(Invited) Study of Barrier Oxide Growth on Impure Aluminum in Aqueous Solution By Chronoamperometry and Electrochemical Impedance Spectroscopy
Authors: Giskeødegård Nils-Håvard , Nisancioglu Kemal 
Published: February 2020

The growth of the barrier oxide layer on aluminum during low-current anodic polarization in aqueous solutions occurs in accordance with the high-field Cabrera-Mott growth law,1
as recently confirmed for pure aluminum by use of simultaneous ellipsometry and chronoamperometry to measure the transient film growth following a potential step.2 Electrochemical impedance spectroscopy (EIS) was used to determine the steady-state barrier layer thickness between each potential step as in situ calibration for the measurement of the film growth rate by the other two methods. Complementarity of the three techniques was demonstrated for studying the kinetics of growth and dissolution of thin passivating oxides in aqueous solution. The purpose of the present study is to investigate whether the same approach can be used for studying the modification of the interface by contamination by the common trace element Pb as a result of heat treatment. Pb is known to activate the surface by destabilizing the passivating barrier layer.3 The microstructure and composition of the contaminated interface has been characterized by ex-situ STEM and various surface analytical techniques for samples heat treated at high (600°C) temperatures, resulting in detectable Pb segregation in the form of both nanosize (up to 10 nm) Pb particles and a few nm thick Pb-rich film along the metal-oxide interface. Pb segregation by heat treatment at lower temperatures was not possible by these techniques. Supplementary data were necessary for samples heat treated at lower temperatures, especially for modification of the electrochemical properties of the interface.
The samples investigated were model AlPb alloys prepared from pure components in the laboratory and cold rolled into 2.2 mm thick plates, followed by polishing metallographically through 1 µm surface finish. They were subsequently heat treated in an air-circulation furnace in the temperature range 300 - 500°C. The amorphous oxide is reported to thicken moderately in the range 10 - 20 nm as a result of 1 h heat treatment at this temperature range.4 Experiments were conducted in a conventional three electrode electrochemical cell in the manner described above. Ellipsometry could not be used because of slow measurement relative to the rate of barrier-layer growth and unexpected loss of specularity of the sample surface during heat treatment. The EIS data were evaluated by use of the equivalent-circuit model derived rigorously by Armstrong and Edmondson,5
which was shown to be applicable to the present electrochemical interface in the case of pure Al.2

Oxide growth was calculated from the current-time data by use of Faraday's law corrected for current leakage (passive current). The growth data were analyzed by use of the integrated form of the Cabrera-Mott law, according to Ghez,6 giving the well-known inverse logarithmic growth law. The changes in the barrier layer thickness appeared to fit the Ghez equation, but not as successfully as pure Al. The EIS data showed that the film thickened with increasing applied potential at the rate of 1.2 nm/V for all heat treatment temperatures except for 600°C, as expected for pure Al. The passive current density increased and the film resistance decreased at a given applied potential, with increasing annealing temperature, indicating reduced passivity. However, the film capacitance, measured by EIS at a given applied potential, increased with annealing temperature, giving an apparent decrease in the film thickness with increasing annealing temperature, in contrast to the expected trend to the opposite.4

The changes in the barrier film properties, measured by EIS, are attributed to the segregation of metallic Pb at the metal-film interface, in the film, and possibly at the film surface, contributing to the increased capacitance of the electrochemical interface. The model used for correlation of the EIS data has to be modified to incorporate the effect of segregated Pb in the form of particles and continuous film for more reliable measurement of the barrier film thickness in future work.
References

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R. D. Armstrong and K. Edmondson, Electrochim. Acta, 18,937 (1973).

R. Ghez, J. Phys. Chem., 58, 1838 (1973).