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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...

A comparison has been made of the influence of two alloy pre-treatments and two coating post-treatments on the formation, composition and corrosion protection of a trivalent chromium conversion coating on AA 2024-T351 alloy. The investigation employed analytical electron microscopies, ion beam analysis, X-ray photoelectron spectroscopy (XPS) and electrochemical tests. The pre-treatments used alkaline etching followed by de-oxidizing in either nitric acid or a commercial de-oxidizer. The conversion coatings were formed in SurTec 650 chromitAL and revealed two-layers, comprising an inner aluminium-rich layer and an outer chromium-and zirconium-rich layer, with a Cr:Zr atomic ratio in the range ~0.73 -0.93. XPS indicated a chromium-enriched near-surface region that contained ~2 at.% of Cr (VI) species. Potentiodynamic polarization and electrochemical impedance spectroscopy revealed an improved corrosion protection for a pre-treatment that left copperrich sponges, probably de-alloyed S phase, and fewer residues of other intermetallic particles on the alloy surface. Post-coating immersion treatments in deionized water at 20 °C or 40 °C resulted in a significant difference in the zirconium species in the region adjacent to the coating surface that is accessible to XPS, with oxide and hydroxide dominating at the respective temperatures.

In the present study, the evolution of hexavalent chromium evolution in a trivalent chromium conversion coating has been investigated by Raman spectroscopy and X-ray photoelectron spectroscopy. In terms of Raman spectroscopy, the presence of CrF3 in the coating (538–540 cm−1) generates an overlapped shift with the trivalent chromium hydroxide and oxide (536–550 cm−1). For the hexavalent chromium (845–866 cm−1) reactivity, post-treatment in sodium sulfite solution was revealed to be an effective process to eliminate Cr(VI) species on the coating surface. However, the diffusion kinetics of sulfite ions into the coating inner layer were found to become a limiting factor. The Raman peak intensity of hexavalent chromium in the air aged coating for 24 h revealed a significant decrease compared to that in the freshly-formed coating after the same conversion treatment process. This was attributed to Cr (VI) degradation at the base of coating cracks as evidenced by scanning electron and atomic force micrographs. Interestingly, 3.5 wt% NaCl droplets induced the trivalent chromium oxidation in the air-aged coating specimen, as evidenced by Raman peak at 845 cm−1. For this reason, the oxygen in the corrosive electrolyte caused the oxidation of trivalent chromium species; the electrolyte might play a role in coating degradation, which, in turn, permits an increase of oxygen ingress. The hexavalent chromium in the freshly-formed, air-aged and corroded trivalent chromium conversion coating on aluminum is an environmental concern for its application.

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