Trivalent chromium conversion coating formation on aluminium

Qi, Jiantao; Hashimoto, Teruo; Walton, John; Zhou, Xiaorong; Skeldon, P.; Thompson, G.E.

doi:10.1016/j.surfcoat.2015.09.024

Cited by 100 publications

(179 citation statements)

References 38 publications

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“…Constant phase elements (CPEs) were used in the EC modelling to represent the capacitance property on the basis of the depressed semicircles of Nyquist plots (not shown here); CPEs have been used to model similar coating systems in Ref. 7,19. Post-coating treatment of immersion in DI water at 40…”

Section: Resultsmentioning

confidence: 99%

An Optimized Trivalent Chromium Conversion Coating Process for AA2024-T351 Alloy

Gao

et al. 2017

J. Electrochem. Soc.

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In this work, a trivalent chromium conversion coating applied on AA2024-T351 alloy has been optimized for corrosion protection in sodium chloride solutions. Scanning electron microscopy, energy dispersive X-ray spectroscopy and electrochemical measurements were employed to characterize the coating. An immersion post-treatment of the coated alloy in 40 • C deionized water for 120 s considerably enhanced the corrosion protection properties, compared with a post treatment at 20 • C, or the absence of a post-treatment. Electrochemical noise measurements, combined with real time imaging, and potentiodynamic polarization experiments indicated a conversion treatment for 300 or 600 s provides optimal corrosion protection. Relatively long conversion treatments decreased the corrosion protection due to an increase in coating defects and cracks, especially around the second-phase particles.

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Section: Resultsmentioning

confidence: 99%

An Optimized Trivalent Chromium Conversion Coating Process for AA2024-T351 Alloy

Gao

et al. 2017

J. Electrochem. Soc.

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“…Notably, our previous paper revealed a copper-enriched layer, a few nanometers thick, at the base of conversion coatings. 4 Its formation is associated with the nobility of copper relative to aluminum. The enrichment develops during pre-treatments of the substrate and is maintained during coating growth.…”

Section: Resultsmentioning

confidence: 99%

“…4 In contrast, some aluminum specimens were immersed in 0.01 M sodium fluoride solution containing either 0.05 g/L or 5 g/L copper sulfate. The specimens were prepared to assist understanding of the coating growth in the copper-containing TCC bath.…”

Section: Methodsmentioning

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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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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“…[34][35][36][37][38][39] The trivalent conversion coating (TCC) typically contains fluoride salts (NaF), trivalent chromium salts (Cr(OH) 3 or Cr 2 (SO 4 ) 3 ) and hexafluorozirconate (ZrF 6 2− ). [40][41][42][43] Surface film formation takes place in an acidic environment where the co-precipitation of hydrated zirconia and trivalent chromium hydroxides occurs, hence providing high barrier properties.…”

Section: Introductionmentioning

confidence: 99%

“…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

177

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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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Trivalent chromium conversion coating formation on aluminium

Cited by 100 publications

References 38 publications

An Optimized Trivalent Chromium Conversion Coating Process for AA2024-T351 Alloy

An Optimized Trivalent Chromium Conversion Coating Process for AA2024-T351 Alloy

Influence of Copper on Trivalent Chromium Conversion Coating Formation on Aluminum

Chromate replacement: what does the future hold?

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