Impedance spectroscopy characterisation of PEO process and coatings on aluminium

Impedance spectroscopy is a comprehensive technique that operates with frequency response (FR) to provide useful information about both surface treatments and layers produced. In this paper, two FR types were obtained for the aluminium samples during treatment by plasma electrolytic oxidation (PEO). The first was obtained in-situ during PEO, using a frequency sweep of bipolar voltage pulses between 20 Hz and 20 kHz. The modulus and phase angle components of the frequency response were analysed asfunctions of voltage and treatment time. With increased frequency, theimpedancemodulusandphaseanglewerefoundtodecreasefrom10 4 to10 Ωandfrom0to −80°respectively.ThesecondFRwasobtainedusing electrochemical impedance spectroscopy (EIS) of PEO coatings, at open-circuit potential. The samples were found to exhibit an active-capacitive load behaviour, whereby the impedance decreased from 10 6 to 10 2 Ω, and phase angle varied between −20° and −80° with the frequency increase. Both FRs show similar behaviour within the frequency range from 5 to 20 kHz. This approach allows estimations of the frequency response of the PEO microdischarges, to justify the optimal frequency for the pulsed bipolar PEO and also suggests concepts for feedback process control. © 2007 Elsevier B.V. All rights reserved.