Elsevier

Corrosion Science

Volume 47, Issue 12, December 2005, Pages 3187-3201
Corrosion Science

Impedance studies of the passive film on aluminium

https://doi.org/10.1016/j.corsci.2005.05.058Get rights and content

Abstract

Impedance spectroscopy has been used to study the characteristics of aluminium in 0.10 M NaCl solution at potentials below the pitting potential (−1.50VSCE to −0.70VSCE). Multiple capacitive (or pseudocapacitive) impedance features have been identified and analyzed over this range of potentials. Higher frequency resistances have been compared with surface analytical data, establishing a correlation between higher frequency impedances and the concentration of chloride in the passive film. The relationship shows that both charging resistance and chloride concentration reach a maximum immediately prior to passive film breakdown. At the onset of metastable pitting, both the impedance and chloride concentration decrease from the measured maxima. In contrast to X-ray photoelectron spectroscopy studies, the impedance results do not reveal oxide thinning prior to the onset of metastable pitting. Modelling from impedance data indicates that the thickness of the oxide layer sampled by impedance is much thinner than the oxide measured with surface analytical techniques, suggesting that the impedance method senses only the space charge layer in the oxide and not the total film thickness.

Introduction

An understanding of the breakdown of the passive film on Al has long been sought [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. Prior work at the US Naval Research Laboratory involved polarizing pure aluminium in deaerated 0.10 M NaCl solution and examining the oxide film formed at various potentials below the pitting potential using X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge structure (XANES) techniques [11], [12]. Comparison of metallic and ionic aluminium (+3) XPS intensities showed the thickness of the oxide layer was ∼5 nm between −1.030 V to −0.80 V, and ∼4 nm between −0.75 V to −0.70 V [11]. The XPS data also revealed the presence of two chloride species in the oxide layer. The total amount of chloride in the oxide film, determined from fluorescence XANES data, reaches a maximum at −0.80 V [12]. This information leads to further questions regarding the overall structure of the layer, and the contribution of the chloride to electrochemical changes in the passive film prior to pitting. To investigate these questions an electrochemical impedance study has been performed on this well-analyzed passive film system.

Several prior studies of the impedance behaviour of aluminium in chloride solutions have been published [13], [14], [15], [16]. In 0.5 M NaCl at −0.90 V (passive region), Bessone et al. [13] obtained two capacitive loops linked by an inductive response. These results were modelled as a single barrier layer on aluminium. With onset of pitting (−0.74 V), only one capacitive loop was observed, whereas active pitting (−0.72 V) produced a closed low-impedance loop, characteristic of corroding systems. Similar results were obtained in 0.5 M NaCl by other investigators [14], although impedance results were reported only in the passive region. These authors observed a lower-frequency capacitive loop which was attributed to film dissolution, in which a soluble aluminium salt was formed. In 0.01 M NaCl solutions, Lee [15] also reported two capacitive loops, with no intervening inductive response, in the passive region. In contrast to Bessone et al.’s work [13] the two-loop behaviour persisted into the pitting region, undoubtedly reflecting the much lower chloride concentrations in the Lee study [15]. The high-frequency loop was taken as evidence of chloride incorporation into the oxide film; however, this loop does not appear to be present in studies at higher chloride concentrations [13], [14]. A study by Brett et al. [16] in solutions of 0.4 M K2SO4 and 0.10 M KCl clearly shows the effect of chloride in promoting pitting corrosion; however, the impedance data were not shown for frequencies below 0.01 Hz. The presence of sulphate also raises some concerns regarding its participation in the film structure. Our previous spectroscopic studies on aluminium surfaces were carried out in 0.10 M NaCl solutions, prompting our present investigations of the impedance behaviour of aluminium in the same medium.

Despite this wealth of impedance information on aluminium in chloride solutions, the experimental conditions employed by other investigators cited above vary considerably and are not those used in our previous surface analytical work. By using the same solution conditions (0.10 M NaCl) in both our previous surface analytical work and our present work, a correlation has been obtained between the impedance of the passive film on aluminium and the concentration of chloride in the passive film.

Section snippets

Experimental

Electrochemical impedance studies were carried out on 99.99% pure aluminium rods (Alfa-Aesar; 0.95 cm nominal diameter), in deaerated (with Ar) 0.10 M NaCl solutions. All specimen surfaces were prepared by dry polishing the aluminium to 600 grit, and degreasing in acetone, ethanol, and methanol followed by rinsing with de-ionized water and drying with nitrogen gas. The working area of each specimen was bounded by 3 M 1490 anti-crevice electrochemistry tape (3 M, St. Paul, MN), shielding both the

Results and discussion

The potentiodynamic curve shown in Fig. 1 demonstrates the overall polarization behaviour of aluminium in 0.10 M NaCl as a function of applied potential. The anodic currents begin at −1.27 V, consistent with the open-circuit potentials observed for aluminium samples in this work. The manifestation of passivity persists as the potential is increased to a value exceeding −0.7 V. The relative insensitivity of current to increasing potential (characteristic of the passive region) changes abruptly as

Conclusions

From a comparison of impedance and X-ray spectroscopic measurements of potentiostatically polarized aluminium in NaCl solutions, it has been shown that a relationship between chloride concentration and impedance characteristics of the passive film exists. Increasing concentrations of chloride in the passive film are linked to an increasing space charge layer resistance. This suggests that the presence of the chloride hinders the motion of oxygen vacancies, the primary charge carriers in this

Acknowledgements

The authors gratefully acknowledge support from the Office of Naval Research and the NRL-USNA Cooperative Program for Scientific Interchange.

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