Influence of Oxygen-Enriched Layer on the Passivity of Ti-Al-Zr Alloy in an Alkaline Environment

Abstract

The manuscript establishes role of an oxygen-enriched layer on the passivity of an alpha Ti alloy, Ti-Al-Zr, in an alkaline environment. The alloy was air-oxidized at 760 °C for different durations up to 700 h. This led to the formation of an oxide layer on the surface and oxygen-enriched layer in the matrix near to the surface. The oxygen-enriched layer had higher hardness as compared to the base metal. A correlation between micro-hardness and normalized oxygen content was established. The influence of oxygen-enriched layer on the passivity of the material was established in ammonia solution (pH: 10.4) at 25 °C by different electrochemical techniques- potentiodynamic polarization, chronoamperometry and electrochemical impedance spectroscopy. Donor density had decreased with increasing oxygen content in the matrix near to the surface. The diffusivity of oxygen vacancies in the passive film was calculated in the order of 10−17 cm2 s−1. The oxygen-enriched layer was shown to promote the formation of a more protective passive film.