Abstract
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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