section: Specimen Characterization-mentioning

confidence: 99%

“…12 It was proposed that hydrogen peroxide, which is formed by reduction of oxygen at sites of copper-rich particles in the alloy, oxidizes the Cr (III) in the coatings to form Cr(VI) species.…”

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confidence: 99%

Formation of a Trivalent Chromium Conversion Coating on AA2024-T351 Alloy

Hashimoto

Walton

et al. 2015

J. Electrochem. Soc.

105

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The formation of a trivalent chromium conversion coating on AA2024-T351 aluminum alloy has been studied using electron microscopy, scanning Kelvin probe force microscopy, ion beam analysis and X-ray photoelectron spectroscopy (XPS). The coating contained oxide, hydroxide, fluoride and sulfate species, and consisted of two main layers: a zirconium-and chromium-rich outer layer and a thinner, aluminum-rich, inner layer. XPS indicated that zirconium and chromium are mainly present as ZrO 2 , Cr(OH) 3 , Cr 2 (SO 4 ) 3 and CrF 3 . In addition, a thin aluminum-rich layer, probably composed of hydrated alumina, occurred transiently at the coating surface. The coating above cathodic second phase particles was usually thicker than that above the matrix due to the locally increased alkalinity. In the early stages of treatment, the thickest coating formed above the S-phase particles. Localized corrosion and copper enrichment of the matrix occurred at the coating base. The localized corrosion is possibly related to the observed accumulation of fluoride ions at the inner layer and the enrichment of copper in the alloy. The thickness of the coating above the alloy matrix was significantly less than that of a coating formed on high purity aluminum. AA2024-T3 aluminum alloy is widely used in the aerospace industry due to its high strength to weight ratio and damage tolerance. However, the presence of copper as a primary alloying element leads to an increased susceptibility to localized corrosion, especially pitting corrosion.2,3 Hence, the alloy requires protective treatments to provide the required corrosion resistance for many applications. Such treatments often involve the use of chromate conversion coatings. However, the toxicity and high disposal costs of Cr(VI) wastes necessitate the development of eco-friendly alternatives. 4,5 One promising candidate treatment uses a trivalent chromium bath, generally consisting of zirconium hexafluoride and trivalent chromium salts. 6,7 The coating formation involves the dissolution of the native oxide on the aluminum surface due to the acidic, fluoride-containing solution, 8 and the subsequent pH-driven deposition of zirconium and chromium species. The increase in interfacial pH is promoted by the cathodic reactions, i.e. hydrogen evolution and/or oxygen reduction. 9Previous work has indicated that the resultant coating consists of two layers. 7,9,10 In one study, it was proposed that a first, dense layer is formed by hydrolysis of Cr(OH) x (3-x)+ and Zr(OH) x (4-x)+ ions that retards oxidation of the alloy and reduction of protons; a second layer then forms by preferential deposition of zirconium hydroxide at transient cathodic sites.10 Other studies revealed a coating composed of an outer, hydrated zirconium-rich layer that also contains chromium species and an inner, aluminum-rich layer. 7,9 Several investigations have been carried out to determine whether Cr(VI) species are formed in trivalent chromium conversion coatings. No Cr(VI) was detected by UV-visible spectroscopy in as-de...

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“…In fact, it was argued that the fluorides species present in the bath (usually added to accelerate film growth and native oxide dissolution) were promoting hydrogen peroxide formation, subsequently oxidizing Cr 3+ species to Cr 6+ . 35,39,[44][45][46] Regardless of the recent findings on TCC chemistry, trivalent chromium-based formulations remain the most common Cr 6+ replacement to date for chemical conversion coatings on aluminum or zinc alloys and are currently commercialized by several coatings suppliers (such as Alodine T 5900 RTU from Henkel, 47 or Socosurf TCS 48 supplied by Socomore). This is often justified by the relative low toxicity of TCC formulations in regards to Cr 6+ , as the proportion of Cr 6+ present in the coating was purported to not exceed the 0.1 wt% set by REACH regulation, 49 although not yet met by all current industrial formulations.…”

Section: Introductionmentioning

confidence: 99%

Chromate replacement: what does the future hold?

Gharbi

Thomas

Smith³

et al. 2018

npj Mater Degrad

178

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The ubiquitous use of chromium and its derivatives as corrosion preventative compounds accelerated rapidly after the second industrial revolution, with such compounds now integral to modern society. However, the detrimental impact of chromium compounds on the environment and human health has prompted the need to revisit the majority of current industrial corrosion protection measures. This review retraces the origins of chromium replacement motivations, introducing the various legislative actions aimed at diminishing the use of chromium compounds, and critically reviews alternative corrosion preventative technologies developed in the recent decades to now. The review, herein, is intended for a broad audience in order to provide a concise update to an increasingly timely issue.

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Transient Formation of Chromate in Trivalent Chromium Process (TCP) Coatings on AA2024 as Probed by Raman Spectroscopy

Cited by 64 publications

References 37 publications

Influence of Copper on Trivalent Chromium Conversion Coating Formation on Aluminum

Influence of Copper on Trivalent Chromium Conversion Coating Formation on Aluminum

Formation of a Trivalent Chromium Conversion Coating on AA2024-T351 Alloy

Chromate replacement: what does the future hold?

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