Structural vs. electrochemical investigation of niobium oxide layers anodically grown in a Ca and P containing electrolyte

Highlights:• Anodizing of niobium promotes the growth of a passive, corrosion resistant oxide.• The anodizing potential modulates the oxide layer morphology and composition.• Anodozing in the ASD regimes leads to significant inclusions of Ca and P ions.• The corrosion behaviour is affected by the oxide thickness and porosity.• EIS allowed to distinguish three different contributions to the corrosion process. -- Abstract: The growth of anodic oxide on niobium has been investigated as a function of the applied potential in a solution containing P and Ca-EDTA complexes. Anodizing below and above the breakdown potential has been explored via morphological (AFM and SEM) and compositional (XPS and EDX) analysis. Below the breakdown potential, thin (few hundreds of nanometers) and uniform oxide layers are formed, with negligible inclusions of electrolyte ions. Conversely, above the breakdown potential, in the so-called ASD regime, thicker and microporous oxide layers are observed, with a significant enrichment of the matrix with Ca and P. At potentials around 250 V the structure evolves towards a highly inhomogeneous and porous layer with the presence of some fractures, potentially affecting the oxide passivation properties. The corrosion resistance properties of the oxide layers have been investigated by electrochemical methods (OCP, anodic polarization and EIS). Data indicate that the corrosion resistance, initially increasing with the anodizing potential, weakens at potentials around 250 V. A potential of about 200 V represents, therefore, a good tradeoff between morphological and compositional properties and resistance to corrosion, which are important issues in view of osteoconductive properties for orthopaedic implant applications.