Study of the Chemical Conversion of Aluminum Alloys by Coupling CFDE and EQCM

Abstract

The ability of the channel flow double electrode (CFDE) technique and electrochemical quartz crystal microbalance (EQCM) for studying in situ chromate phosphate conversion coating on 5182 aluminum alloys was explored. It was first demonstrated that aqueous Cr(VI) can be analyzed quantitatively with the CFDE technique by reduction into Cr(III) on a graphite electrode. Samples used for EQCM were quartz plated by physical vapor deposition using a 5182 alloy target, allowing thin layers of aluminum alloys with a similar chemical composition to be obtained. EQCM was adapted in order to account for the hydrodynamic conditions in an industrial process, by placing the quartz in a flow cell. In order to be able to control, and then study, the early stage of the coating, the aluminum electrode used for the CFDE technique was covered with a mask (a thin layer of varnish) that was removed in situ by a laser pulse. With EQCM the triggering of the coating was performed by injecting the required amount of hydrofluoric acid. Since the time duration of the transient responses was different in both techniques, we focused our discussion on the steady state. Measurements of the electroacoustic impedance of the aluminum-alloy-coated quartz in the conversion solution showed that the chromate phosphate film can be considered as acoustically thin, allowing the use of the Sauerbrey’s equation to calculate mass variations. Combining in the steady state the rate of mass variation measured by EQCM, and the rate of Cr(VI) reduction by CFDE as functions of the bath composition, it was possible to calculate a chemical yield for the film growth. The results in agreement with the literature indicate strong synergetic effect between chemical species present in the solution. © 2004 The Electrochemical Society. All rights reserved.