Oxidation and electrochemical characterization of oxide formed on Ti-Al-Zr in a neutral steam environment

Abstract

The study focuses on the oxidation and the electrochemical behavior of the oxide formed on an alpha titanium alloy, Ti-Al-Zr, in a neutral steam environment. The alloy was exposed in a static autoclave at 400 °C, 10 MPa for durations up to 90 days. The oxidation kinetics and the rate laws were established by the discontinuous gravimetric method. The characterization of the oxide was done by scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), glow discharge optical emission spectroscopy (GDOES) and Raman spectroscopy. The electrochemical characterization of the high-temperature oxide film was done by a potentiodynamic polarization experiment and ex-situ electrochemical impedance spectroscopy (EIS) measurement. The defect density of the oxide was calculated using the Mott-Schottky (M-S) analysis. A physical model was developed based on the impedance response, within the framework of point defect model (PDM) theory. The oxidation kinetics followed a nearly fourth power rate law followed by a linear rate law. The oxide consisted of an inner barrier layer and an outer layer containing crystallized grains. The transition in the oxidation kinetics was attributed to the recrystallization of the outer layer. The defect density for all the oxidized specimens was of the order of 1021 cm−3. The EIS study, in conjunction with the PDM, established that the oxidation kinetic was controlled by both titanium interstitials and oxygen vacancies. A correlation between the oxidation kinetics and the kinetics of the interfacial reactions was established and the oxide growth mechanism in the steam environment is discussed.