Kinetic and mechanistic studies of rare earth-rich protective film formation using in situ ellipsometry

Bohm, S., Greef, R., McMurray, H. N., Powell, S. M. and Worsley, D. A. (2000) Kinetic and mechanistic studies of rare earth-rich protective film formation using in situ ellipsometry. Journal of the Electrochemical Society, 147 (9), 3286-3293. (doi:10.1149/1.1393897).

Abstract

The deposition of rare earth metal (REM) rich (hydr)oxide films on pure iron and zinc surfaces has been studied using in situ ellipsometry. Iron and zinc surfaces freely corroding in near-neutral 0.86 mol dm(-3) aerated aqueous sodium chloride were found to be covered with 6-15 nm thick native (hydr)oxide films. The kinetics of REM-rich film growth for one redox active [Ce(III)] and two redox inactive [Y(III) and La(IU)] cations were measured. Addition of 2.5 x 10(-3) mol dm-3 REM chloride salts to the electrolyte resulted in rapid growth of REM-rich (hydr)oxide films on both metal substrates. Similar film growth rates were observed for each REM cation. REM-rich film deposition proceeds via the precipitation of REM hydroxides [M(OH)(3)] produced by cation hydrolysis proximal to the substrate-solution interface driven by increased interfacial pH resulting from cathodic oxygen reduction. When iron was cathodically polarized to near the free corrosion potential of zinc (-1.05 V vs. SCE) the pre-existing iron (hydr)oxide film was removed within 400 s and no deposition of REM-(hydr)oxide films was observed. On the basis of these findings it is proposed that REM-(hydr)oxide deposits only nucleate efficiently on the native (hydr)oxide covered metal surfaces.

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