Microstructure of Trivalent Chromium Conversion Coating on Electrogalvanized Steel Plate

Wen, Niann-Tsyr; Chen, F. J.; Ger, Ming-Der; Pan, Yung-Ning; Lin, Chao-Sung

doi:10.1149/1.2932053

Cited by 20 publications

(20 citation statements)

References 14 publications

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“…In the negative ion depth profiles (Figures 2a and 2b) 16 O − , which saturates the detector. The intensities of each ion are plotted as a function of the sputtering time.…”

Section: Resultsmentioning

confidence: 94%

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Role of Post-Treatment in Improved Corrosion Behavior of Trivalent Chromium Protection (TCP) Coating Deposited on Aluminum Alloy 2024-T3

Ely

Światowska

Seyeux

et al. 2017

J. Electrochem. Soc.

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The effects of post-treatment applied on Trivalent Chromium Protection (TCP) coatings deposited on aluminum alloy 2024-T3 was studied by electrochemical and surface analytical techniques: X-ray Photoelectron Spectroscopy (XPS) and Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). The aim of the post-treatment was to improve the corrosion resistance of the TCP coating. To understand the influence of the post-treatment, using a bath containing H 2 O 2 and a lanthanum salt as an inhibitor, our approach was to study the effect of these two components separately. It was found that the improved corrosion protection provided by the post-treated TCP was related to a synergetic effect of the two components. XPS and ToF-SIMS analyses showed that (i) the thickness of the TCP coating is not modified by the post-treatment, (ii) lanthanum is present on the surface and in the bulk of the TCP and (iii) is found in very small amount only on the extreme surface. The total concentration of Cr(VI) (oxide/hydroxide) was estimated to be below 0.1 wt% in the post-treated TCP layer. It was concluded that lanthanum plays a significant role in improving the coating density and homogeneity and has a beneficial effect on the TCP coating cracking as observed by Scanning Electron Aluminum alloys are used for aerospace applications for their low weight and good mechanical properties.1-3 To improve the corrosion properties of aluminum alloys and to provide good paint adhesion, the first layer of coating applied on the aluminum alloy surface used to be a chromate conversion coating (CCC).4-6 However, hexavalent chromium used in the CCC coatings is highly toxic, 4,7 its use is very much restricted in Europe 8 and the United States 9 and will be forbidden by 2024 by the European Community with respect to the Registration, Evaluation, Authorization and Restriction of Chemicals (REACh).8 Therefore, new conversion coatings have been developed, and one of the most promising solutions is the Trivalent Chromium Protection coating called also as the Trivalent Chrome Process (TCP).10-14 TCP conversion coatings are formed by immersion in a solution containing Zr 4+ , Cr 3+ and F − ions, at a pH between 3.8 and 4.0. As it has already been widely discussed, the coating formation mechanism occurs by multiple chemical steps. 10,11,13,[15][16][17] The initial step involves dissolution of the aluminum oxide layer by fluorides. The removal of the oxide layer exposes the metal surface where cathodic reactions take place. Oxygen reduction reaction, and possibly hydrogen discharge, which counterbalance aluminum oxidation, consume protons and causes a pH increase at the interface between the metallic substrate and the solution, as measured by Li et al. 15 The high pH favors the precipitation of ions present in the solution, forming a hydrated layer of zirconium and chromium oxide. The thickness of the TCP layer is usually of the order of 100 nm, depending on the nature of the aluminum alloy, the concentration of the solution and the immersion time.11,12,18 ...

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“…In the negative ion depth profiles (Figures 2a and 2b) 16 O − , which saturates the detector. The intensities of each ion are plotted as a function of the sputtering time.…”

Section: Resultsmentioning

confidence: 94%

“…As it has already been widely discussed, the coating formation mechanism occurs by multiple chemical steps. 10,11,13,[15][16][17] The initial step involves dissolution of the aluminum oxide layer by fluorides. The removal of the oxide layer exposes the metal surface where cathodic reactions take place.…”

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confidence: 99%

Role of Post-Treatment in Improved Corrosion Behavior of Trivalent Chromium Protection (TCP) Coating Deposited on Aluminum Alloy 2024-T3

Ely

Światowska

Seyeux

et al. 2017

J. Electrochem. Soc.

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“…8,21 As a consequence, the concentration of H 2 O 2 after 5400 s in the stirred solution containing 0.01 M Cr(NO 3 ) 3 4− species during hydrolysis of ZrF 6 2− in the H 2 O 2 -containing medium, the F − ions promoted the formation of the hydroperoxyl ions for the peroxide ionization for the dimer compound formation. 32 The reaction Equations 3-4 and 5-7 are the H 2 O 2 -self-ionization and the fluorine-promoted dimer formation process respectively as follows, (the dimer formation process) [7] In addition, the chemical exchange between ZrF 6 2− and F − possibly generated the fluoride intermediate ZrF 7 3− , which would reduce the concentration of F − ions in the reaction solution. 35,36 Based on the present study, the previous hypothesis of the transient formation of H 2 O 2 during a trivalent chromium conversion treatment has been strongly supported by the spectrophotometric measurements.…”

