Interaction of Chromate with Aluminum in NaCl Solutions

The influence of solution pH on chromium(VI) deposition from 0.1 M NaCl + 0.1 mM Na 2 CrO 4 solution onto Al 1100 alloy was studied under open circuit potential (OCP) condition. To monitor the deposition of chromium(VI) on the aluminum surface we applied an experimental methodology based on the energy selective measurement of the characteristic K α , β X-radiation emitted by the 51 Cr -labeled chromate species. Data obtained from this in situ radiochemical analysis were complemented by X-ray photoelectron spectroscopy (XPS) and electrochemistry. The results of these analyses indicated that both the kinetics and the extent of deposition are strongly pH dependent. Specifically, after 2 h of solution exposure, chromate deposition exhibited a maximum surface coverage at pH 2. Chromate, even in 0.1 mM concentration, was found to effectively reduce the chloride content of the oxide film. In addition, the presence of chromate in the solution phase was found to alter the hydration and thickness of the passive film. The XPS analyses revealed that chromate not only adsorbed but also electrochemically interacted with aluminum, creating a Cr(III,VI)-rich layer on the surface. The Cr(III,VI) species is bound to the passive film in a largely irreversible manner.