Electrochimica Acta 51 (2006) 1443–1452 Electrochemical impedance spectroscopy in the presence of non-linear distortions and non-stationary behaviour Part II. Application to crystallographic pitting corrosion of aluminium E. Van Gheem a, ∗ , R. Pintelon b , A. Hubin a , J. Schoukens b , P. Verboven c , O. Blajiev a , J. Vereecken a a Department of Metallurgy, Electrochemistry and Materials Science, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium b Department of Fundamental Electricity and Instrumentation, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium c Department of Mechanical Engineering, Acoustics and Vibration Research Group, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium Received 23 June 2004; received in revised form 19 October 2004; accepted 25 February 2005 Available online 24 August 2005 Abstract The framework about the odd random phase multisine with a random harmonic grid [E. Van Gheem, R. Pintelon, J. Vereecken, J. Schoukens, A. Hubin, P. Verboven, O. Blajiev, Electrochim. Acta, submitted for publication] developed in Part I of this series of two papers, is now applied to aluminium (99.5 wt.%) in an aerated 0.5 M NaCl solution. Due to the special design of this excitation signal it becomes possible to obtain information about the non-stationary behaviour of the aluminium system within successive measurements, and to correct the impedance spectra for this behaviour. In addition, the proposed methodology determines the type and level of non-linearities in the measured spectra. From these results the investigator has two choices. On the one hand, optimal EIS measurement conditions for linear modelling purposes can be derived, based on a tradeoff between the minimization of the level of the non-linearities and an acceptable noise level. On the other hand, from the results with the best signal to noise ratio, the knowledge about the non-linear distortions in the impedance spectra can be further elaborated as a useful tool during the modelling procedure. Finally, the standard deviations of the corrected impedance spectra are calculated, which are generally used as a weighting factor to improve the modelling procedure. © 2005 Elsevier Ltd. All rights reserved. Keywords: Aluminium; Corrosion; Electrochemical impedance; Non-linear distortions; Non-stationary behaviour 1. Introduction To obtain the transfer function of a system with electro- chemical impedance spectroscopy (EIS), implies that the in- vestigated system should meet the conditions of causality, linearity and time-invariance [1] (cf. Part I [2]). The method most commonly used, to perform EIS is based on a step- wise change of the frequency of an imposed sinusoidal volt- age [3–16]. The requirement about the linearity is in these studies often approximated by decreasing the amplitude of the excitation signal, but it is not explicitly proven. In or- ∗ Corresponding author. Tel.: +32 2 629 35 36; fax: +32 2 629 32 00. E-mail address: evgheem@vub.ac.be (E. Van Gheem). der to fulfill the condition of time-invariance, most studies present long-term (i.e. more than 2 h) [4–15] EIS data of alu- minium since they consider the electrode–electrolyte inter- face to be in a macroscopically steady-state. An alternative is a broadband signal which imposes all frequencies at the same time, having the important advantage of a significant reduction of the measurement time. Recently, Darowicki et al. [17–19] developed a method to perform impedance mea- surements with a periodic broadband excitation. However, the method is apparently not applied for low frequencies (<1 Hz), which are often of interest when corrosion processes are studied. Another attempt is performed by Popkirov et al. [20–22] who proposed a sum of properly defined sine waves to obtain the frequency response function (FRF). The final 0013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2005.02.096