Structures and characteristics of Cr(III)-based conversion coatings on electrogalvanized steels

Wen, Niann-Tsyr; Lin, Chao-Sung; Bai, Ching-Yuan; Ger, Ming-Der

doi:10.1016/j.surfcoat.2008.09.006

Cited by 55 publications

(23 citation statements)

References 20 publications

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“…34 Notably, the coatings displayed the popular presence of defects and cracks with the evident spallings after a prolonged conversion treatment. 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.…”

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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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 feature is often considered as the response of an inhomogeneous film composed of a compact inner layer and a less compact (porous) outer layer [22][23][24][25]. However, other authors have proposed that the unfinished capacitive arc observed in coatings on electroplated steels is mainly due to the presence of a corrosion-resistant coating and a diffusion effect [26]. In this way, the modelling of the impedance data using an equivalent circuit is necessary to explain the differences in the structure and corrosion resistance of the Zn electroplated steel passivated in the baths under study.…”

Section: Eis Measurementsmentioning

confidence: 99%

“…The equivalent electric circuit (EEC) represented in Figure 7 was used to describe the impedance behaviour of a porous coating [3,26,29]. In the present work, this EEC has been used to model the results of the Zn electroplated steel samples passivated in the different baths.…”

Section: Eis Measurementsmentioning

confidence: 99%

Improvement of the electrochemical behaviour of Zn-electroplated steel using regenerated Cr (III) passivation baths

Garcı́a-Antón

Fernández‐Domene

Sánchez-Tovar

et al. 2014

Chemical Engineering Science

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ElsevierGarcía Antón, J.; Fernández Domene, RM.; Sánchez Tovar, R.; Escrivá Cerdán, C.; Leiva García, R.; García, V.; Urtiaga, A. (2014). Improvement of the electrochemical behaviour of Zn-electroplated steel using regenerated Cr(III) passivation baths. AbstractConversion coatings based on trivalent chromium are more sensitive to the presence of zinc and iron impurities than the chromate formulations. This fact contributes to a decrease in the quality of passivation and to the generation of a significant amount of hazardous liquid waste. Recently, a new eco-innovative process based on Emulsion Pertraction Technology (EPT) is being implemented at industrial scale for selectively removing Zn and Fe from spent passivation baths in order to enhance the lifetime of the Cr (III) baths. In this study, the effect of Zn and Fe removal on the electrochemical behaviour of Zn-electroplated steel samples was evaluated by means of polarisation curves and electrochemical impedance spectroscopy measurements at open circuit potential conditions in 3.5 g/L NaCl solutions. The main objective was to assess the benefits brought by EPT using electrochemical methods. Cr (III) passivation baths regenerated using the EPT process have been compared to the bath used in a local industry as well as to fresh and spent baths. According to the results, the samples passivated in the EPT regenerated bath showed a significant improvement in their electrochemical behaviour compared to the samples passivated in the spent baths. This study concluded the suitability of EPT for regenerating Cr(III) passivation baths.

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“…Because of its excellent corrosion resistance, chromate conversion coating is widely used in the surface treatment of aluminum alloys, Mg, Zn and steel [1][2][3]. However, due to the toxicity of hexavalent chromium in the conversion coating, RoHS directive of European Union (EU) has have imposed severe restrictions on the use of chromate.…”

Section: Introductionmentioning

confidence: 99%

Preparation of Trivalent Chromium Coating on 6063 Aluminum Alloy

Huang¹,

Luo²

2017

dtmse

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Abstract. The trivalent chromium coating was deposited on the surface of 6063 aluminum alloy with Cr 2 (SO 4 ) 3 , H 3 PO 4 and K 2 ZrF 6 . The performances of trivalent chromium conversion coating were analyzed by CuSO4 dropping corrosion test, electrochemical polarization curve, scanning electron microscope (SEM) and energy dispersive X-ray analysis (EDAX). The conversion coating shows gray color and consists of Cr, Zr, K, Al, P, O. The analytical results indicate that the aluminum alloy coated with trivalent chromium exhibits better corrosion resistance than the pure aluminum alloy.

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Structures and characteristics of Cr(III)-based conversion coatings on electrogalvanized steels

Cited by 55 publications

References 20 publications

Chromate Formed in a Trivalent Chromium Conversion Coating on Aluminum

Chromate Formed in a Trivalent Chromium Conversion Coating on Aluminum

Improvement of the electrochemical behaviour of Zn-electroplated steel using regenerated Cr (III) passivation baths

Preparation of Trivalent Chromium Coating on 6063 Aluminum Alloy

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