Chromate Formed in a Trivalent Chromium Conversion Coating on Aluminum

Abstract

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...

Protection of Aluminum Alloy 3003 in Sodium Chloride and Simulated Acid Rain Solutions by Commercial Conversion Coatings Containing Zr and Cr

“…The nodules on the 25% ST650 coated surface were larger (Figure 9b), up to 180 nm. Cracks were observed, as has been previously reported [6,7,[29][30][31] and ascribed to the cracks that formed during coating growth and/or subsequent drying in vacuum. Figure 9c corresponds to the conversion coating with 50% ST650.…”

Review—Conversion Coatings Based on Zirconium and/or Titanium

“…121,122 Based on that information, Zrand Cr(III)-containing conversion coatings on AA2024-T3 were prepared to achieve self-repairing. 75 After 3 days exposure to ammonium sulfate and sodium chloride solution in an artificial scratch test cell, only Cr species were present at the surface, proving that these species were responsible for the self-repairing effect.…”

Chromate replacement: what does the future hold?

“…95 Buchheit et al 103 used Zn-Al layered double hydroxides (LDHs) as containers for inhibiting decavanadate ions (V 10 O 28 6-), within epoxy coatings upon AA2024-T3. It was observed that firstly, decavanadate ions Several studies demonstrated the presence of Cr(VI) within the conversion coating 45,46 Suspected to be carcinogenic 50 RE-based (Ce or La) AA2024-T3 57,58,63 Mainly works as cathodic inhibitors, precipitation of Ce-oxide on cathodic sites, induced by the local pH increase Vulnerability of RE market as mainly dominated by Chinese production 66,67 Galvanized steel 62 Only minor improvement in pitting potential and corrosion current was noted 56,65 Vanadate-based AA2024-T3 zinc carbon steel 69 Adsorption of inhibitor on cathodic sites (intermetallic particles in the case of AA2024-T3) and stabilization of passive film. Inhibition of oxygen reduction reaction [20][21][22]70 Do not meet environmental and health restrictions as Vanadium and its compounds were proven to be carcinogenic [72][73][74] Li-containing Only tested on AA2024-T3 Good barrier properties provided by both systems Organic: effective inhibitor is still needed to achieve good corrosion resistance Nanocomposites: higher density of inhibitor can be loaded into the system Nanocomposites: cost towards implementation could be too important 99,100 Phosphate-based Steel carbon steel Al alloys zinc were controllably released from the LDH containers and also that the aggressive chloride ions were also entrapped within such containers.…”

“…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.…”