Anodic oxidation of pure niobium in salt water and sulphuric acid solutions and characterization of oxides

Anodic oxidation (anodization) is a widely used thin film deposition technique. Anodic oxidation is the process of forming an oxide (MxOy) on a metal surface (M) by applying an electrical current or potential through a cell containing a suitable electrolyte. Depending on the experimental oxide growth conditions, barrier type films are produced as well as porous type films. Both types are used in different technological applications. Being economical, simple and clean are among the advantages of anodic oxidation method. Thin films which were produced with this method are used in different industrial applications such as electronics industry, biocompatible materials, electrochromic materials etc. In this study, the electrochemical corrosion behavior of pure niobium in 3.5 % NaCl, 0.1 M H2SO4 and 0.1 M H2SO4 including 0.5 % HF solutions were analyzed and the oxide was grown with anodic oxidation by potentiostatic mode in these solutions. According to polarization curves, it was found that niobium showed passive behavior under these three conditions and the passive region reached to 2V. Due to passivation of niobium in NaCl solution and wide passivation region, chlorine ions did not cause pitting on niobium surface. Oxide growth process on niobium surface was carried out at 0.15V, 0.3V, 0.5V, 0.65V, 1V and 2V in 3.5 % NaCl; 0.5V, 0.65V, 1V, 2.5V, 5V and 7.5V in 0.1 M H2SO4; 1V and 5V in 0.1 M H2SO4 consisting 0.5 % HF solutions. Afterwards, EIS measurements of anodic oxides were performed and Nyquist curves which were obtained before and after anodic oxidation were compared. XRD analysis showed that oxides consists of NbO, NbO2 and Nb2O5 complex phases. AFM analysis showed that oxides have different surface roughness which increases as the anodizing voltage increases. Anodic films which were grown in HF containing solutions observed to be porous because of the fluorine ions in the solution.