Section: Sputtering-deposited Almentioning

confidence: 99%

Chromate Formed in a Trivalent Chromium Conversion Coating on Aluminum

Gao

Liu

et al. 2017

J. Electrochem. Soc.

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This present study employs aluminum substrates to investigate the formation of Cr(VI) species during a trivalent chromium conversion coating process. The study had a particular focus on understanding the influences of copper in the substrate and O 2 , ZrF 6 2− and F − in the bath on the formation of Cr(VI) species, which were detected by Raman spectroscopy. Comparison of electropolished aluminum and sputtering-deposited aluminum substrates revealed greatly increased rates of coating growth associated with an enrichment of copper impurity in the electropolished substrate that was revealed by transmission electron microscopy. With respect to chromium chemistry in a developed coating, the presence of dissolved oxygen and long conversion treatment times promoted the formation of Cr(VI) species that are generated by oxidation of Cr(III) species. The Cr(III) species are oxidized by H 2 O 2 , which was produced by oxygen reduction reaction. The generation of H 2 O 2 was demonstrated by analysis of the treatment bath using UV spectrophotometry. Conversion coating processes are employed as surface pretreatments of aluminum and aluminum alloys to improve the adhesion with organic primers and increase corrosion protection. Chromate conversion coating (CCC) has been widely used as a conventional and effective process in the aerospace and automotive industries.1-3 However, the toxicity of chromates to human beings and the detrimental impact of chromates on the environment are giving rise to greater regulation of their use. 4,5 In contrast, trivalent chromium species are relatively eco-friendly and, hence, trivalent chromium conversion (TCC) coating processes are being investigated as promising alternatives to CCC processes. 6The TCC coating bath generally contains hexafluorozirconate, sodium fluoride and trivalent chromium sulfate, which results in coatings formed on aluminum that contain Zr-/Cr-rich oxides, hydroxides and fluorides.6-8 Interestingly, a freshly-formed TCC coating after a conversion treatment for 1200 s in a naturally-aerated bath displayed chromate presence by a Raman shift at 866 cm −1 , but the peak intensity was negligible in a coating formed in a deoxygenated bath. 8 Notably, the oxidation of Cr 3+ to Cr 6+ in the aqueous electrolyte is not due solely to the presence of dissolved oxygen. This conclusion is supported by experiments in which no significant chromate species were found in the solution bubbled with only air for more than 300 h. 9 In contrast, the Cr 3+ /Cr 6+ reaction has been reported to occur in atmospheric oxygen at high temperature during a bush fire 10 and also with the presence of MnO 2 oxidant in the natural sea water. 11,12 In the case of TCC coatings, chromate formation has been attributed to the transient formation of hydrogen peroxide generated by oxygen reduction. The influence of H 2 O 2 has been explored by Li et al.,13 who immersed TCC coated AA2024 alloy in 0.5 M Na 2 SO 4 solutions with addition of 0.01, 0.1 and 1 v/v % H 2 O 2 for 1 h. A small level (0.01% v/v) of H 2 O 2 ca...

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“…25 The driving force for Cr(III) deposition comes from the rise in pH near the metal surface [26][27][28][29] caused by redox reactions on the heterogeneous, heat-treated alloy. 30 The oxidation reaction occurs on Al-rich matrix (similar to our alloy substrates) accompanied by proton reduction on Cu-rich inclusions, which are absent in our films.…”

Section: Film Structure Of Tcp Immersion Filmsmentioning

confidence: 99%

Morphology and Mechanism of Benign Inhibitors

Schaefer¹,

Wang²,

Xin³

2012

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Microstructure of Trivalent Chromium Conversion Coating on Electrogalvanized Steel Plate

Cited by 20 publications

References 14 publications

Role of Post-Treatment in Improved Corrosion Behavior of Trivalent Chromium Protection (TCP) Coating Deposited on Aluminum Alloy 2024-T3

Role of Post-Treatment in Improved Corrosion Behavior of Trivalent Chromium Protection (TCP) Coating Deposited on Aluminum Alloy 2024-T3

Chromate Formed in a Trivalent Chromium Conversion Coating on Aluminum

Morphology and Mechanism of Benign Inhibitors

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