Large amplitude voltage oscillations during plasma electrolytic oxidation (PEO) of niobium and their correlation with the porous morphology

Abstract

Plasma electrolytic oxidation (PEO) is a technique that has been widely studied to obtain thick anodic oxide coatings based on aluminum, titanium, niobium, and other valve metals. In the case of niobium oxides, they are attractive because of their properties as semiconductors, biocompatibility, and high corrosion resistance. The electrochemical behavior during this process can vary according to the PEO parameters and influence characteristics such as coating thickness, adherence, morphology, and crystalline structure of the oxide film. In this study, unusual voltage oscillation transients are observed during the anodization of niobium under a galvanostatic PEO regime. The niobium substrates were anodized in three different electrolytic solutions (H3PO4, KOH, and C2H2O4), and the influence of temperature on the oscillation aspects in each one was investigated. In addition, three different current densities were tested to check the effect of this parameter on voltage oscillations. Scanning Electron Microscope (SEM), Energy Dispersive Spectrometer (EDS), and X-ray Diffraction (XRD) are used to characterize the samples. The results indicate a relationship between the exposed surface area and the pore morphology during the film growth in the oscillatory regime. A simultaneous process of oxide dissolution/pore filling and the micro-discharges observed during the PEO process is proposed.