Abstract
In this work a systematic study of plasma electrolytic oxidation (PEO)-coated alloys, namely AA2024, AA5083 and AW6082, was carried out using electrochemical impedance spectroscopy (EIS) to understand how different electrolytes and aluminum grades affect the properties and corrosion protective behavior of PEO coatings. Preliminary microstructural and phase composition characterization of PEO-coated samples was done using electron microscopy (SEM) and X-ray diffraction (XRD). EIS measurements were used to evaluate the behavior of different coatings during immersion in NaCl solutions. Only the dense part of the PEO coatings contributed to the impedance at high frequencies, and the calculated thickness of the dense part from fitting results was consistent with the thickness of the dense part obtained from SEM images of PEO cross-sections. Such a situation was found on all PEO-coated alloys having thin (about 5 µm) and thick (about 40 µm) PEO coatings. Additional model EIS experiments were conducted to help in evaluating low frequency parts in the EIS spectra and assign the diffusion and corrosion processes to their respective time constants. The results were rationalized and degradation models were developed and discussed with the help of corrosion, microstructural, chemical and elemental analyses. A proper analysis of EIS fitting parameters is imperative for establishing degradation modes of PEO layers and quantifying the coating degradation and corrosion progress.